C-multiplier again
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From: Winfield Hill on 23 May 2010 12:58 John Larkin wrote... > > Since the problem is the Early effect, namely the effective C-E > resistance bleeding ripple through, it didn't seem to me like the > Sziklai thing would help. The PNP doesn't insulate the NPN from the > ripple. So I spiced it. If the LT Spice transistor models are to be > trusted, it's actually worse. The optimum value for the PNP's b-e > resistor is zero. Thanks for checking that. I'd say look at their Early effect values. They'll often throw in something like VA=50 volts, which is really crazy, IMHO. It's also true that subtracting the PNP's Vbe pushes the NPN's Vce down toward 0V, where it'd be saturated and not working well. So to use the PNP, you need a bit more DC operating room (like I said), and you have to lower the NPN base voltage with another resistor to ground. Just for kicks, try another 200mV. Spice isn't on my computer right now, so I can't try it myself. -- Thanks, - Win
From: Vladimir Vassilevsky on 23 May 2010 13:38 John Larkin wrote: > I'm thinking about an R-C after the opamp, 10 or 20 ohms and a 120 uF > polymer aluminum cap. That only costs 150-300 mV and has a corner > frequency in the 100 Hz ballpark, so fixes the opamp's PSRR falloff at > high frequencies and rolls off the wideband noise. The DC feedback can > still be from the output, so regulation stays good. This is > practically my existing circuit, without the transistor! To take advantage of the output C, the regulating element should have output with high impedance. Voltage controlled current source, ideally. VLV
From: John Larkin on 23 May 2010 14:03 On 23 May 2010 09:58:14 -0700, Winfield Hill <Winfield_member(a)newsguy.com> wrote: >John Larkin wrote... >> >> Since the problem is the Early effect, namely the effective C-E >> resistance bleeding ripple through, it didn't seem to me like the >> Sziklai thing would help. The PNP doesn't insulate the NPN from the >> ripple. So I spiced it. If the LT Spice transistor models are to be >> trusted, it's actually worse. The optimum value for the PNP's b-e >> resistor is zero. > > Thanks for checking that. I'd say look at their Early > effect values. They'll often throw in something like > VA=50 volts, which is really crazy, IMHO. It's also > true that subtracting the PNP's Vbe pushes the NPN's > Vce down toward 0V, where it'd be saturated and not > working well. So to use the PNP, you need a bit more > DC operating room (like I said), and you have to lower > the NPN base voltage with another resistor to ground. > Just for kicks, try another 200mV. Spice isn't on my > computer right now, so I can't try it myself. What's a more realistic Early voltage for a gumdrop NPN transistor? It would be silly if I were using a bad transistor model. I will be running at low Vce, as low as I can manage, so maybe I won't be in the really flat part. I suppose I should breadboard, grumble. John
From: John Larkin on 23 May 2010 15:07 On Sun, 23 May 2010 11:29:24 -0500, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > > >John Larkin wrote: > >> >> >> I need a super-low noise power supply. I have a 15 volt switching >> wall-wart input and want as close to 15 volts, regulated, as I can >> get; 14 would be nice, 13.5 is OK. >> >> The LDOs that I can find are all pretty noisy and have mediocre PSRR. > > >http://www.abvolt.com/misc/psrr.jpg > >The topology like this is stable and provides for ~100dB of PSRR. That's cute. And complex. John
From: Jim Thompson on 23 May 2010 16:05
On Sun, 23 May 2010 12:07:44 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sun, 23 May 2010 11:29:24 -0500, Vladimir Vassilevsky ><nospam(a)nowhere.com> wrote: > >> >> >>John Larkin wrote: >> >>> >>> >>> I need a super-low noise power supply. I have a 15 volt switching >>> wall-wart input and want as close to 15 volts, regulated, as I can >>> get; 14 would be nice, 13.5 is OK. >>> >>> The LDOs that I can find are all pretty noisy and have mediocre PSRR. >> >> >>http://www.abvolt.com/misc/psrr.jpg >> >>The topology like this is stable and provides for ~100dB of PSRR. > >That's cute. And complex. > >John > And I wonder about that wording, "stable". ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy |