Electronic – Op amp has a biased output


I'm designing an op amp circuit to amplify 20mV, 18kHz square wave up to measurable levels (~2-3V). Why is the output signal:

  • Biased at ~400mV
  • Triangular (high output capacitance? is that a thing?) ?

Op amp is chosen to have maximum slew rate (1V/uS).

Schematic here

As suggested, I've tried a couple of higher-GBWP op amps and none of them work for some reason. Why is that?

Another schematic here

Best Answer

Question 1 (biasing): It is no surprise that the output signal contains a dc part because the input signal is centered upon a dc voltage. However, the dc ouput level cannot be verified simply by amplifying the input dc voltage because the form of the output signal has changed.

Question 2 (form): There are two different effects which influence the output form: (a) Limited and frequency-dependent small signal gain of the opamp and (b) limited slew rate of the opamp. The slew rate may be the reason for the observed behaviour in case of a wideband opamp model.

EDIT 1: Simulations based on the LF411 model show pretty good results (as expected): Output 2.3Vpp (between 0.7 and 2v) with slight distortions and a very small noise component (5 Mhz) riding on the top of the signal.

EDIT 2: I think, both questions are answered: DC portion and signal form distortion.