The Transimpedance Amplifier: Heart of the Barcode Reader Signal Chain |
Executive Summary |
This article provides a comprehensive, accessible exploration of the transimpedance amplifier (TIA), the critical circuit that converts the tiny current from a photodiode into a usable voltage in barcode readers. We examine why this amplifier is the heart of the reader, affecting everything from signal quality and reading range to noise performance and speed. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real products from industry leaders including Texas Instruments, Microchip Technology, and various pioneering patent holders. We explore the fundamental challenge of converting current to voltage, the critical role of the feedback resistor in setting gain, the necessity of compensation for stability, and the practical design considerations for real-world barcode reading applications. The article covers both simple discrete designs and sophisticated integrated solutions, with special attention to the practical trade-offs between gain, bandwidth, and noise. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing or selecting transimpedance amplifier circuits for barcode reader systems. |
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Chapter 1: The Current-to-Voltage Problem |
The photodiode at the heart of a barcode reader produces a current, not a voltage. When light strikes the photodiode, it generates electron-hole pairs, producing a current that flows through the device. This current is directly proportional to the incident light intensity---more light means more current, and less light means less current. |
But the rest of the electronic world expects voltages. The comparators that digitize the signal need voltage inputs. The analog-to-digital converters that sample the signal need voltage inputs. The microcontrollers that decode the barcode need digital signals derived from voltages. So the current from the photodiode must be converted to a voltage before it can be processed. |
The photodiode current is tiny. Even with a strong reflection from a white space, the current might be only a few microamps. From a black bar, it might be nanoamps or less. The transimpedance amplifier must take this minuscule current and produce a usable voltage---typically from zero to five volts or more---without adding significant noise or distortion. |
This is the task of the transimpedance amplifier, or TIA. It is the first active signal processing stage in the reader after the photodiode itself. Its performance sets the foundation for everything that follows. A well-designed TIA preserves the linearity and speed of the photodiode, providing a clean, strong signal that can be digitized and decoded. A poorly designed TIA can swamp the signal in noise, distort the amplitude information, or limit the bandwidth to the point that the barcode pattern is blurred. |
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Chapter 2: The Basic Transimpedance Amplifier |
The transimpedance amplifier is a specialized operational amplifier configuration. In its simplest form, it consists of an op-amp with a feedback resistor connected between the output and the inverting input. The non-inverting input is connected to a reference voltage, typically ground or a mid-supply voltage. The photodiode is connected between the inverting input and ground or a bias voltage. |
The principle of operation is straightforward. The op-amp's negative feedback forces the inverting input to be at the same voltage as the non-inverting input. This means the photodiode sees a nearly constant voltage across it. The photodiode current cannot flow into the op-amp's input because the input impedance is very high. So the current must flow through the feedback resistor, creating a voltage drop. The output voltage of the op-amp is equal to this voltage drop, which is the photodiode current multiplied by the resistance of the feedback resistor. |
This is why the transimpedance amplifier is often called a current-to-voltage converter. The output voltage is directly proportional to the input current, with the proportionality constant set by the feedback resistor. A larger feedback resistor gives more gain---a larger output voltage for a given input current. A smaller feedback resistor gives less gain. |
Microchip Technology provides a concise description of this principle in their application note on amplifying high-impedance sensors: 'At low frequencies, the op amp's inverting input is forced to be at ground potential and the source current must flow through the feedback resistor. This combination of effects creates an output voltage equal to the source current multiplied by the feedback resistance.' |
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Chapter 3: The Feedback Resistor and Gain |
The feedback resistor is the most important component in the transimpedance amplifier because it sets the gain. For a photodiode current of 1 microamp and a feedback resistor of 100 kilohms, the output voltage is 0.1 volts. For the same current with a 1 megohm resistor, the output voltage is 1 volt. |
The choice of feedback resistor is a balancing act. A larger resistor gives more gain, making it easier to detect weak signals. But a larger resistor also adds more noise. Thermal noise, also called Johnson noise, increases with the square root of the resistance. A 1 megohm resistor has more than three times the thermal noise of a 100 kilohm resistor. |
A larger resistor also limits the bandwidth. The feedback resistor and the photodiode's capacitance form a low-pass filter. A larger resistance means a lower cutoff frequency, reducing the circuit's ability to respond to fast changes in light. This is particularly important for barcode reading, where the pattern changes rapidly as the reader moves across the symbol. |
The Texas Instruments TIPD176 reference design illustrates this trade-off. The design uses a feedback resistor that gives a gain of 53.6 kilovolts per amp, producing output voltages from 0.1 to 4.9 volts for photodiode currents of 0 to 90 microamps. The bandwidth is greater than 1 megahertz, sufficient for barcode scanning. This design shows how careful component selection can balance gain, bandwidth, and noise. |
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Chapter 4: The Photodiode Capacitance Problem |
The photodiode is not just a current source. It also has capacitance. This capacitance comes from the depletion region of the PN junction, and it is the primary factor limiting the speed of the photodiode. In reverse bias, the capacitance is reduced, which is why photoconductive mode is preferred for high-speed applications. |
The photodiode's capacitance creates a problem for the transimpedance amplifier. The capacitance and the feedback resistor form a pole in the amplifier's frequency response. If this pole is not properly compensated, the amplifier can oscillate or ring. |
Microchip Technology's application note on photodiode amplifiers describes this problem and the solution: 'Placing a negative bias on the photodiode significantly reduces its junction capacitance, which allows the circuit to operate at a much higher speed. This reverse bias also increases the dark current and current noise, however. A capacitor can be used to limit the bandwidth and help stabilize the circuit when the photodiode's junction capacitance is large.' |
The compensation capacitor is placed in parallel with the feedback resistor. It creates a pole in the noise gain that can stabilize the amplifier. But the compensation capacitor also limits the bandwidth. A larger compensation capacitor provides more stability but reduces the speed. A smaller compensation capacitor provides less stability but allows higher speed. |
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Chapter 5: The Compensation Capacitor and Stability |
The compensation capacitor is the key to a stable transimpedance amplifier. It prevents oscillation while allowing the maximum possible bandwidth. Selecting the right value is one of the most critical steps in the design. |
Microchip Technology's application note provides a method for selecting the compensation capacitor. The design starts with the compensation capacitor set to zero and then analyzes the circuit's stability using Bode plots. Three cases are identified based on the relationship between the noise gain zero and the op-amp's gain-bandwidth product. |
The first case occurs when the photodiode capacitance is small. The circuit is potentially stable without compensation. The compensation capacitor may not be needed, or it may be added to improve the phase margin. |
The second case occurs when the photodiode capacitance is moderate. The circuit has poor stability, and the compensation capacitor must be increased to improve the phase margin. |
The third case occurs when the photodiode capacitance is large. The circuit is stable, and the compensation capacitor may be reduced to improve the noise performance. |
The application note provides specific guidelines for selecting the compensation capacitor to achieve a stable circuit while minimizing noise. These guidelines are based on the relationship between the compensation capacitor and the op-amp's gain-bandwidth product. |
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Chapter 6: The Feedback Capacitor in the Compensation Network |
The compensation capacitor is connected in parallel with the feedback resistor. It is often called the feedback capacitor, and its value is typically just a few picofarads. A patent from a barcode reader manufacturer describes a transimpedance amplifier with a compensation capacitor of 10 picofarads, used to stabilize the amplifier for barcode signals over a wide frequency range. |
The patent notes that the bar code signal frequencies are below the frequency that is carried through the compensation capacitor, because of its small capacitance value. This means the capacitor does not affect the bar code signal but provides stability at higher frequencies where oscillation might occur. |
The compensation capacitor is part of a more complex network that includes the feedback resistor and the photodiode's capacitance. This network's behavior determines the amplifier's stability and bandwidth. Proper design of this network is essential for reliable barcode reading. |
Microchip Technology's application note emphasizes that the compensation capacitor is chosen 'to keep the transimpedance amplifier stable' and that 'the capacitors also limit the bandwidth of the transimpedance amplifier.' This trade-off between stability and bandwidth is central to transimpedance amplifier design. |
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Chapter 7: Texas Instruments' TIPD176 Reference Design |
Texas Instruments provides a practical reference design for a transimpedance amplifier suitable for barcode scanners. The TIPD176 is a single-supply photodiode amplifier with a bandwidth greater than 1 megahertz. |
The design is configured as a transimpedance amplifier with a small bias voltage derived from the positive supply and applied to the op-amp's non-inverting input. This prevents the output from saturating at the negative supply rail in the absence of input current. It is designed to produce output voltages of 0.1 to 4.9 volts for photodiode currents of 0 to 90 microamps. |
The TIPD176 includes theory, component selection, simulation, PCB schematic and layout, bill of materials, and measured results. This comprehensive documentation makes it easier for designers to implement a high-performance transimpedance amplifier in their barcode readers. |
The TIPD176's 1 megahertz bandwidth is typical for barcode scanner applications. The feedback resistor provides a gain of 53.6 kilovolts per amp, producing a usable output voltage from the small photodiode current. The design's single-supply operation makes it suitable for battery-powered handheld readers. |
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Chapter 8: Microchip's Photodiode Light Detector Demo Board |
Microchip Technology offers a Photodiode Light Detector Demo Board that showcases transimpedance amplifier design. The board features two different circuit implementations: photovoltaic and photoconductive mode. |
The demo board uses the MCP6021 or MCP6491 operational amplifiers and external resistors to convert photocurrent to output voltage. In the photoconductive topology, the photodetector is reverse-biased by an amplifier that provides a bias voltage. In the photovoltaic topology, the amplifier generates a reference voltage so that the circuit provides an output referenced to the supply midpoint. |
The board supports both photovoltaic and photoconductive modes, with a jumper to select between them. This allows designers to evaluate the performance of each mode for their specific application. The board also includes test points for connecting lab equipment, making it easy to measure the circuit's performance. |
The demo board includes an on-board photodiode connector with two or three pin configurations, and it supports the high precision operational amplifiers MCP6021 and MCP6491. This flexibility makes it a useful tool for evaluating transimpedance amplifier designs for barcode readers. |
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Chapter 9: Microchip's MCP6031 Photodiode PICtail Plus Demo Board |
Microchip also offers the MCP6031 Photodiode PICtail Plus Demo Board, which demonstrates how to use a transimpedance amplifier to convert photocurrent to voltage. The board uses the MCP6031 high precision operational amplifier and a PIN photodiode as the light detector. |
The MCP6031 is a high precision op-amp that is well-suited for photodiode applications. It has low input offset voltage and low input bias current, which are essential for accurate current-to-voltage conversion. The PICtail Plus form factor allows the board to be connected to Microchip's development systems. |
The demo board includes test points for connecting lab equipment, making it easy to measure the circuit's performance. The board's user guide provides instructions for setup and operation, and the firmware is available for download. This makes the board a useful starting point for designers developing barcode reader circuits. |
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Chapter 10: A Patent from Micro Video, Inc. on Enhancing Signal-to-Noise Ratio |
A patent from Micro Video, Inc. describes a technique for enhancing the signal-to-noise ratio of a transimpedance operational amplifier. The patent addresses a fundamental trade-off in transimpedance amplifier design: using a large feedback resistor to increase the gain and signal-to-noise ratio, and compensating for the lost bandwidth without reducing the signal-to-noise ratio. |
The patent describes a compensation network that provides a zero which cancels the upper 3 dB pole of the transimpedance amplifier, and also provides a high frequency pole defining the restored bandwidth of the amplifier. This allows the designer to use a large feedback resistor for high gain and low noise while still achieving the bandwidth needed for barcode reading. |
This technique is particularly important for barcode readers because it enables high sensitivity without sacrificing speed. The ability to use a large feedback resistor improves the signal-to-noise ratio, making it easier to detect weak reflected light from distant or low-contrast barcodes. The compensation network restores the bandwidth, ensuring that the reader can still capture fast-moving barcodes. |
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Chapter 11: The Patent on Using a Field Effect Transistor as a Variable Resistor |
A patent from Symbol Technologies describes a technique for preventing high-gain transimpedance amplifiers from saturating in bright ambient light. The patent addresses a problem that affects all barcode readers: ambient light adds a DC current to the photodiode signal, which can drive the amplifier into saturation. |
The technique uses an offset adjustment circuit with a field effect transistor as a variable resistor. The FET's resistance is controlled by the output amplified signal. When the output voltage approaches a level that would cause saturation, the FET's resistance decreases, applying an offset current to the amplifier input that forces the amplifier away from saturation. |
The patent explains that 'the brighter the ambient light, the larger the offset current and, hence, the amplifier will not saturate even under the brightest sunlight.' This allows the designer to use a high-gain transimpedance amplifier without fear of saturation, maximizing the signal-to-noise ratio. |
The patent also notes that 'the variable resistor does not affect the resistance of the feedback resistor, does not change the amplifier gain, and does not introduce shot or white noise into the output amplified signal.' This is a significant advantage over simply reducing the gain to prevent saturation. |
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Chapter 12: The Two-Channel Detector Patent |
A patent from a barcode reader manufacturer describes a multi-channel detector that uses multiple photodiodes and transimpedance amplifiers. The detector includes an array of photodiodes, each with its own transimpedance amplifier. The outputs of the amplifiers are summed to produce a single output signal. |
The patent explains the advantages of this approach. For a given field of view, each photodiode generates about half the signal level and half the noise level of a single-channel detector. The total signal level at the output of the summing unit is approximately equal to the signal level of a single-channel detector. However, the total noise level is only about 70% of the noise level of a single-channel detector. The signal-to-noise ratio is improved by a factor of about 1.4. |
This improvement is because 'noise adds statistically' while 'signals add algebraically.' The patent also notes that the use of a multi-channel detector allows a larger feedback resistor to be used, because there is less ambient light received by each photodiode. This further improves the signal-to-noise ratio. |
The patent describes both transimpedance and equalizing modes of amplification. Transimpedance amplification is generally preferred for non-retroreflective readers, which have larger fields of view and collect more ambient light. Equalizing amplification is generally preferred for retroreflective readers, which have lower ambient light levels. |
Chapter 13: The Role of the Op-Amp in Transimpedance Amplifiers |
The operational amplifier is the active component in the transimpedance amplifier, and its characteristics are critical to the circuit's performance. The op-amp must have low input bias current and low input offset voltage to accurately convert the small photodiode current. |
Microchip Technology's application note on amplifying high-impedance sensors discusses the key op-amp characteristics needed for transimpedance amplifiers: 'low input offset voltage, low input bias current, high input impedance and an input common mode range that includes ground. The low input offset voltage and low input bias current support a very low voltage drop across the photodiode; this gives the best photodiode linearity.' |
The MCP6031 op-amp, used in Microchip's demo board, is a high precision device with low input offset voltage and low input bias current. The MCP6021, used in the Photodiode Light Detector Demo Board, is another example of an op-amp well-suited for photodiode applications. |
The op-amp's gain-bandwidth product is also important. A higher gain-bandwidth product allows the amplifier to achieve higher bandwidth at a given gain. This is particularly important for barcode readers, where high speed is essential. |
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Chapter 14: Photovoltaic vs. Photoconductive Mode in Transimpedance Amplifiers |
The transimpedance amplifier can be used with photodiodes in either photovoltaic or photoconductive mode. The choice of mode affects the circuit's speed, noise, and linearity. |
In photovoltaic mode, the photodiode is biased at zero volts. The photodiode's capacitance is higher, limiting the speed. But there is no dark current, so the noise is lower. The Microchip demo board's photovoltaic circuit uses an amplifier to generate a reference voltage that allows the output to be sampled by an analog-to-digital converter. |
In photoconductive mode, the photodiode is reverse-biased. The reverse bias reduces the photodiode's capacitance, increasing the speed. But the reverse bias also introduces dark current, which adds noise. The Microchip demo board's photoconductive circuit uses an amplifier to provide the reverse bias. |
The Microchip user guide notes that photovoltaic topology has advantages of optimal accuracy (both linear and sensitive) and simple diode biasing, but disadvantages of slower operation and the need for a considerably faster operational amplifier. Photoconductive topology has advantages of fast operation and compatibility with PWM communications, but disadvantages of reduced accuracy due to dark current and the need for a considerably faster operational amplifier. |
Chapter 15: The Differentiator and Transimpedance Amplifier |
A patent from a barcode reader manufacturer describes a transimpedance amplifier stage followed by a differentiator. The transimpedance amplifier converts the photodiode current to a voltage, and the differentiator finds the edges of the barcode signal. |
The patent explains that the transimpedance amplifier's output voltage is 'relatively low and requires amplification in order to enable the bar code video signal to be digitized.' The differentiator stage amplifies the signal and differentiates it, producing a signal with peaks at the barcode edges. |
The differentiator's time constant is determined by a capacitor and a resistor. The patent notes that 'at high frequencies, there is a phase delay between the bar code video signal and the derivative thereof.' The phase delay depends on the frequency of the signal and must be compensated for. |
This approach to barcode signal processing uses the transimpedance amplifier as the first stage, followed by a differentiator that emphasizes the edges of the barcode. The differentiated signal is then digitized and decoded. |
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Chapter 16: The Logarithmic Compression Amplifier |
A patent from a barcode reader manufacturer describes a transimpedance amplifier followed by a logarithmic compression amplifier. The transimpedance amplifier converts the photodiode current to a voltage, and the logarithmic compression amplifier compresses the dynamic range of the signal. |
The patent explains that the output of the photodetector stage is applied to a logarithmic compression amplifier similar to the amplifier. The amplified and compressed signal is then converted into a stream of digital values by an analog-to-digital converter. |
The logarithmic compression amplifier reduces the dynamic range of the signal, making it easier to digitize with a fixed number of bits. The compressed signal still preserves the barcode information because the edges---the transitions between bars and spaces---are preserved in the compressed signal. |
The patent notes that the processor may process the bytes from the ADC by 'performing an inverse logarithmic function and providing an output which is the inverse log of the input.' This restores the original signal, undistorted and with precise leading and lagging edges. |
Chapter 17: The Summing Amplifier in Multi-Channel Detectors |
The multi-channel detector patent describes a summing amplifier that combines the outputs of multiple transimpedance amplifiers. The summing amplifier adds the signals from all the photodiodes, producing a single output signal. |
The summing amplifier is a critical component in the multi-channel detector. It must add the signals without introducing significant noise or distortion. The patent describes a summing circuit that provides an output signal representative of the algebraic sum of the output values of all of the photodiodes. |
The summing amplifier's input is coupled to the outputs of the transimpedance amplifiers. Low-pass filters may be employed at the output of each transimpedance amplifier to attenuate high-frequency noise signals. This ensures that the summing amplifier receives clean signals from each channel. |
The use of a summing amplifier is a key advantage of the multi-channel detector. It combines the individual signals into a single output that can be processed by the rest of the reader's circuitry. |
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Chapter 18: The Equalizing Amplifier |
The multi-channel detector patent also describes an equalizing amplifier. The equalizing amplifier is used to offset the bandwidth-limiting effects of the front-end low-pass filters and thereby increase the bandwidth of the reader. |
The equalizing amplifier is a specialized type of summing amplifier that includes a resistor and capacitor in parallel across the inverting feedback loop of an operational amplifier. This network provides high-frequency boost, compensating for the low-pass filtering of the preamplifiers. |
The patent explains that the equalizing amplifier is generally preferred for retroreflective readers that have low ambient light levels. The preamplifiers in retroreflective readers use large resistors to achieve high sensitivity, but these resistors create low-pass filters that limit the bandwidth. The equalizing amplifier compensates for this bandwidth limitation, restoring the high-frequency response. |
The use of an equalizing amplifier is a trade-off. It increases the bandwidth, but it also amplifies noise. The choice of equalizing amplification versus transimpedance amplification depends on the specific requirements of the reader. |
Chapter 19: The Logarithmic and Inverse Logarithmic Processing |
The patent on logarithmic compression describes how the digital values from the analog-to-digital converter can be processed by a processor. The processor may operate by 'looking for a change in the value of the byte and selecting a change which reflects accurately the transitional location.' |
The patent also describes an alternative method where the bytes from the ADC are processed by 'performing an inverse logarithmic function and providing an output which is the inverse log of the input.' This restores the original signal, undistorted and with precise leading and lagging edges. |
The inverse logarithmic processing converts the compressed signal back to its original form. The patent notes that 'by converting (Digital to analog conversion) the BC signal can be restored so as to provide the digitized, digital data signal.' |
This approach demonstrates the flexibility of digital signal processing in barcode readers. The signal can be compressed for efficient analog-to-digital conversion and then decompressed in the digital domain. |
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Chapter 20: The Lead-Lag Compensation Network |
A patent from Micro Video, Inc. describes a compensation network that provides a lead-lag response. The network includes a zero that cancels the upper 3 dB pole of the transimpedance amplifier and a high-frequency pole that defines the restored bandwidth. |
The lead-lag compensation network allows the use of a large feedback resistor, which improves the signal-to-noise ratio, while still achieving the bandwidth needed for the application. The zero cancels the low-pass pole caused by the feedback resistor and photodiode capacitance, extending the bandwidth. The high-frequency pole limits the bandwidth to prevent instability. |
This compensation technique is a sophisticated approach to transimpedance amplifier design. It addresses the fundamental trade-off between gain, bandwidth, and noise by actively shaping the frequency response of the amplifier. |
The patent does not specify the component values for this compensation network, but the technique is well-known in analog circuit design. It is a powerful tool for designers who need to achieve both high sensitivity and high speed. |
Chapter 21: The FET Variable Resistor in Detail |
The Symbol Technologies patent provides a detailed description of how a field effect transistor can be used as a variable resistor to prevent amplifier saturation. The FET is connected across the amplifier's feedback network, and its resistance is controlled by the output voltage. |
The patent explains that 'the resistance is decreased at the amplifier input when the output amplified signal is increased in high intensity ambient light to drive the amplifier away from saturation.' This is a negative feedback mechanism: as the signal increases, the FET resistance decreases, reducing the effective gain and preventing saturation. |
The FET's resistance is controlled by the output amplified signal. The brighter the ambient light, the larger the offset current and, hence, the amplifier will not saturate even under the brightest sunlight. The variable resistor does not affect the resistance of the feedback resistor, does not change the amplifier gain, and does not introduce shot or white noise. |
This technique is a clever solution to a common problem in barcode readers. It allows the use of a high-gain transimpedance amplifier without sacrificing performance in bright ambient light. |
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Chapter 22: The Saturation Problem in Barcode Readers |
The Symbol Technologies patent addresses a specific problem in barcode readers: saturation of the transimpedance amplifier in bright ambient light. The patent explains that the output voltage of the amplifier 'increases or decreases in dependence on increases or decreases in the collected light intensity. If the collected light intensity is bright enough, the output voltage will go so high that the amplifier is incapable of going any higher. This is called saturation and, when this happens, the reader will not function because the data signal derived from the symbol will be lost.' |
Standard practice to prevent saturation is to decrease the gain of the transimpedance amplifier. But this reduces the signal faster than the noise, degrading the signal-to-noise ratio. This is why non-retroreflective readers, which collect more ambient light, have had poorer signal-to-noise ratios than retroreflective readers. |
The patent's solution---using a FET variable resistor to control an offset current---allows the amplifier to maintain high gain while preventing saturation. The offset current forces the amplifier away from saturation, but the gain remains high, preserving the signal-to-noise ratio. |
Chapter 23: The Barcode Reader Transimpedance Amplifier Circuit |
A patent from a barcode reader manufacturer provides a specific circuit diagram for a transimpedance amplifier. The circuit includes the photodiode, the op-amp, the feedback resistor, and the compensation capacitor. |
The patent explains that the photodiode current is 'principally through the feedback resistor.' The compensation capacitor 'is used to stabilize the amplifier and the bar code signal frequencies are below the frequency that is carried through the compensation capacitor, because of its small capacitance value.' |
The photodiode current is 'effectively multiplied by the value of the feedback resistor and appears at the output.' The current is in the hundred nanoamp range so that the voltage is relatively low and requires amplification in order to enable the bar code video signal to be digitized. |
This circuit is a classic transimpedance amplifier design. The op-amp provides the gain, the feedback resistor sets the gain, and the compensation capacitor ensures stability. The circuit's simplicity belies its importance in the barcode reader signal chain. |
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Chapter 24: The Transimpedance Amplifier in the Hackaday Supercon Badge Reader |
A practical, hobbyist-level example of a transimpedance amplifier can be found in the Supercon 2022 Badge Card Reader project. This project used LEDs reverse-biased as photodiodes and a transimpedance amplifier with a 1 megohm feedback resistor. |
The project's designers, Ben Hencke and Zach Fredin, used arrays of red LEDs harvested from a donor badge. One set of LEDs served as receivers; they were wired in parallel, reverse-biased, and fed into a transimpedance amplifier with a 1 megohm gain. This technique was based on experiments by Forrest Mims III. |
The transimpedance amplifier converted the photocurrent from the LEDs into a voltage that could be read by an analog-to-digital converter. The ADC was part of a Pixelblaze Sensor Expansion Board, which provided 12-bit, 1 megasample per second conversion. |
This project demonstrates that transimpedance amplifiers are not just for commercial barcode readers. They are a fundamental technique for any application that needs to detect small currents from optical sensors. The use of LEDs as photodiodes is a clever, low-cost alternative to commercial photodiodes. |
Chapter 25: The Photodiode Amplifier Demo Board and PICtail Plus |
Microchip provides multiple demo boards for photodiode amplifiers. The Photodiode Light Detector Solutions Demo Board features two photodetection topologies: photovoltaic and photoconductive. The MCP6031 Photodiode PICtail Plus Demo Board demonstrates how to use a transimpedance amplifier to convert photocurrent to voltage. |
The Photodiode Light Detector Solutions Demo Board uses a photodiode with two or three pin configurations. An example of a photodiode circuitry implementation is found in application note AN1494. The board supports the MCP6021 or MCP6491 operational amplifiers and external resistors to convert photocurrent to voltage. |
The MCP6031 Photodiode PICtail Plus Demo Board uses a high precision operational amplifier and a PIN photodiode as the light detector. The circuit is not calibrated for absolute accuracy, but it includes an RC low-pass filter to remove high-frequency noise and interference from the signal path prior to analog-to-digital conversion. |
These demo boards provide practical examples of transimpedance amplifier circuits for barcode readers. They are useful tools for learning about photodiode amplification and for developing custom circuits. |
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Chapter 26: The Open-Loop Gain of the Op-Amp |
The open-loop gain of the operational amplifier is an important factor in transimpedance amplifier performance. A higher open-loop gain allows the amplifier to achieve higher closed-loop accuracy. However, the open-loop gain decreases with frequency, limiting the bandwidth. |
Microchip Technology's application note on amplifying high-impedance sensors discusses the effect of op-amp gain on stability. The open-loop gain can be approximated as a single-pole response, with a gain-bandwidth product that determines the frequency response of the amplifier. |
The gain-bandwidth product is the product of the open-loop gain and the frequency at which it is measured. For a given op-amp, the gain-bandwidth product is constant. A higher gain-bandwidth product allows the amplifier to achieve higher bandwidth at a given gain. |
The selection of an op-amp with sufficient gain-bandwidth product is essential for achieving the required bandwidth in a transimpedance amplifier. The TIPD176 reference design, for example, uses an op-amp with sufficient gain-bandwidth product to achieve a bandwidth greater than 1 megahertz. |
Chapter 27: The Noise Gain in Transimpedance Amplifiers |
The noise gain is a concept that simplifies the analysis of transimpedance amplifier stability and noise. It is the gain of the amplifier from the input noise source to the output. |
Microchip Technology's application note defines the low-frequency noise gain as 1 volt per volt, and the high-frequency noise gain as 1 plus the ratio of the photodiode capacitance to the compensation capacitance. The noise gain increases with frequency, which affects the amplifier's stability. |
The noise gain's zero and pole are determined by the feedback resistor, the compensation capacitor, and the photodiode capacitance. The relationship between the zero and pole determines the amplifier's phase margin and stability. |
The application note provides a detailed analysis of the noise gain for three cases, corresponding to small, moderate, and large photodiode capacitance. This analysis guides the selection of the compensation capacitor to achieve stability while minimizing noise. |
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Chapter 28: The Bandwidth of the Transimpedance Amplifier |
The bandwidth of the transimpedance amplifier determines its ability to respond to rapidly changing light signals. For barcode reading, sufficient bandwidth is essential to capture the pattern as the reader moves across the symbol. |
The bandwidth is primarily limited by the feedback resistor and the photodiode capacitance. A larger feedback resistor gives more gain but less bandwidth. A smaller feedback resistor gives less gain but more bandwidth. |
The compensation capacitor also affects the bandwidth. A larger compensation capacitor provides more stability but reduces the bandwidth. A smaller compensation capacitor provides less stability but allows higher bandwidth. |
The TIPD176 reference design achieves a bandwidth greater than 1 megahertz with a gain of 53.6 kilovolts per amp. This bandwidth is sufficient for most barcode reading applications, where the pattern changes at rates up to several hundred kilohertz. |
Chapter 29: The Stability Analysis of the Transimpedance Amplifier |
Stability analysis is a critical step in transimpedance amplifier design. An unstable amplifier can oscillate or ring, corrupting the barcode signal and making decoding impossible. |
Microchip Technology's application note provides a method for stability analysis using Bode plots. The method involves calculating the noise gain zero and pole and comparing them to the op-amp's gain-bandwidth product. Three cases are identified: |
Case 1: The circuit is potentially stable without compensation. |
Case 2: The circuit has poor stability and requires compensation. |
Case 3: The circuit is stable, and the compensation can be reduced to improve noise. |
The application note provides guidelines for selecting the compensation capacitor in each case. The goal is to achieve a stable circuit with maximum bandwidth and minimum noise. |
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Chapter 30: The DC Performance of the Transimpedance Amplifier |
The DC performance of the transimpedance amplifier is determined by the op-amp's input offset voltage and input bias current. These parameters contribute to a DC offset at the output. |
Microchip Technology's application note states that 'the op amp will contribute a DC offset voltage to the output' equal to the input offset voltage plus the input bias current multiplied by the feedback resistance. The output offset voltage can be significant if the feedback resistance is large. |
The application note recommends selecting an op-amp with low input offset voltage and low input bias current to minimize the DC offset. The MCP6031, for example, is a high precision op-amp with low input offset voltage and low input bias current. |
The DC offset can also be reduced by using a bipolar supply or by adding a DC offset cancellation circuit. The TIPD176 reference design uses a bias voltage to prevent saturation in the absence of input current. |
Chapter 31: The Noise Performance of the Transimpedance Amplifier |
The noise performance of the transimpedance amplifier determines its ability to detect weak signals. The noise sources in the amplifier include thermal noise from the feedback resistor, shot noise from the photodiode, and noise from the op-amp. |
The thermal noise of the feedback resistor is proportional to the square root of the resistance. A larger feedback resistor gives more gain but also more noise. The shot noise of the photodiode is proportional to the square root of the photocurrent. The op-amp's noise is specified in the datasheet. |
Microchip Technology's application note recommends minimizing the compensation capacitor to reduce noise. A smaller compensation capacitor reduces the noise gain, reducing the noise at the output. |
The TIPD176 reference design includes measured results that confirm the noise performance of the circuit. The design achieves low noise while maintaining high gain and bandwidth. |
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Chapter 32: The Transimpedance Amplifier with a PIN Photodiode |
The PIN photodiode is the most common type of photodiode used in barcode readers. Its low capacitance and fast response make it ideal for high-speed applications. |
The Microchip PICtail Plus demo board uses a PIN photodiode as the light detector. The PIN photodiode is connected to the transimpedance amplifier, which converts the photocurrent to a voltage. The PIN photodiode's low capacitance allows high-speed operation. |
The PIN photodiode's low dark current also improves the noise performance. The dark current is typically in the nanoamp range, which is negligible compared to the photocurrent at typical light levels. |
The use of a PIN photodiode with a transimpedance amplifier is the standard approach in modern barcode readers. It provides the speed, sensitivity, and linearity needed for reliable barcode reading. |
Chapter 33: The Transimpedance Amplifier with a BPW34 Photodiode |
The BPW34 family of PIN photodiodes is among the most popular for barcode scanning. These devices offer a good balance of speed, sensitivity, and cost. |
The Microchip Photodiode Light Detector Solutions Demo Board uses a BPW34 photodiode. The board's user guide provides instructions for setting up the BPW34 photodiode and connecting it to the transimpedance amplifier. |
The BPW34 has a fast response time, low dark current, and low capacitance. These characteristics make it ideal for transimpedance amplifier circuits. The BPW34 is available in both through-hole and surface-mount packages. |
The use of a BPW34 photodiode with a transimpedance amplifier is a common combination in barcode readers. The BPW34 provides the optical performance, and the transimpedance amplifier provides the signal conditioning. |
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Chapter 34: The Transimpedance Amplifier in the TIPD176 Reference Design |
The TIPD176 reference design from Texas Instruments is a complete transimpedance amplifier circuit. It includes the op-amp, the feedback resistor, the compensation capacitor, and the bias network. |
The TIPD176's design is optimized for barcode scanners. The bandwidth is greater than 1 megahertz, and the gain is 53.6 kilovolts per amp. The design includes theory, component selection, simulation, PCB schematic and layout, bill of materials, and measured results. |
The TIPD176's single-supply operation is a key feature. It uses a small bias voltage derived from the positive supply to prevent saturation at the negative rail. This makes it suitable for battery-powered handheld readers. |
The TIPD176 is a practical example of a high-performance transimpedance amplifier. Its comprehensive documentation makes it a valuable resource for designers developing barcode readers. |
Chapter 35: The Transimpedance Amplifier in Optical Networking |
Transimpedance amplifiers are used not only in barcode readers but also in optical networking. In fiber-optic receivers, the transimpedance amplifier converts the photodiode current to a voltage for further processing. |
The TIPD176 reference design is explicitly designed for optical networking as well as barcode scanners. The high bandwidth and low noise requirements are similar for both applications. |
The use of transimpedance amplifiers in optical networking is well-established. The fundamental principles of current-to-voltage conversion, gain, bandwidth, and noise are the same as in barcode readers. |
This commonality makes transimpedance amplifier design a transferable skill. Designers who understand transimpedance amplifiers for barcode readers can also design them for optical networking and other applications. |
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Chapter 36: The Transimpedance Amplifier in Spectrometers |
Transimpedance amplifiers are also used in spectrometers, where they measure the light intensity at different wavelengths. The photodiode produces a current proportional to the light intensity, and the transimpedance amplifier converts this current to a voltage for measurement. |
The TIPD176 reference design is designed for spectrometers as well as barcode scanners. The high linearity and low noise requirements are essential for accurate spectral measurements. |
The use of transimpedance amplifiers in spectrometers is another example of the versatility of this circuit. The fundamental principles are the same as in barcode readers, but the application requirements may differ. |
This versatility makes the transimpedance amplifier a valuable circuit for any application that requires converting current to voltage with high accuracy and low noise. |
Chapter 37: The Op-Amp Selection for Transimpedance Amplifiers |
The selection of the operational amplifier is a critical step in transimpedance amplifier design. The op-amp must have low input bias current, low input offset voltage, and sufficient gain-bandwidth product. |
Microchip Technology's MCP6031 is a high precision op-amp suitable for photodiode applications. It has low input offset voltage and low input bias current, which are essential for accurate current-to-voltage conversion. The MCP6021 and MCP6491 are also suitable. |
Texas Instruments offers a range of op-amps for photodiode applications. The TIPD176 reference design uses an op-amp with sufficient gain-bandwidth product to achieve a bandwidth greater than 1 megahertz. |
The op-amp's supply voltage is another consideration. The transimpedance amplifier's output voltage range is limited by the supply voltage. A single-supply design, like the TIPD176, produces output voltages from 0.1 to 4.9 volts. |
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Chapter 38: The Compensation Capacitor Selection in Practice |
The selection of the compensation capacitor in practice often involves a combination of calculation and empirical testing. The calculated value provides a starting point, and the circuit is then tested to ensure stability. |
Microchip Technology's application note provides guidelines for selecting the compensation capacitor. The equations are based on the op-amp's gain-bandwidth product and the photodiode's capacitance. The goal is to achieve a stable circuit with maximum bandwidth and minimum noise. |
The compensation capacitor is typically a ceramic capacitor with a value of a few picofarads. The capacitor should be placed as close as possible to the op-amp's inverting input to minimize parasitic effects. |
The compensation capacitor's value can be adjusted after testing the circuit. If the circuit oscillates or rings, the compensation capacitor should be increased. If the circuit is stable but the bandwidth is too low, the compensation capacitor can be decreased. |
Chapter 39: The Feedback Resistor Selection in Practice |
The selection of the feedback resistor is a trade-off between gain, bandwidth, and noise. A larger resistor gives more gain but also more noise and less bandwidth. A smaller resistor gives less gain but also less noise and more bandwidth. |
The feedback resistor is typically a precision resistor with a tolerance of 1% or better. The resistor should be a surface-mount component to minimize parasitic capacitance. The resistor's temperature coefficient should be low to minimize drift. |
The TIPD176 reference design uses a feedback resistor to achieve a gain of 53.6 kilovolts per amp. This gain produces output voltages from 0.1 to 4.9 volts for photodiode currents of 0 to 90 microamps. |
The feedback resistor's value is determined by the required gain and the expected photodiode current. The designer must also consider the noise and bandwidth requirements. |
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Chapter 40: The Input Capacitance and Its Effects |
The input capacitance of the transimpedance amplifier includes the photodiode's capacitance and the op-amp's input capacitance. This capacitance affects the amplifier's stability and bandwidth. |
The photodiode's capacitance is specified in the datasheet. The BPW34FAS has a capacitance of 7 picofarads, and the Hamamatsu S8385 has a capacitance of 12 picofarads. The op-amp's input capacitance is also specified in the datasheet. |
The input capacitance creates a pole in the amplifier's frequency response. If the pole is not properly compensated, the amplifier can oscillate or ring. The compensation capacitor is used to cancel this pole. |
The input capacitance is a key parameter in the noise gain analysis described in Microchip Technology's application note. The noise gain zero and pole are determined by the input capacitance, the feedback resistor, and the compensation capacitor. |
Chapter 41: The Photodiode Biasing and the Transimpedance Amplifier |
The photodiode biasing---whether photovoltaic or photoconductive---affects the transimpedance amplifier circuit. In photovoltaic mode, the photodiode is biased at zero volts. In photoconductive mode, the photodiode is reverse-biased. |
The Microchip Photodiode Light Detector Solutions Demo Board supports both modes. A jumper selects between photovoltaic and photoconductive mode. In photovoltaic mode, the amplifier generates a reference voltage that allows the circuit to provide an output referenced to the supply midpoint. In photoconductive mode, the amplifier provides a reverse bias. |
The biasing affects the photodiode's capacitance, which in turn affects the transimpedance amplifier's speed and stability. Photoconductive mode reduces the capacitance, increasing the speed but introducing dark current noise. Photovoltaic mode eliminates dark current but limits the speed. |
The choice of biasing mode is one of the fundamental decisions in barcode reader design. The Microchip demo board allows designers to evaluate both modes. |
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Chapter 42: The Loop Gain and Phase Margin |
The loop gain and phase margin are the key metrics for transimpedance amplifier stability. The loop gain is the product of the op-amp's open-loop gain and the feedback factor. The phase margin is the difference between the loop gain's phase shift and 180 degrees at the crossover frequency. |
Microchip Technology's application note defines the crossover frequency as the point where the loop gain is unity. The phase margin is defined at this frequency. A phase margin of 45 degrees or more is typically considered stable. |
The compensation capacitor affects the loop gain and phase margin. A larger compensation capacitor increases the phase margin but reduces the bandwidth. A smaller compensation capacitor reduces the phase margin but increases the bandwidth. |
The loop gain and phase margin can be analyzed using Bode plots. The application note provides examples of Bode plots for three cases, showing how the compensation capacitor affects the stability. |
Chapter 43: The Step Response and Ringing |
The step response is a practical measure of transimpedance amplifier stability. A stable amplifier has a well-behaved step response with minimal ringing. An unstable amplifier has a step response with excessive ringing or oscillation. |
The compensation capacitor affects the step response. A properly compensated amplifier has a step response with a fast rise time and minimal overshoot. An over-compensated amplifier has a slow rise time. An under-compensated amplifier has overshoot and ringing. |
The step response can be measured with an oscilloscope. The photodiode is illuminated with a step change in light, and the amplifier's output is observed. The rise time, overshoot, and settling time are measured. |
The step response is a useful tool for tuning the compensation capacitor. The capacitor is adjusted until the step response is clean and well-behaved. |
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Chapter 44: The Output Voltage Range |
The output voltage range of the transimpedance amplifier is determined by the op-amp's output swing capability and the supply voltage. The output must be able to swing from near the negative rail to near the positive rail to provide the maximum dynamic range. |
The TIPD176 reference design produces output voltages of 0.1 to 4.9 volts from a single 5-volt supply. This range is sufficient for most analog-to-digital converters. |
The output voltage range is limited by the op-amp's output stage. Rail-to-rail output op-amps can swing close to the supply rails, maximizing the dynamic range. The MCP6021 and MCP6491 are rail-to-rail output op-amps. |
The output voltage range is important for barcode readers because the signal amplitude varies with the barcode's contrast. A wider output range allows the reader to handle a wider range of contrast levels. |
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Chapter 45: Summary --- The Transimpedance Amplifier in Perspective |
The transimpedance amplifier is the heart of the barcode reader's signal chain. It is the circuit that converts the tiny current from the photodiode into a usable voltage, setting the foundation for all subsequent processing. |
We have examined how different companies and technologies have approached the challenges of transimpedance amplifier design: |
Texas Instruments provides the TIPD176 reference design, a complete transimpedance amplifier with a bandwidth greater than 1 megahertz and a gain of 53.6 kilovolts per amp. The design is optimized for barcode scanners and includes comprehensive documentation. |
Microchip Technology offers the Photodiode Light Detector Solutions Demo Board and the MCP6031 Photodiode PICtail Plus Demo Board. These boards demonstrate transimpedance amplifier design for photovoltaic and photoconductive modes. |
Patent literature reveals a wealth of innovative techniques: using a FET as a variable resistor to prevent saturation, employing a compensation network to enhance signal-to-noise ratio, using multi-channel detectors with summing amplifiers, and implementing logarithmic compression. |
The fundamental principles of transimpedance amplifier design are well-established: the feedback resistor sets the gain, the compensation capacitor ensures stability, and the op-amp must have low input bias current and low input offset voltage. |
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The key lessons from our exploration are: |
The transimpedance amplifier is a current-to-voltage converter. It transforms the photodiode's tiny current into a usable voltage. Its performance determines the reader's sensitivity, speed, and noise. |
The feedback resistor sets the gain and bandwidth. A larger resistor gives more gain but also more noise and less bandwidth. The choice is a trade-off that depends on the application requirements. |
The compensation capacitor ensures stability. It prevents oscillation and ringing. Its value is a critical design parameter that must be carefully selected. |
The op-amp selection is critical. The op-amp must have low input bias current, low input offset voltage, and sufficient gain-bandwidth product. These characteristics affect the amplifier's accuracy, noise, and speed. |
The circuit layout matters. The photodiode, the op-amp, and the feedback components must be placed close together to minimize parasitic effects. The PCB layout must be carefully designed to reduce noise and maintain stability. |
Integration is the trend. Many barcode readers now use integrated analog front ends that include the transimpedance amplifier on a single chip. This simplifies the design and reduces the component count. |
In the end, the transimpedance amplifier is a testament to the ingenuity of analog circuit designers. It is a circuit that achieves high performance with remarkable simplicity. Its design is a blend of art and science, requiring careful attention to every component and every connection. For the barcode reader, the transimpedance amplifier is the foundation upon which reliable reading is built. |
cs@easiersoft.com