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?
Nodes:
- V[n004]: Input signal (green)
- V[n005]: Signal at non-inverting input (cyan)
- V[n006]: Signal at opamp output (red)
Best Answer
Of course it is "biased": you cannot leave amp input "hanging" in the air. In real life your schematic will potentially destroy itself and everything connected to it because opamps usually have very large input impedance and will pick up static electricity from the air, resulting in complete malfunctioning of the device. You have to have the resistor between the input and ground.