I was fiddling around in LTSpice for a project I'm working on.
This circuit is a basic preamplifier and a low-pass filter. But this question refers solely to the amplifier stage. The opamp is the "universal opamp2" model.
However, the resulting waveform has a 4.5V DC bias. Since I intend to run this from a single-supply source I was planning on biasing the input signal at 4.5V so the opamp can swing the signal to 0 and 9V. But I hadn't added the bias yet.
Without C3 there is no bias and the output waveform is as expected. If I add a, say, 1M resistor from the noninverting input to ground, creating a high-pass filter, the bias is gone.
The C3 is supposed to remove any DC component from the input signal.
I figured this was just SPICE being stupid, but is this something that would be expected in practice?
- V[n004]: Input signal (green)
- V[n005]: Signal at non-inverting input (cyan)
- V[n006]: Signal at opamp output (red)