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Design >> Analog Design >> Op amp open loop gain measurement
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Message started by baab on Jan 11^th, 2016, 5:32am

Title: Op amp open loop gain measurement
Post by baab on Jan 11^th, 2016, 5:32am

I would like to ask about open loop gain measurement for Op amp.
I tried two methods with configurations as in the picture below and got the results totally different. Could anyone explain why? Thanks.

Method 1: arrange the circuit test as in the picture then sweep Vin for example from -2.5 to 2.5V. Plot DC response (Vout vs Vin).
Open loop gain is the slope of linear region of that characteristic.

Method 2:
arrange the circuit test as in the picture. Run ac analysis and then plot Vout in db20. This is the open loop gain.

With method 1 I got Ao = 20dB.
With method 2 I got Ao = 75dB.

So what is the problem here?
Note that voltage supply and R, C values are the same.

Title: Re: Op amp open loop gain measurement
Post by ULPAnalog on Jan 11^th, 2016, 11:53am

Hello

In the first method, you are not only changing the differential input to the opamp but also the common mode, which is why I suspect the amplifier is not biased properly. Notice that in your method 2, both inverting and non-inverting input terminals of the opamp are biased at 1.65V and incremental analysis is done around this operating point. I would suggest you to try the testbench shown in the attachment, with Vin,d being a small differential voltage applied. The derivative of the output wrt to Vin,d should match with the gain obtained from AC analysis.

Thanks and regards