What signal does a transimpedance amplifier output?
A transimpedance amplifier (transimpedance amplifier, TIA) outputs a voltage signal. It is used to convert an input current signal, for example the photocurrent produced by a photodiode, into a corresponding voltage for amplification and subsequent processing.
Basic principle
The TIA converts input current to output voltage; the conversion gain is set by the feedback resistor. When the input current flows through the feedback resistor Rf, a proportional voltage appears at the amplifier output. By choosing the feedback value appropriately, the output voltage can meet the requirements of the application.
The transimpedance circuit typically consists of a photodiode, an amplifier, and a feedback capacitor/resistor pair (Figure 1). Although the circuit appears simple, parasitic effects can cause instability if not addressed.
Figure 1: A zero-bias transimpedance amplifier using an operational amplifier. Although the schematic is simple, parasitic capacitances and other effects can lead to instability. Image source: Digi-Key Electronics.
Light incident on the photodiode generates a photocurrent Ipd that flows from the diode cathode to anode. This current also flows through the feedback resistor Rf. The product Ipd times Rf produces the output voltage Vout at the amplifier output. In this configuration, increasing incident light increases the output voltage magnitude.
The term "zero reverse bias" in Figure 1 indicates that the photodiode has 0 V reverse bias across it. With zero reverse bias, leakage current or dark current is relatively low, but the photodiode junction capacitance is higher compared with a configuration that applies a larger reverse bias.
Frequency response and stability
The AC gain of the TIA depends mainly on the resistor and capacitor in the amplifier feedback loop. The ideal AC and DC transfer functions for the circuit in Figure 1 show a single-pole response determined by the feedback elements. Although the single-pole formula explains the basic bandwidth, it does not by itself explain why a TIA can sometimes oscillate. Oscillation often arises from additional poles and zeros introduced by the photodiode junction capacitance, amplifier input capacitance, PCB parasitics, and the interaction with the feedback network. Proper compensation, including selection of feedback capacitance and layout considerations, is needed to ensure stability.
Differences between transimpedance amplifiers and ordinary op amps
Transimpedance amplifiers (TIAs) and standard operational amplifier configurations have different input/output characteristics and are optimized for different tasks:
- Input and output characteristics: A TIA accepts an input current, commonly from a photodiode, and provides a proportional voltage output. In contrast, typical op amp configurations take a voltage input and produce a voltage output.
- Feedback method: TIAs use feedback to convert input current to output voltage, often with a resistive feedback element that sets the current-to-voltage gain. Ordinary op amp stages usually use voltage-mode feedback.
- Gain type: TIAs provide a current-to-voltage conversion gain (transimpedance), suitable for amplifying very small currents such as photodiode currents. General op amp stages provide voltage gain, which is adjustable by the feedback network.
- Frequency response: TIAs are often designed for wide bandwidths and fast response to support optical detection and communication. Ordinary op amps can be designed for high frequency as well, but many general-purpose op amp circuits target mid-to-low frequency ranges.
TIAs are commonly used in photodetection, optical communication, and optical sensing applications, while standard op amp circuits are widely used in analog signal processing, filter design, and general-purpose amplification.
