Why AC analysis result doesn't agree with the transient resu

[Allen]

I've done several transient and AC analysis simulations for a wideband
amplifier we designed. However I got different results from two types
of simulations. The AC analysis shows a much higher gain than
transient does. Could anyone explain this? Thanks.

Best,
Allen

 

[S. Badel]

I've done several transient and AC analysis simulations for a wideband
amplifier we designed. However I got different results from two types
of simulations. The AC analysis shows a much higher gain than
transient does. Could anyone explain this? Thanks.

AC simulation results depend on the operating point. Before AC is run, there's a DC simulation to
determine the DC operating point, then, the circuit is linearized around that point. Linearization
is an idealization, where your circuit is considered to be perfectly linear and it holds
approximately as long as your signals are small enough.

During transient analysis, the signal swing may be large enough to shift the operating point, thus
inducing effects such as what you describe. Also during transient, there's slew rates and other
time-dependant effects that will affect the results.

In order to get the same results during AC and transient, the different stages of your amplifier
would need to be perfectly linear over the whole dynamic range, which is probably not the case.
However, if you apply slowly varying and very small amplitude signals, the results should agree in
theory.

So, to make it short, what I'm suspecting is that it's due to limited linear range of your
amplifier, and not to a wrong simulation setup... however in closed loop the linear range extends.

I'm no amplifier expert anyway, but hope this may help.


Good luck,

St¨Śphane

 

[Allen]

On 4ÔÂ13ČŐ, ÉĎÎç4Ęą24ˇÖ, "S. Badel" <stephane.ba...@REMOVETHISepfl.ch>
wrote:

I've done several transient and AC analysis simulations for a wideband
amplifier we designed. However I got different results from two types
of simulations. The AC analysis shows a much higher gain than
transient does. Could anyone explain this? Thanks.

AC simulation results depend on the operating point. Before AC is run, there's a DC simulation to
determine the DC operating point, then, the circuit is linearized around that point. Linearization
is an idealization, where your circuit is considered to be perfectly linear and it holds
approximately as long as your signals are small enough.

During transient analysis, the signal swing may be large enough to shift the operating point, thus
inducing effects such as what you describe. Also during transient, there's slew rates and other
time-dependant effects that will affect the results.

In order to get the same results during AC and transient, the different stages of your amplifier
would need to be perfectly linear over the whole dynamic range, which is probably not the case.
However, if you apply slowly varying and very small amplitude signals, the results should agree in
theory.

So, to make it short, what I'm suspecting is that it's due to limited linear range of your
amplifier, and not to a wrong simulation setup... however in closed loop the linear range extends.

I'm no amplifier expert anyway, but hope this may help.

Good luck,

St¨Śphane

Hi St¨Śphane,
It totally make sensen to me, I did some more simulations, the results
proved your theory, thanks very much.

Best,
Allen