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Design >> Analog Design >> Question on large signal gain
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Message started by nobody on Mar 17^th, 2009, 6:31am

Title: Question on large signal gain
Post by nobody on Mar 17^th, 2009, 6:31am

Hello, all

The schematic is a fully differential opamp and an ideal cmfb is used and biased @ half VDD. VDD is 3V.
I was wondering if it is natural or right to test that or any opamp with
a differential dc voltage which ranges from -1v to 1v because I observed the gain is around 240. I sweep the opamp using dc analysis and check the slope of output voltage. The gain or small signal gain is quite good when the input differential voltage ranges from -20mV to 20mV.
Thanks.

Title: Re: Question on large signal gain
Post by jiesteve on Mar 18^th, 2009, 10:31pm

This is a high gain amp, so the differential input needs to be limited to a small range to keep the outputs from railing.

Title: Re: Question on large signal gain
Post by nobody on Mar 19^th, 2009, 12:46am

Thanks for your reminding. Actually the large signal gain is measured when output is in transition and the input ranges from -10mV to 10mV
@ that time when a large signal is applied from -1V to 1V. I tried ways to increase all length or add a booster but the large signal was the same. However, when a small differential signal like 300mV or an input voltage from -10mV to 10mV is applied, a high gain can be expected. One more question shows up.
Why the gain is different even the gain is measured when vid is between -10mV to 10mV under large and small input signal ?

Title: Re: Question on large signal gain
Post by subgold on Mar 19^th, 2009, 1:44am

nobody wrote on Mar 19^th, 2009, 12:46am:

for such a high gain amp with limited output swing at 3V supply, i would assume its input linear range to be between +/-1.5mV at maximum. +/-10mV is still too large to be considered as small signal, let alone 300mV (it is way too huge). another possible error is, you need to guarantee the same step size (and small enough) for dc sweep through different range, in order to get the correct information.

Title: Re: Question on large signal gain
Post by nobody on Mar 19^th, 2009, 3:47am

Hello, subgold

There you go. I just found that the error is due to less step size, which causes inconsistent result for small and large signal.
Thanks.