# Three level PWM modulator circuit feedback simulation

I'm trying to design a three level PWM circuit with feedback and i'm struggling to understand the feedback aspect.

The design is based on this design by Texas instruments and modified to suit three level.

The design works if B is not added to the feedback loop.

As soon as i connect B to the 10k resistor, the circuit completely deteriorates.

I'm certain I don't understand feedback properly and am hoping someone can tell me what the issue is here or can direct me to relevant feedback literature.

The PWM is for a class D amp and i'm taking the feedback before the switching stage currently but intend to take pre filter feedback later on. Am I okay in assuming the feedback in this circuit is a good indication of what it would be like in a full amplifier circuit minus the switching error?

• Post your schematic? Clearly label this "B" you're talking about. There's a lot of schematics in that app note, it's not clear which one you're talking about. It also appears that $10\mathrm{k\Omega}$ is the designer's favorite value, so it's not clear which $10\mathrm{k\Omega}$ you're talking about, either. – TimWescott Mar 13 at 23:54
• Apologies, completely forgot to add schematic. – dos584 Mar 14 at 0:05
• Is your sine wave centered around 2.5V? – TimWescott Mar 14 at 0:33
• Adding B to the feedback loop effectively cancels A, and causes you to run open loop (ish). Try adding B not instead. – TimWescott Mar 14 at 0:36
• Seems to get better but still not correct: imgur.com/a/MD8pmOP Sine wave input is is 2v signal with zero offset. Offset is added with U2 and vref. – dos584 Mar 14 at 1:33