C-multiplier again
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From: dagmargoodboat on 23 May 2010 17:11 On May 23, 3:20 pm, Vladimir Vassilevsky <nos...(a)nowhere.com> wrote: > John Larkin wrote: > > On Sun, 23 May 2010 11:29:24 -0500, Vladimir Vassilevsky > > <nos...(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. > > PhD design. What do you expect :))) > > Here is a simpler idea: > > http://www.abvolt.com/misc/feedfwd_psrr.jpg > A.c. couple the f.f. input and adjust the gain to cancel the output ripple. Yep. A shunt regulator can do that with less headroom. -- Cheers, James Arthur
From: John Larkin on 23 May 2010 17:37 On Sun, 23 May 2010 15:20:01 -0500, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > > >John Larkin 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. > >PhD design. What do you expect :))) > >Here is a simpler idea: > >http://www.abvolt.com/misc/feedfwd_psrr.jpg > > That's not simpler! At least over frequency. John
From: John Larkin on 23 May 2010 17:43 On Sun, 23 May 2010 16:25:02 -0400, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_(a)charter.net> 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. >> >> 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. >> >> John >> >Did you set the ESR with C4 to some low value ? > > I just used the default cap, zero ESR and maybe some Spice thing lurking in the background. The bad PSRR is at low frequencies when the cap isn't doing anything. John
From: John Larkin on 23 May 2010 18:07 On Sun, 23 May 2010 13:26:26 -0700 (PDT), dagmargoodboat(a)yahoo.com wrote: >On May 23, 11:29�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On 23 May 2010 04:28:01 -0700, Winfield Hill >> >> >> >> <Winfield_mem...(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 > >Win's idea looks pretty decent to me, IIUIC: > >FIG. 1 (View in fixed font) >====== > > Q1 > 2n3906 >Vin >--+----. .-------+---+------+--> +13.3v > | V / | | | > R1 ------ | R2 --- C1 > 470 | Q2 | 1k --- 15uF > | | 2n3904 | | | > '------+---. / === === > \ ^ > ----- > | > R3 > 33 > | > +14v >---' > >LT Spice says 31uV of the 50mV 1KHz ripple gets through (32dBv), >and the load step is 340uV. That's a lot stiffer than the original, >which >had a 4.5mV load step (d(i) = 2mA for both). > >The Sziklai version has the same ripple; I don't quite understand >how Early explains that--Early should wreck the load step response >too, shouldn't it? > >FIG 1's load step is only 60uV if you replace R1 with a 5mA current >source, >the 1KHz ripple stays the same. > > >This shunt filter only needs 200mV headroom: > > >FIG. 2 >====== > R1 >+15V >--+------------------/\/\/\--------+--> Vout = 14.8v > | 5 | > | | > | | > | | > | .-------+------+--------+ > | | | | | > | | | R6 | > | | | 1k | > | R3 R5 | |<' Q3 > | 2.7M 10K +------| 2n3906 > | | | | |\ > | | | |/ Q2 | > | | +----| 2n3904 | > | | | |>. | > | C1 | |<' | | > '---||---+----| Q1 '--------+ > 10uF |\ 2n3906 | > | R4 > | 4.7R > | | > === === > >LT Spice says 20dBV rejection @ 1KHz, zero @ d.c., natch. Only 100 dB to go! But I don't understand Q1s biasing. But I can replace all that stuff after R1 with a big polymer aluminum cap and get about the same rolloff, probably better at high frequencies. > >I used transistors because they're fast--for canceling wideband noise. > >You could use op-amps or TLV431 or such for accuracy and get make a >shunt regulator / noise canceler with much better 1KHz rejection, plus >load regulation. > >Silliness, but fun. *I'm* not having much fun. I've got a circuit that needs nV/fA noise levels and it's beseiged from all directions. Johnson noise. Shot noise. Power supplies coupling in through diode junctions. Switchers inches away. And I'm supposed to Gerber it tomorrow. Wish you were here. John
From: David Eather on 23 May 2010 18:28
On 24/05/2010 8:07 AM, John Larkin wrote: > On Sun, 23 May 2010 13:26:26 -0700 (PDT), dagmargoodboat(a)yahoo.com > wrote: > >> On May 23, 11:29 am, John Larkin >> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>> On 23 May 2010 04:28:01 -0700, Winfield Hill >>> >>> >>> >>> <Winfield_mem...(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 >> >> Win's idea looks pretty decent to me, IIUIC: >> >> FIG. 1 (View in fixed font) >> ====== >> >> Q1 >> 2n3906 >> Vin>--+----. .-------+---+------+--> +13.3v >> | V / | | | >> R1 ------ | R2 --- C1 >> 470 | Q2 | 1k --- 15uF >> | | 2n3904 | | | >> '------+---. / === === >> \ ^ >> ----- >> | >> R3 >> 33 >> | >> +14v>---' >> >> LT Spice says 31uV of the 50mV 1KHz ripple gets through (32dBv), >> and the load step is 340uV. That's a lot stiffer than the original, >> which >> had a 4.5mV load step (d(i) = 2mA for both). >> >> The Sziklai version has the same ripple; I don't quite understand >> how Early explains that--Early should wreck the load step response >> too, shouldn't it? >> >> FIG 1's load step is only 60uV if you replace R1 with a 5mA current >> source, >> the 1KHz ripple stays the same. >> >> >> This shunt filter only needs 200mV headroom: >> >> >> FIG. 2 >> ====== >> R1 >> +15V>--+------------------/\/\/\--------+--> Vout = 14.8v >> | 5 | >> | | >> | | >> | | >> | .-------+------+--------+ >> | | | | | >> | | | R6 | >> | | | 1k | >> | R3 R5 | |<' Q3 >> | 2.7M 10K +------| 2n3906 >> | | | | |\ >> | | | |/ Q2 | >> | | +----| 2n3904 | >> | | | |>. | >> | C1 | |<' | | >> '---||---+----| Q1 '--------+ >> 10uF |\ 2n3906 | >> | R4 >> | 4.7R >> | | >> === === >> >> LT Spice says 20dBV rejection @ 1KHz, zero @ d.c., natch. > > > Only 100 dB to go! But I don't understand Q1s biasing. > Improved ripple response (but I think a little defective - it only works when Vin drops). When Vin drops Q1 turns on via base current drawn out through C1. Q1 robs base current from Q2 turning it off, which in turn turns off Q3 and reduces the current flow and hence voltage loss through R1. Since you have only a volt to play with and the noise requirement you have I think a shunt regulator of some kind is your only chance. > But I can replace all that stuff after R1 with a big polymer aluminum > cap and get about the same rolloff, probably better at high > frequencies. > >> >> I used transistors because they're fast--for canceling wideband noise. >> >> You could use op-amps or TLV431 or such for accuracy and get make a >> shunt regulator / noise canceler with much better 1KHz rejection, plus >> load regulation. >> >> Silliness, but fun. > > *I'm* not having much fun. I've got a circuit that needs nV/fA noise > levels and it's beseiged from all directions. Johnson noise. Shot > noise. Power supplies coupling in through diode junctions. Switchers > inches away. And I'm supposed to Gerber it tomorrow. > > Wish you were here. > > John > |