Transimpedance amplifier
A transimpedance amplifier (TIA) turns a current into a voltage: the amplifier a bare photodiode needs. Its gain is the feedback resistance, in V/A.
Ports
One input, in, a current, and one output, out, a voltage.
The model
Like the voltage amplifier it is a single pole, of gain \(G\) (V/A) and bandwidth \(B\):
\[H(f) = \frac{G}{1 + i f/B}, \qquad V = G\,I \text{ at low frequency}.\]Its noise, referred to its input as a current, is the amplifier’s input current noise \(i_n\) and the Johnson noise of the feedback resistor \(R = G\) at 295 K:
\[S_\text{out}(f) = \lvert H(f)\rvert^2\left(S_\text{in}(f) + i_n^2 + \frac{4kT}{G}\right).\]A larger gain lowers the resistor’s noise current, \(\sqrt{4kT/G}\), which is why sensitive photodetectors use a large one, and pay for it in bandwidth.
Wire a photodetector set to a bare photodiode into it. Its input should be a current: a voltage is read as amps, and the TIA notes it. Past its output limit, it says it would clip.
Settings
| Field | Default | Meaning |
|---|---|---|
gain | 100000 | Gain (V/A) |
bw | 100000 | Bandwidth (Hz) |
inoise | 1 | Input noise (pA/√Hz) |
vmax | 10 | Output limit (V) |
From the tool's own tables, file format version 1: the names are those in a .jekray file.
Readouts
The panel gives its output in the steady state: the DC level, the five strongest tones (each one’s amplitude, rms and the phase of its cosine), and the noise, as a density at 1 kHz and in rms from 1 Hz to 1 GHz. Any notes, such as an input left unwired or an output past its limit, are listed under it.
Assumptions
- One pole: the photodiode’s capacitance does not shape the bandwidth or raise the noise at high frequency (no \(e_n C\) noise).
- Linear up to its limit, and flagged, not clipped, past it.
Related
- Amplifier.
- Photodetector and fibre photodetector, which have one built in.