Transimpedance

Transimpedance is the conversion of input current into an output voltage in an electronic circuit. It is defined as the transimpedance Zt = Vout / Iin, with units of ohms. This metric is especially important in systems that detect small currents and present the result as a voltage for further processing.

The most common implementation is the transimpedance amplifier (TIA): an operational amplifier with a feedback element, typically a resistor Rf, often in parallel with a capacitor Cf, connected from the op-amp output back to its inverting input. The non-inverting input is held at a reference potential, usually ground. The input current is injected into the inverting input, and the op-amp maintains the inverting node at the reference, forcing the current through the feedback network. For an ideal op-amp, Vout = -Iin × Rf, so the transimpedance magnitude equals Rf and the sign is inverted relative to the input current.

In real circuits, the op-amp has finite open-loop gain and input capacitances, making the transimpedance frequency dependent and prone to instability at high frequencies. A feedback capacitor Cf is commonly added to limit bandwidth and improve stability by shaping the response and mitigating the effects of input capacitance and phase shift. The usable bandwidth is influenced by the op-amp’s gain-bandwidth product as well as the RC time constant formed by Rf and Cf.

Noise and dynamic range considerations include the detector’s dark current, shot noise from the signal current, and the op-amp’s voltage and current noise. The low input impedance of a TIA helps in accurate current-to-voltage conversion but must be managed to avoid saturation at high photocurrents.

Applications include photodiode readouts in optical communications, astronomy, spectroscopy, LIDAR, and radiation detectors, where small currents must be faithfully converted to usable voltages for processing.