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Design >> Analog Design >> OP DC gain and AC gain
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Message started by uncle_ezra on Nov 2^nd, 2004, 5:28pm

Title: OP DC gain and AC gain
Post by uncle_ezra on Nov 2^nd, 2004, 5:28pm

Hi,

This might be a stupid question but shouldn't the DC gain of a OP be the same as the low frequency ac gain? For the DC gain I am sweeping the non-inverting input from one rail to the other for a fixed inverting input. Then I extract the DC gain from the slope of the curve. For AC I supply a 1 V AC into the noninverting input and find the output response.

So am I doing something wrong? Any suggestion will be appreciated.

Thanks

Title: Re: OP DC gain and AC gain
Post by Rajarshi Paul on Nov 3^rd, 2004, 10:58am

U r doing the correct thing but give a small ac signal of 1mv mag instead of 1V ac. Then perform an ac analysis. The ac gain gain u will obtain will be same as the slope of the DC sweep that you have talked about.

Title: Re: OP DC gain and AC gain
Post by ywguo on Nov 5^th, 2004, 5:50am

Hi,

I think 1V input for AC analysis is good, because the output is voltage gain when the input is 1V.

Dont worry. :) For SPICE-like simulator, 1V or 1mV has no difference for AC analysis. It dervies linear model through Operating Point analysis first, then run AC analysis based on that linear model. That is different from the transient analysis.

Best regards
Yawei

Title: Re: OP DC gain and AC gain
Post by sheldon on Dec 10^th, 2004, 7:19pm

Hi,

Yes, the open loop gain measured by looking at the
slope of the dc analysis should match the results from low
frequency ac analysis. However, the test fixturing for the
dc test is complex. You might want to look at how ATE
machines measure dc open loop gain for the details.
Basically you need to put the op amp in a servo loop to
keep it in the linear region. There are also some other
simpler methods. The other issue is that you need to be
careful about the error tolerances in the dc analysis and
the dc stepping, since you are differentiating small errors
can create a lot of noise. BTW, you did not mention how
you are doing the ac measurement. If you are using
open loop measurements, there that could also be an
issue. Have you tried the approaches Ken suggests in
his book?

Sheldon

Title: Re: OP DC gain and AC gain
Post by uncle_ezra on Dec 14^th, 2004, 8:22pm

Can you provide page number or outline the method?

Thanks

Title: Re: OP DC gain and AC gain
Post by Art Schaldenbrand on Dec 18^th, 2004, 5:24pm

Hi,

Try section 3.4, Applications of AC Analyses, page 67
to 108. There is a good summary chart on page 107. The
extensions for diffferential amplifiers are discussed in,
section 3.4.5, pages 113-119. BTW if memory serves,
stability analysis directly calculates loop gain, T. You can
use loop gain to calculate Open Loop Gain.
Aopen,loop = Aclosed,loop + Tloop,gain

Best Regards,

::)