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Old Yesterday, 02:43 PM   #1
diyAudio Member
Join Date: Nov 2014
Default Class d loop gain determination using simulation

Hello everybody,

I'm new here so bare with me....
I'm working on a class d amplifier which uses pre filter feedback for oscillation. Now i want to incorporate the lc filter with a second feedback loop which gives a flat frequency response. (not the ucd method).

I have some practical applications where this has worked (at least the no load part...). But that doesnt say anything about stability etc..

How do i determine the stability of a simulated amplifier (or a real one) ? I have used a single opamp with lc filter and feedback and tried the middlebrook approach to get the loop gain response. But i am missing the gain in the response and i'm not even sure if i am looking at the right picture....

Maybe someone can give me some hints on finding the loop gain of a class d amp. Which steps to take and if looking at the loop gain from the closed loop system is the right way to go...

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Old Yesterday, 06:31 PM   #2
diyAudio Member
Join Date: Dec 2003
Location: Munich
In the averaged model approach the modelled forward gain of the circuit section from input of the comparator to output of the halfbridge is given by the ratio:
(Pos rail - Neg rail) / Vpp triangle

The triangle does not need to be the triangle of a clocked PWM generator.
The method is also applicable to the carrier signal which you can find at the comparator input of a self oscillating design.
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Old Today, 06:35 AM   #3
diyAudio Member
Join Date: Nov 2014
Hello Chocoholic,

Thanks for the reply, maybe i should clarify something...
I have a model with comparator, delay and switching stage but when i do an ac simulation the gain is not correct....
Ideally this model i want to use to work on...
But because of this problem i tried to simplify things and used an opamp for the gain which drives a LC stage.
When i want to look at the loop gain of this model the gain isnt correct either.

So my main problem is how to get a model which gives a reasonable real response and look at the loop gain to determine the stability.
I want to compare this with real live data...

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