Two feedback paths (again)

George Herold · 2013-01-29T02:07:11+00:00

Hi guys (which includes women in my vernacular) I've got what looks like another two feedback loop thingie. (It's simpler so maybe I can get a result.) I use this as a low noise voltage source. Vin-+------------------+ | | +Vref-+ | p | | o~Rb-| | +|-/ |> GND ||/ | +---+-R-+--+---+-- ^---out/load. | +-C-+ R | --- R/4 idle +Rf-+ | | | GND---------+ Cf +-------+ Rb~500, R = 10k, C = 10-100nF, Ridle-10k, Rf = 500, Cf = 100uF(Al) It's a voltage reference/ opamp/ pass element/ filter. There's some RC filtering between pot and opamp that is not shown. So this has some problems. It's got an output impedance of Rf/beta several ohms, and the temperature variations in the uncontrolled cap mult. output filter. So I tried taking the RC feedback from the output. Putting the cap mult in the loop. Worked, but lots more noise at low frequency. (broad peak at ~2kHz.) The now uncontrolled node between pass transistor emitter and cap mult collector/base now had about the same broad 2 kHz. noise peak. So If I could split up the feed back task so everything slower than (say) 100ms was from the output, with the faster stuff via the pass element.... that might be nice. So as I add HF feedback from the pass element node, I've got to subtract it from the DC path. Is that right? (sorry, just thinking out loud at the SED.) George H.