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.

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

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

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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?

  • \$\begingroup\$ 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. \$\endgroup\$ – TimWescott Mar 13 '19 at 23:54
  • \$\begingroup\$ Apologies, completely forgot to add schematic. \$\endgroup\$ – dos584 Mar 14 '19 at 0:05
  • \$\begingroup\$ Is your sine wave centered around 2.5V? \$\endgroup\$ – TimWescott Mar 14 '19 at 0:33
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    \$\begingroup\$ Adding B to the feedback loop effectively cancels A, and causes you to run open loop (ish). Try adding B not instead. \$\endgroup\$ – TimWescott Mar 14 '19 at 0:36
  • \$\begingroup\$ 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. \$\endgroup\$ – dos584 Mar 14 '19 at 1:33

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