Zener diode configuration question.

agfa

Could someone explain what the purpose of the two zener diodes labeled D1 & D2 in the attached circuit diagram?

Tracy Allen

Those are probably there to stabilize the feedback voltage. This is an older op-amp, and it's output is not rail-to-rail, that is, with +/- 15 volt supplies, the output might swing +/-13 volts, but that is not a well controlled parameter and depends on operating conditions. The back-to-back series zeners clamp the feedback at +/- 7 volts, and the temperature coefficients in zener mode and forward mode tend to cancel out. The value of PWM depends on having a stable feedback voltage in relation to the control input.

With a more modern rail to rail op-amp, the circuit might dispense with the zeners.

agfa

Thank you Tracy for taking the time to share your expertise. You actually provided more insight than I expected.