C-multiplier again
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From: Phil Hobbs on 24 May 2010 14:41 On 5/23/2010 12:29 PM, Vladimir Vassilevsky 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. > > 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 That's assuming that your op amp is running off batteries? Because otherwise its PSR is going to dominate above ~10 kHz. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: Vladimir Vassilevsky on 24 May 2010 14:52 Phil Hobbs wrote: > On 5/23/2010 12:29 PM, Vladimir Vassilevsky 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. >> >> You can also simulate huge LC filter with gyrators. >> >> Oh, and the trivial solution would be cascading regulators one after the >> other. >> >> > That's assuming that your op amp is running off batteries? Opamp is running from the output. Autostart is no problem. > Because > otherwise its PSR is going to dominate above ~10 kHz. Even if it is, there is RC on the power rail anyway. VLV
From: George Herold on 24 May 2010 15:03 On May 24, 2:39 pm, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > On 5/24/2010 8:09 AM, Mike wrote: > > > > > > > Phil Hobbs<pcdhSpamMeSensel...(a)electrooptical.net> wrote: > > >> Cap multipliers are magic--especially two-pole ones. It's 0.7 volts > >> well spent IMO. If Early is a worry, use a slower transistor--the > >> ripple rejection is basically C_CE/C_BFC, with some degradation due to > >> Early voltage and capacitor ESR. > > >> Cheers > > >> Phil Hobbs > > > Phil, you mentioned earlier being able to reach 140 db in one stage: > > >http://groups.google.com/group/sci.electronics.design/msg/143f77519fe... > > > That's a ratio of 10 million to one. How do you do it? > > > Thanks, > > > Mike > > Like I said, it's basically C_CE/C_BFC. You pick a transistor with > reasonable characteristics at frequencies you care about, drive its base > from a really really filtered version of V_CC--with a resistor in series > to make sure it doesn't oscillate and doesn't blow up if the input or > output gets shorted Yeah a bit of resistance is needed. I make these with 2N3904's.. and at certain voltage/ current levels they would sing at 300MHz or so. (Actually I now use 2N4401's and 4403's, these don't seem to sing.. but I've kept the base R. ) I've never measured the rejection ratio, but you can make a ~1nV/rtHz power supply able to deliver several hundered mA's of current, driven from a switcher. George H. --and put a BFC at the output. If the transistor has > 10 pF C_CB and the BFC is 100 uF, that's 140 dB, provided you look after > other stuff such as the Early voltage and the ESR of the output cap. > Generally if your application needs more than 100 dB of ripple > rejection, you have to be pretty careful. > > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal > ElectroOptical Innovations > 55 Orchard Rd > Briarcliff Manor NY 10510 > 845-480-2058 > hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text - > > - Show quoted text -
From: David Eather on 24 May 2010 16:57 On 24/05/2010 12:11 PM, dagmargoodboat(a)yahoo.com wrote: > On May 23, 6:56 pm, David Eather<eat...(a)tpg.com.au> wrote: >> On 24/05/2010 8:45 AM, John Larkin wrote: >> >>> On Mon, 24 May 2010 08:28:03 +1000, David Eather<eat...(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), 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 | >>>>>> R2 | R4 >>>>>> 5k | 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 > > If by 46dB you mean power, i.e. 20log(Vin/Vout) = 46, yes, that's > easily possible--that implies 0.5% gain accuracy. > > If you mean 46dBv, i.e. 10log(Vin/Vout) = 46, i.e. Vout / Vin = 25ppm, > no, that ain't happening, not unless you use op amps and some mighty > fine resistors. > > -- > Cheers, > James Arthur Um, I thought power was 10*log(what ever / what ever else) and voltage was 20*log(what ever / what ever else) e.g. a reduction in ripple of 46 db = a factor of 200, no? (I still like my idea of lm334 feeding LM431/tl431 - 4 small components - 6 if you use 2 x 334)
From: Mike on 24 May 2010 17:04
Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > On 5/24/2010 8:09 AM, Mike wrote: [...] > Like I said, it's basically C_CE/C_BFC. You pick a transistor with > reasonable characteristics at frequencies you care about, drive its > base from a really really filtered version of V_CC--with a resistor in > series to make sure it doesn't oscillate and doesn't blow up if the > input or output gets shorted--and put a BFC at the output. If the > transistor has 10 pF C_CB and the BFC is 100 uF, that's 140 dB, > provided you look after other stuff such as the Early voltage and the > ESR of the output cap. Generally if your application needs more than > 100 dB of ripple rejection, you have to be pretty careful. > > Cheers > > Phil Hobbs Is that spiceable? I made a simple circuit with a voltage source driving the base and a cap on the emitter. I tried various transistors such as 2N2222 and 2N2369, and various ESR and ESL values for the cap. The capacitance had little effect on the attenuation floor, but mainly moved the low frequency corner. No reasonable combination of transistors or cap values got below -120dB. The base resistance had little effect. Here's the file if you'd like to show me how it should work: Version 4 SHEET 1 1140 1108 WIRE -304 -448 -384 -448 WIRE -160 -448 -224 -448 WIRE -128 -448 -160 -448 WIRE -128 -400 -128 -448 WIRE -192 -352 -256 -352 WIRE -384 -320 -384 -448 WIRE -256 -320 -256 -352 WIRE -128 -288 -128 -304 WIRE -32 -288 -128 -288 WIRE 16 -288 -32 -288 WIRE -128 -256 -128 -288 WIRE -32 -256 -32 -288 WIRE -384 -224 -384 -240 WIRE -256 -224 -256 -240 WIRE -32 -176 -32 -192 WIRE -128 -160 -128 -176 WIRE -32 -80 -32 -96 WIRE -32 16 -32 0 FLAG -256 -224 0 FLAG -384 -224 0 FLAG -128 -160 0 FLAG -160 -448 Vin FLAG -32 -288 Vout FLAG -32 16 0 SYMBOL npn -192 -400 R0 SYMATTR InstName Q1 SYMATTR Value 2N2369 SYMBOL voltage -384 -336 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 15 SYMBOL voltage -256 -336 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 38 Left 0 SYMATTR SpiceLine Rser=1 SYMATTR InstName V2 SYMATTR Value 10 SYMBOL current -128 -256 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value 20ma SYMBOL voltage -208 -448 R90 WINDOW 0 49 39 VRight 0 WINDOW 123 -48 40 VRight 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value2 AC 1 SYMATTR Value "" SYMBOL cap -48 -256 R0 SYMATTR InstName C1 SYMATTR Value 1000�f SYMATTR SpiceLine Rser=1u Lser=1n SYMBOL res -48 -96 R0 SYMATTR InstName R1 SYMATTR Value 100m SYMBOL ind -48 -192 R0 SYMATTR InstName L1 SYMATTR Value 10n TEXT -216 -536 Left 0 ;'BJT Ripple Filter TEXT -224 -504 Left 0 !.ac oct 100 1 1e7 |