Signal buffer with op-amp clamping and voltage bias
My comments

If &quot;SIG&quot; is the output of a full rectifier, then capacitor C1 is going to drop the DC component before it reaches your clamp circuit. You're going to have a signal at bufferOUT that is varying above and below ground. Exactly how much above and below depends on the exact shape of the waveform at SIG.
This is also what's causing your output to trend back toward 5 V when the SIG input stays constant for a long time.

60 V input and 4 kohms at R1 implies you're working with 10's of mA of current. The op-amp will have to sink all of this current when it works as a clamp. The LM324 is only guaranteed to sink about 10 mA. Typically it should do 20 mA, but no promises.
You can reduce the need of the op-amp to sink large currents by replacing D2 with the b-e junction of a pnp transistor, and tying the collector to ground.

This clamp circuit depends on the op-amp being able to switch between saturated and active operation quickly. Most modern op-amps aren't good at this, though I don't know about the '324 specifically.


Why not just use a 5 V zener and be done with it?