# How to simulate fully differential amplifier

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#### walker5678

##### Full Member level 3
how to simulate differential amplifier gain

Hi ,
I am designing a fully differential amplifier, and want to simulate the frequency response of the Open Loop Gain and the Phase. Any one know how to set up the simulation circuit? Is it the same as simulating single output amplifier?

Thanks a lot!

#### sengyee88

##### Advanced Member level 4
ac simulation differential amplifier

I think it is the same as simulating single-ended amp.
Just that you place 1V AC to Vin_p and -1V AC to Vin_n and construct the feedback path that open to AC but closed to DC.

#### walker5678

##### Full Member level 3
how to simulate open loop amplifier

"place 1V AC to Vin_p and -1V AC to Vin_n"

in the single ended simulation, we only place 1V AC to Vin_p and feedback the DC signal to Vin_n. So if also place AC signal to Vin_n, then the AC signal will be shorted by the large capacitor connected to GND, so how to implement it ?

#### mince

##### Member level 5
Put 0.5 VAC to the in+ and -0.5 VAC to in-. Measure the output voltage as out+ - out-.

#### sengyee88

##### Advanced Member level 4
Walker,
Remove the two caps at the input and the resistor at the output.
Put a big (10H) inductor to the feedback paths, the feedback resistors should be big too.
I think this works if there is no current flow through the feedback path. If there is, u need to reduce the feedback resistors to avoid saturation, but your open loop gain plot will not be accurate.

Hope this helps.

Put 0.5 VAC to the in+ and -0.5 VAC to in-. Measure the output voltage as out+ - out-.
You are correct, but it has the same effect as "place 1V AC to Vin_p and -1V AC to Vin_n" and measure out+ only.

#### jeffsky520

##### Member level 4
Yes, of cource you can simulate as single-ended OP AMP.
put AC 1 on in+ and connect in+ and out- with large resistor, and like single-ended, conect in- and out+ with large resistor and large cap, then measure (out+-out-) for open loop Gain and PM.

#### walker5678

##### Full Member level 3
Hi, sengyee

I think you are right, that's the way to feedback DC signal while avoid AC signal. Thanks.

Added after 13 minutes:

Another question, why should i remove the resistor from the output? I found that when this resistor is connected , the Open Loop Gain simulation result becomes much lower than that without the resistor.

#### sengyee88

##### Advanced Member level 4
Because you form another feedback loop. If your opamp output do not have buffer, it actually reduces output impedance, hence reduces overall open loop gain.

#### beckchm

##### Banned
use ideal cm

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