C-multiplier again
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From: MooseFET on 23 May 2010 18:33 On May 23, 9:01 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Sun, 23 May 2010 07:39:01 -0700 (PDT), MooseFET > > > > <kensm...(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. You can use 2 op-amps with the first low noise and the second R-R If you were at lower voltages, I'd suggest the LT6200 or the like. With a dual op-amp, you could use the second section to servo a PNP transistor's current to let the first one always run with a low output current. > > 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: John Larkin on 23 May 2010 18:45 On Mon, 24 May 2010 08:28:03 +1000, David Eather <eather(a)tpg.com.au> wrote: >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. So what's the quiescent current of Q1? Of Q3? John
From: David Eather on 23 May 2010 19:33 On 24/05/2010 8:45 AM, John Larkin wrote: > On Mon, 24 May 2010 08:28:03 +1000, David Eather<eather(a)tpg.com.au> > wrote: > >> 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. > > So what's the quiescent current of Q1? Of Q3? > > John > > IQ q1 = 0 IQ Q3 is intended to be dependent on the load 0 ma to 40 ma. I didn't say I liked the circuit - I don't think it will even work very well or at all. As a suggestion how about using a shunt regulator made of 2 x NSI50010 constant current sources feeding into a TL431? My back of the envelope suggests -70db with no bypass caps. Low ESR / ceramic caps on the output should get you very close to what you want.
From: Tim Williams on 23 May 2010 19:50 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:1u8jv51o8m9n30hb95cr6vs05jumactm5k(a)4ax.com... > *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. http://www.mouser.com/ProductDetail/Hammond/193D/?qs=sGAEpiMZZMsg%252by3WlYCkUz0dxeSDrDZIbvMJGFxR5SE%3d Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Winfield Hill on 23 May 2010 19:54
dagmargoodboat(a)yahoo.com wrote... > > 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 > | | > ------+----------------+---- Nice ASCII art. Is fig 2 from your feverish brain? I see your idea, invert the ripple and subtract it out. Good. To do that the cancellation amplifier needs to be biased class A, so it can work over the entire ripple range. It should continuously draw current from the supply through R1, and superimpose the inverted ripple signal on top of that. R4 can be trimmed to optimize. The new R7 should be sized to handle the p-p ripple. Then John's delicate C-multiplier filter can follower, with all the heavy lifting having been done. +15V >--+-----------------/\/\/\--------+--> Vout 14.8v | 5 | | | | .------+------+--------+ | | | | | | | | R6 | | | | 1k | | R3 R5 | |<' Q3 | 2.7M 10K +------| 2n4403 | | | | |\ | | | |/ Q2 | | C1 | +----| 2n3904 | '---||---+ | |>. | 10uF | |<' | | +----| Q1 '--------+ | |\ 2n3906 | R7 | R4 TBD 27k | 4.7R | | | --+------+---------------+---- -- Thanks, - Win |