High Ripple-Rejection Linear Regulator

LTspice simulation shows that, with some modifications, a ripple-rejection of 81dB can be achieved.

I’d love to see a bode plot of this but that pesky “Cannot find solution” keeps it from generating one. Any ideas on which part of the circuit is causing it and how to modify it to make the sim happier while preserving the over all operation of the circuit?

Or is it overall? Over all? Overall? Gosh, even as a native english speaker this language is difficult sometimes.

Overall I think. And I'm not sure you can do a bode plot of this circuit, because in order to do so, the circuit must not crash on startup with the switch closed. But as we all know, that's impossible.

You can always do a frequency sweep on LTspice. You can find the .asc file in the easyeda link.

Ok

I found that by replacing the switch with an auto-closing relay (just a relay with a constant voltage source across the coil) allows the bode simulation to happen, although it looks like it’s not simulating quite right because the attenuation is way too high. I tested putting multiple relays in series each one powered by the previous and they all close in normal simulation but only the first one closes in bode simulation so I think it doesn’t properly simulate stuff after an auto-closing relay. Perhaps it does a DC analysis, then closes the powered relays, then does the small signal analysis without re-applying the DC analysis. I think this is because if it re-did the DC analysis after closing the relay it could cause the same relay to open again and then keep re-applying the DC analysis over and over as the relay has no stable point and keeps opening and closing.

I found that when the switch is open, the DC voltage is 115μV. By closing the switch and changing the DC bias of the voltage source to 115μV I get the same bode plot as if I used an auto-closing relay instead of decreasing the DC voltage. This seems to confirm my theory.

I see