# AC Analysis of single opamp differential amplifier Pspice simulation

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#### themaccabee

##### Full Member level 4
Hi,
Im trying to do a ac analysis of a opamp differential amplifier the circuit is shown below.
The input is fed with a common mode ac signal 1 vpp.
The differential stage is given a 6V offset too..
So for the common mode signal the output ideally will be 6V dc signal only.But when i do the AC analysis im getting something like this..
Can somebody tell me why

#### steve.hunch

##### Member level 1
The transient in the waveform is around BW cutoff of the circuit. Can you elaborate more, what exactly are you showing? Is is written there "N differential..?"

#### FvM

##### Super Moderator
Staff member
Your circuit measures common mode rejection ratio, as expectable for any real OP, it's below infinity...
The measurement correspondends almost exactly to the datasheet's CMRR specification of about 60 dB at 1 MHz.

themaccabee

### themaccabee

Points: 2

#### themaccabee

##### Full Member level 4
Your circuit measures common mode rejection ratio, as expectable for any real OP, it's below infinity...
The measurement correspondends almost exactly to the datasheet's CMRR specification of about 60 dB at 1 MHz.
Thats true..but ive a question about the dc point..May be this is due to my ignoance about ac analysis...
what im asking is that,,,,Isn't the output voltage should rise frm the offsetted 6v dc rather than from 0v dc,after 100kHz?
For zero differential input i ve offsetted the opamp at 6v dc.
Thanks For any help
Regards

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#### themaccabee

##### Full Member level 4
or does the ac analysis will kill all the dc sources?? and show only relative values?? or is there some error in my circuit?

#### FvM

##### Super Moderator
Staff member
or does the ac analysis will kill all the dc sources??
For the displayed response, yes. If the DC component would be shown at all (it is e.g. in a fourier transformation), it's written at frequency 0, which is never included in an AC analysis. A reasonable DC operation point inside the linear voltage range is however a required for the AC analysis devices like OPs.

themaccabee

### themaccabee

Points: 2

#### pmr9

##### Junior Member level 2
Your sim results are as expected. AC analysis doesnot kill dc sources, it will take dc points before calculating AC analysis data. You are not offsetting the opamp as you mentioned but giving common mode signal on dc 6V . Are u clear?

themaccabee

### themaccabee

Points: 2

#### themaccabee

##### Full Member level 4
Your sim results are as expected. AC analysis doesnot kill dc sources, it will take dc points before calculating AC analysis data. You are not offsetting the opamp as you mentioned but giving common mode signal on dc 6V . Are u clear?

I was thinking that since the zero differential input voltage will provide an output of 6V , i thought the 6v dc will level shift the output of the opamp & thinking that the equation for the output is
Vo=Vref+Vdiff*G
Vref=6V
G=1
So thats becoz i asked so.
I think now it is becomming clear..
Thanks..

#### FvM

##### Super Moderator
Staff member
AC analysis is small signal analysis. It determines the change in output voltage generated by a small (or as the mathematicans say infinitesimal) change in input voltage. So the DC level is always ignored.

themaccabee

Points: 2