C-multiplier again
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From: Vladimir Vassilevsky on 23 May 2010 16:20 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 Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Jamie on 23 May 2010 16:25 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 ?
From: dagmargoodboat on 23 May 2010 16:26 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. 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. -- Cheers, James Arthur
From: Vladimir Vassilevsky on 23 May 2010 16:28 Jim Thompson wrote: > 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". Unconditionally stable with proper parameters. I.e. stable at any positive load impedance. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Jamie on 23 May 2010 16:44
Vladimir Vassilevsky 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 > > > > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultant > http://www.abvolt.com > kind of reminds me of a simple reg I made (age 14) using an incandescent lamp as part of the bias on the Power transistor. Those were the good old days!. |