C-multiplier again
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From: MooseFET on 23 May 2010 10:39 On May 22, 7:57 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> 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. > > So I thought about using a Phil Hobbs-ian c-multiplier transistor, an > R-C lowpass and an emitter follower, with a slow opamp loop wrapped > around it for DC regulation. It looks fine on paper, simple loop to > stabilize, but I figured I may as well Spice it and be sure. > > What I'm seeing is mediocre PSRR. Stripping out the opamp and such, I > have... > > ftp://jjlarkin.lmi.net/C-multiplier.gif > > which has psrr of about 70 dB at low frequencies, improving as the > output cap finally kicks in at around 5 KHz. The transistor equivalent > seems to look like the expected dynamic Re of about 2 ohms, with a C-E > resistor of around 6.6K. Reducing Vb (and Vout) doesn't help much. > > I'm using the LT Spice 2N3904 model, which I take to be a sort of > generic small-signal NPN. The 33r base resistor value doesn't seem to > matter. > > There must be a better way, ideally one that doesn't throw away 0.7 > perfectly good volts. How much current do you need to produce? At low currents, a fast rail-rail op-amp can make a good clean power supply. They work with as little as 0.3V of head room. If you get the ones that are stable into a capacitive load, like the LT1498, you can bypass the output.
From: John Larkin on 23 May 2010 12:01 On Sun, 23 May 2010 07:39:01 -0700 (PDT), MooseFET <kensmith(a)rahul.net> wrote: >On May 22, 7:57�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> 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. >> >> So I thought about using a Phil Hobbs-ian c-multiplier transistor, an >> R-C lowpass and an emitter follower, with a slow opamp loop wrapped >> around it for DC regulation. It looks fine on paper, simple loop to >> stabilize, but I figured I may as well Spice it and be sure. >> >> What I'm seeing is mediocre PSRR. Stripping out the opamp and such, I >> have... >> >> ftp://jjlarkin.lmi.net/C-multiplier.gif >> >> which has psrr of about 70 dB at low frequencies, improving as the >> output cap finally kicks in at around 5 KHz. The transistor equivalent >> seems to look like the expected dynamic Re of about 2 ohms, with a C-E >> resistor of around 6.6K. Reducing Vb (and Vout) doesn't help much. >> >> I'm using the LT Spice 2N3904 model, which I take to be a sort of >> generic small-signal NPN. The 33r base resistor value doesn't seem to >> matter. >> >> There must be a better way, ideally one that doesn't throw away 0.7 >> perfectly good volts. > >How much current do you need to produce? 15 mA maybe, fairly steady. I'm running photodiodes and discrete jfets and such. > >At low currents, a fast rail-rail op-amp can make a good clean >power supply. They work with as little as 0.3V of head room. >If you get the ones that are stable into a capacitive load, like >the LT1498, you can bypass the output. Right, I've been considering that. I have the LM8261 in stock, a rrio C-load amp that has low frequency psrr of about 100 dB. Noise is a little high, 10 nv per, but that's already 15x better than your typical voltage regulator. And I can get a better opamp by applying money. 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! John
From: Vladimir Vassilevsky on 23 May 2010 12:29 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. You can also simulate huge LC filter with gyrators. Oh, and the trivial solution would be cascading regulators one after the other. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: John Larkin on 23 May 2010 12:29 On 23 May 2010 04:28:01 -0700, Winfield Hill <Winfield_member(a)newsguy.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. >> >> So I thought about using a Phil Hobbs-ian c-multiplier transistor, an >> R-C lowpass and an emitter follower, with a slow opamp loop wrapped >> around it for DC regulation. It looks fine on paper, simple loop to >> stabilize, but I figured I may as well Spice it and be sure. >> >> What I'm seeing is mediocre PSRR. Stripping out the opamp and such, I >> have... ftp://jjlarkin.lmi.net/C-multiplier.gif >> which has psrr of about 70 dB at low frequencies, improving as the >> output cap finally kicks in at around 5 KHz. The transistor equivalent >> seems to look like the expected dynamic Re of about 2 ohms, with a C-E >> resistor of around 6.6K. Reducing Vb (and Vout) doesn't help much. > > You're complaining about a 70dB improvement? There is a simple > way to use your 0.7 volts, well maybe 0.8 volts, to get even > more rejection: change your simple NPN follower into a Sziklai > connection (AoE page 95). The base resistor across the added > PNP creates a relatively-fixed collector current for your NPN, > which means a fixed Vbe, for improved AC ripple rejection. 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. John
From: Mike on 23 May 2010 12:39
Winfield Hill <Winfield_member(a)newsguy.com> wrote: [...] > You're complaining about a 70dB improvement? Phil promised 140dB: http://groups.google.com/group/sci.electronics.design/msg/143f77519fed66e8 |