AC analysis of double ended OP AMP

[ytliang]

First of all, I am not sure if I posted in the right place. I apologize if I didnt.

How do you measure the dc gain of a double ended OP AMP using AC analysis?
I tried taking the difference between the 2 outputs, but the waveform does not seem to come out correct.

Thanks,

 

[LvW]

How do you measure the dc gain of a double ended OP AMP using AC analysis?
I tried taking the difference between the 2 outputs, but the waveform does not seem to come out correct.

You are going to simulate the dc gain with the help of an ac analysis ?
And you are surprised about the waveform ?
This sounds a bit confusing.

 

[ytliang]

You are going to simulate the dc gain with the help of an ac analysis ?
And you are surprised about the waveform ?
This sounds a bit confusing.

AC analysis shows the frequency response of the circuit. When u plot your ac analysis waveform, u can see your dc gain at freq=0 as well as unity gain freq, ...etc.

I also plotted my dc analysis by sweeping Vin1 (with Vin2 tied to a constant dc voltage source and measure the slope of the output). This gain I obtained, which seems more correct, is very different from the result I got from AC analysis.

sorry for the confusion and please correct me if I am wrong.

Thanks,

 

[LvW]

AC analysis shows the frequency response of the circuit. When u plot your ac analysis waveform, u can see your dc gain at freq=0 as well as unity gain freq, ...etc.
I also plotted my dc analysis by sweeping Vin1 (with Vin2 tied to a constant dc voltage source and measure the slope of the output). This gain I obtained, which seems more correct, is very different from the result I got from AC analysis.

1.) OK, I see what you mean: gain for low frequencies is identical to dc gain. Agreed.
But you should avoid the term "ac analysis waveform", because the ac response is no "wave" and it creates misunderstandings.

2.) In principle, your dc simulation set up is correct. This way you get the amplifier transfer curve - and its slope is identical to the dc gain. If the results don´t match the value derived from ac analyses, it could be a problem of incorrect or different bias points for both simulation runs.