unity gain amplifier output higher than input?

[jiesteve]

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?

 

[A.Anand Srinivasan]

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

 

[yxo]

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

 

[jiesteve]

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.

 

[yxo]

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

 

[Fahmy]

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

 

[jiesteve]

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

Thanks!