op-amp AC magnitude analysis

ac magnitude

Hi guys,

does anybody know what input i should apply to m y differtial opamp for AC magnitude analysis?
I have this setting:
i applied a sin source and i set AC mag=1, Ac phase=0.
Can someobody please confirm this?

cadence ac magnitude

1. Before AC you must determine offset voltage and DC-gain of opamp in DC analysis
2. In AC: one input is biased offset voltage by DC-source to compensate offset of opamp, other input is biased by AC-source. The voltage of AC-source I set usually as (Vout max-Vout min)/(2*Gain op amp)

what is dc analysis of op-amp

when you use AC analys you feed infinite small signal, so dc points in opamp linearizing. Nowise witch magnitude set in "AC magnitude". Ratio outputAC/inputAC will be equal for any "AC magnitude". For simplicity often "AC magnitude" set to 1 V.
It is important
1. You must set dc operation points at input (often vcc/2)
2. You must have only one ac source in schem

what is ac magnitude

thanks YXo, what if my opamp is differential. Then i have to apply -1 and 1 as my AC sourcs?

QT_GIRL said:

thanks YXo, what if my opamp is differential. Then i have to apply -1 and 1 as my AC sourcs?

Click to expand...

no, You must apply only one AC source at any input. But DC operation voltage must be equal at both inputs (often vcc/2)

do you mean to apply the AC source signgle ended and DC source differntial?

Just for general information on ac analysis. Why do we set AC magnitude =1?

This is because when we take 20log(1) =0dB, so we can look for PM when the gain plot crosses the 0dB line. This make to easy/simple and can use the PM formula in Cadence calculator.

If you can also use AC magnitue equal to any number (x). But you need to look for PM when the gain plot crosses 20log(x) line. Slighly more complicated but still get the same results if you know where to look for.

Similarly for the phase input.

As for the setup, connect a ac source between the positive input and the negative input with AC=1. Then use the formula 20log(Vf("out+")/Vf("out-")). You so able to get what you want, I think.

Added after 3 minutes:

"This is because when we take 20log(1) =0dB, so we can look for PM when the gain plot crosses the 0dB line. This make thing easy/simple and can use the PM formula in Cadence calculator. "

Correct one.

ur setup is fine.

0.5,-0.5