Electronic – Spice Transimpedance amplifier AC analysis

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I'm trying to analyse the AC response of a transimpedance amplifier with LTSpice. I have a current source set to "small signal analysis" with 1mV as the input of the amplifier stage. I run and probe the output but I get 0dB at low frequencies. So I conclude that LTSpice is using the input voltage as reference and not the input current.

I believe I need to explicitly tell LTSpice to plot amplitude and phase of outputVoltage/inputCurrent signals. How can this be done?

If you need more information, please let me know.

Here's a picture of my circuit and resulting analysis. With a resistor of R1=1k I'd expect to get an amplitude of 20log(210mV/210uA) = 60dB at low frequencies. Instead I get 0dB.

enter image description here

Best Answer

Let me point out your main misconception. From there, the answers to your question should be obvious.

In comments you said:

The AC analisys presents data in dB, it's a Bode plot. This means it's a relative amplitude given by 20log(Vo/Vi). So it gives you a relative amplitude and phase.

This is not correct.

When plotting a voltage in dB scale, LTSpice plots in dBV (\$20*\log_{10}\left(\frac{V}{1 \rm V}\right)\$), and when plotting a current on a dB scale it plots dBA (\$20*\log_{10}\left(\frac{I}{1 \rm A}\right)\$).

LTSpice doesn't in general know which source you are considering as the input, so it can't be expected to automatically plot a voltage or current relative to the input voltage or current.

If you want to plot the gain of an amplifier, you can enter formula to be plotted.

Even easier, simply set the AC amplitude of your input source to 1 V or 1 A. Since the SPICE AC analysis is a linearized analysis, this won't cause any problems, even if these amplitudes would cause severe distortion if applied to your real circuit.

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