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Sources of AC elements in Feedback loop

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shauns87

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For an error amplifier, the stability is defined based on the response of the amplifier for a range of sine wave frequencies.

However, when we have DC-DC conversion - linear or switching, when the source is pure DC, what is the source of AC frequency at the input of the error amplifier?

For the simplest case - converting 12V to 5V using an LDO - what could be the source of AC at the input of the error amplifier because we will have pure 12V DC at the input and all along the DC supply line.
 

For an error amplifier, the stability is defined based on the response of the amplifier for a range of sine wave frequencies.
or something like that...

Stability can be determined by analyzing the loop gain magnitude and phase versus frequency. For simple cases, the phase margin can be used as stability criterion.

The loop gain can be measured empirically by injecting a sine signal of variable frequency into the feedback loop and observe the response. There are dedicated instruments for this purpose like the Ridley AP300 analyzer. In terms of your question, you superimpose a small AC voltage to the DC feedback signal.

Another popular test method is applying a load current step to the DC/DC converter or amplifier and observing the response.
 

do you feel confused if there is no AC component in the power source ,and where the AC component to EA comes from ,right?
the AC component to EA side not comes from the source only,but also and mainly comes from the loading side,because the purpose of keeping the feedback loop stabilize is to make the power module satisfy the rapid changing of loading requirement and the "change" of the loading has form the AC component which as the input of EA.
 

There's no such thing as "pure DC." Thermal noise and transients both contain broadband AC which will perturb the feedback loop.
 

It sounds to me that you are trying to understand the theory in absolute sense before getting on and designing it...but that IMHO, is a mistake, the way to learn is to get on and do it, then see what youve got, and youll soon see what you have to do to make it work....and youll see those theoretical things as interesting and from your practical experience and observation of them, you will then develop an understanding of it.

Here is a simulation of a linear regulator to help you get more of a feel for it.

As you know, any wavform can be represented by sine waves of various A and f and phi, so any disturbance at the output is sinusoidal in that sense and can be represented by sines.
 

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