In my last post I simply described the approaches I explored. This post shows some simulation results. The first image below shows the performance of a simple capacitance-multiplier circuit. It attenuates 60Hz ripple by about 24 dB:
Its main advantage is that it is very simple, using a single transistor to do the job.
And here is the simulation result for my opamp-based version:
This design attenuates 60 Hz ripple a bit more than 72 dB. That's pretty good. The design is interesting because the capacitors C2 and C3 are the primary determiners of the circuit's gain, and it is independent of the frequency -- because the ratio of their impedances over frequency remains constant. The 20 megohm resistor is mostly there to provide bias current for the opamp's inverting input. It does introduce some rolloff in the circuit's performance at very low frequencies but its impact on noise+ripple at or above 60Hz is negligible.
In a real-world design I would add some protection diodes. One across the transistor's collector and emitter pins and two connected to the opamp's inverting input. One connected to the opamp's positive supply and one to either ground or its negative supply. I probably would choose the negative supply to balance the two diode's leakage currents, but it probably isn't too critical since the opamp is AC coupled to the transistor's base -- so a DC offset on its output wouldn't affect circuit operation. Unless the offset is so large that the opamp output voltage is close to railing. But a good diode's leakage current won't be high enough to do that, even with the 20 megohm feedback resistor.
Not shown here, I also looked at the circuit's response to a transient current load to check its response. I didn't want the circuit to exhibit a lot of ringing, or, worse, burst into oscillation. It does exhibit a bit of undershoot/overshoot but that disappears when I place a 10nF capacitor between the output and ground.
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