C-multiplier again
in [Design]
|
Prev: Futuristic weapon question: Anti-matter-gun, would it have a signature ?
Next: EEVblog Live Event
From: David Eather on 23 May 2010 19:56 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 > > R2 is missing - from the base of Q1 to GND - I suggest a value of 18k but it is a weird circuit I think a ripple reduction of no more than 46db
From: David Eather on 23 May 2010 20:21 On 24/05/2010 9:33 AM, David Eather wrote: > 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 Better would be 2 x LM334 > 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: Joerg on 23 May 2010 20:46 John Larkin wrote: > 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! > Since you don't need much current, how about a cheap low-noise non-RR opamp such as the LM833 followed by a JFET with its drain tied to the input rail? Ok, then you have your transistor back :-) Has anyone ever checked the noise performance of logic inverters used as analog amplifiers? -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jamie on 23 May 2010 20:53 John Larkin wrote: > 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 > Being that circuit is acting more like a current source with a max voltage of 13.5- 0.6 = ~12.9. I don't think you are going to get the desire affects. Yup. A precision feed back is in order here. But then again, who knows.
From: dagmargoodboat on 23 May 2010 21:01
On May 23, 5:07 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Sun, 23 May 2010 13:26:26 -0700 (PDT), dagmargoodb...(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! That's 20dBV, e.g., (ripple in)/(ripple out) = 100. Pretty good for standard parts with no trimming, I thought. Better cancellation needs more accurate parts, e.g. op amp + precision resistors. Do you really need that ratio >= 10^12 ? Or do you mean dB(power), i.e. (ripple in)/(ripple out) >= 10^6? > But I don't understand Q1s biasing. Nope, you understand it fine! R2 got overlooked in the ASCII-art conversion--5k from Q1(b) to GND. > 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. If you've got the room that sounds fine. > >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. It sounds like a lovely challenge, with a few shields tossed in the mix. Not unlike RF receivers, methinks--those might inspire. -- Best, James Arthur |