unity gain amplifier output higher than input?

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jiesteve

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In some corners of my DC simulation of a unity gain amplifier, I sometimes see the output being higher than the input. I thought that the output could never be higher, because DC gain is T0 = Av/(Av+1), which is always less than 1.... Why is this?
 

can u post ur simulation output and circuit.... i hope this might be due to feedthrough or coupling from other part of the circuit...
 

You need attach schematic of you amplifier and test bench. probably, amplifier is not stable
 

Here's a screen shots of the circuit and DC operating point. It is just a current mirror OTA driving an NMOS to form a voltage regulator. The input (inp) is 800mV
and the output is 802mV (inn).

Its not coupling because it's a DC sim.
 

You have noninverting amplifier with dc gain
(Rf/Rg + 1). Most likely, series resistance of source follower = 1/gm - Rf. And more large resistance to ground, that you not shown (or in real world Rg is finite). As a result you have gain more then 1
 

If the o/p is higher than the input in DC level only , then it is a systematic offset due to current mirror mismatch.
 

    J

    jiesteve

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Thanks everybody. I've convinced myself that it is systematic offset.

Thanks!
 

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