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Differential output opamp

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chunhang

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differential output op-amp

HI all,
For the single-ended output opamp, we need to feed the output to the Neg input (The Pos input is short to GND) in order to find the offset voltage.
BUt what could we do for the differential output opamp?
In addition, if i want to plot the freq. response of the differential output opamp, is it use the command: .probe ac vdb(vo+, vo-) ?

THx
 

opamp differential output

If you designed your fuly-diff amplifier in the right way, your common mode output level is fixed, whatever is the differential input voltage. So, leave you amplifier in open loop and just scan the differential input voltage. The input offset is the voltage at which the differential output becomes zero. Repeat the test for several values of the common mode input voltage
 

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    chunhang

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differential output op amp

Firstly in a differential amplifier there is no systematic offset. The offset will be random and has to be taken care in the layout. So, there is no point in testing the circuit for offset in the differential amplifier case because both the outputs will be at the same common mode level. What ever common mode level you want at the output is taken care by the CMFB loop.

Secondly, to do an ac analysis, you can put the feedback and test it like a normal amplifier. Of course, you can probe the difference, like you have given.

I hope that it helps.......
 

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    chunhang

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