# Low gain transimpedance amplifier

I have an op-amp based current-to-voltage converter, where the input current is summed from a number of LM13700 OTA's. The signal is audio, so the bandwidth of interest is up to $$\20\mathrm{kHz}\$$. The output gain I want is quite small, which naturally leads to a small feedback resistor, in this case $$\1.5\mathrm k \Omega\$$. Due to other constraints, I'd like to use the rather modest TL074, but the general rule of thumb seems to be that the load on the '07x should be at least $$\3.3 \mathrm k \Omega \$$ to avoid undue distortion.

My question is: is it really so that a low gain (i.e. small resistor) transimpedance amplifier distorts worse than a high gain one? Intuitively I'd expect that a small feedback resistor means less work for the op-amp, as the output voltage swing becomes smaller...

The circuit: simulate this circuit – Schematic created using CircuitLab

Circuit usage details: the input current is very rarely the full $$\5\mathrm{mA}\$$, the typical case is more like $$\0.5\mathrm{mA}\$$. When the input is hitting full $$\5\mathrm{mA}\$$, the device is being intentionally driven to distortion (this is an effect-type device, not HiFi), so minor details matter less then anyway. However the normal case should not have obvious distortion.

• A low feedback resistor means more work for the op amp, as it means higher output current. Dec 8, 2022 at 16:00

is it really so that a low gain (i.e. small resistor) transimpedance amplifier distorts worse than a high gain one? Intuitively I'd expect that a small feedback resistor means less work for the op-amp, as the output voltage swing becomes smaller...

Due to negative feedback, the op-amp is trying to maintain its inverting input at a virtual ground potential (0 volts) so, in effect the feedback resistor is from output to 0 volts. If everything else was equal it means that the output amplitude is smaller for a lower value feedback resistor but, by no means is it less-work for the op-amp AND, when trying to drive bigger output signals, the output waveform inevitably clips at a lower level.

• This is indeed how I understood it, it just seemed so silly that getting less gain is more difficult that I had to check.
– Timo
Dec 9, 2022 at 6:44