# 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).

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

• What exactly is the question? Nov 10, 2014 at 12:59
• "Why is the output: -biased, -triangular?" Nov 10, 2014 at 13:18
• Andrey-do you realize that the noisy signal as shown is NOT used as an input for the opamp (R8_C1 provide lowpass filtering).
– LvW
Nov 10, 2014 at 14:41
• @LvW Yeah, that's kinda useless at the moment. Nov 10, 2014 at 14:47
• The reason for offset is simple: you have a low-pass filter on the input to your amplifier. You probably want to block DC with a high-pass filter. Nov 10, 2014 at 17:13

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.

• But all wideband op amps i've used have order of magnitude higher GWBP than the original one. Nov 10, 2014 at 14:50
• What about slew rate? Try to confiorm my results with LF411.
– LvW
Nov 10, 2014 at 14:56
• @LvW: You may have a typo, the Vpp should 1.3V or so. With MCP601, the Vpp is 2V, with DC level 1V. Nov 10, 2014 at 14:57
• Sorry-you are right: 1.3Vpp.
– LvW
Nov 10, 2014 at 15:00
• But andrey g's result is not reasonable. There must be something wrong. Nov 10, 2014 at 15:03

I would say the output was offset at 1 volt and this is approximately 100 times higher than the offset on the input waveform. Given your amp is configured to have a gain of about 100, there is no surprise here.

The triangulated output is due to the poor gain-bandwidth-product of the op-amp you chose. You want a gain of 100 and you are feeding it 18kHz and it has a GBWP of 1 million Hz.

If the gain is 100, then the "usable" bandwidth of the circuit is 10 kHz but you are exceeding that with your 18 kHz signal hence the circuit acts like a low pass filter at about 10 kHz.

• I don't understand what you mean by saying that the input signal is offset. Is it really? Also i've updated original question with different op amp simulations. Nov 10, 2014 at 13:25
• @andreyg - ask yourself - what is the average DC level of the input signal as shown on the pictures you provide. Nov 10, 2014 at 13:44
• @andreyg: I think you may have some error in your simulations. It seems your simulator is "proteus", right? From your simulation graphs, it seems there are some "ripple" on your op amp's out. Can you split the MIC and OUT into two different graph? It seems your scales not right. Nov 10, 2014 at 14:00
• @diverger Tried that, there are actually no ripples, it's a perfectly level signal. Yes, i'm using proteus. Nov 10, 2014 at 14:10
• Please check your LF411's result, i can see small ripples on OUT in your picture. That is, OUT isn't perfect DC. Nov 10, 2014 at 14:12