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Do we always have to open the loop for AC analysis?

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Advanced Member level 4
Mar 21, 2006
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Is it true that for any kind of ac analysis, we have to open the loop ?


ac analysis

No, It depends your circuit and design.
but transient analysis in close loop can help you more.

where to break the loop for ac analysis

for opamp, i did not see anything wrong to do open loop when you sim ac (i mean for simulation purpose only). for transient, you'd better do close loop especially when you deal with high gain opamp.

how to break the loop for open loop ac simulation

when the opamp has very high gains(around 80 dB or more) opening the loop can saturate opamp. you must change the gate of one transistors of input stage very carefully(around microvolts)

loop break for ac analysis

It definitely depends on the circuit. So the answer would be no. For oscillator analysis we break the loop and we analyze the phase of the loop gain.

sweep frequencyin ac analysis

The AC simulation only a way to check the Vout/vin relate to the frequency. it don't depends on the loop close or open. such as some circuit's PSRR simulation, it is only AC simulation. Such as .noise analysis. So run AC simulation, you must know what spec you are checking. Check the loop stability you must break the loop, check phase margin and gain. If you only want to know the close loop's Vout/Vin VS. frequency in one stage system, you must close loop.

Re: AC analysis

Not all.

Open loop is needed for stability test.

But some needn't such as PSRR test of a LDO.

Re: AC analysis

AC analysis is the frequency sweep. It is used to get frequency response. It depends on what kind of frequency response, then the system can be closed loop or open loop or even off for switch ( such as rejection or cross coupling,etc).

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