John Larkin wrote...> > 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.Thanks for checking that. I'd say look at their Early effect values. They'll often throw in something like VA=50 volts, which is really crazy, IMHO. It's also true that subtracting the PNP's Vbe pushes the NPN's Vce down toward 0V, where it'd be saturated and not working well. So to use the PNP, you need a bit more DC operating room (like I said), and you have to lower the NPN base voltage with another resistor to ground. Just for kicks, try another 200mV. Spice isn't on my computer right now, so I can't try it myself. -- Thanks, - Win
C-multiplier again
Started by ●May 22, 2010
Reply by ●May 23, 20102010-05-23
Reply by ●May 23, 20102010-05-23
John Larkin wrote:> 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!To take advantage of the output C, the regulating element should have output with high impedance. Voltage controlled current source, ideally. VLV
Reply by ●May 23, 20102010-05-23
On 23 May 2010 09:58:14 -0700, Winfield Hill <Winfield_member@newsguy.com> wrote:>John Larkin wrote... >> >> 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. > > Thanks for checking that. I'd say look at their Early > effect values. They'll often throw in something like > VA=50 volts, which is really crazy, IMHO. It's also > true that subtracting the PNP's Vbe pushes the NPN's > Vce down toward 0V, where it'd be saturated and not > working well. So to use the PNP, you need a bit more > DC operating room (like I said), and you have to lower > the NPN base voltage with another resistor to ground. > Just for kicks, try another 200mV. Spice isn't on my > computer right now, so I can't try it myself.What's a more realistic Early voltage for a gumdrop NPN transistor? It would be silly if I were using a bad transistor model. I will be running at low Vce, as low as I can manage, so maybe I won't be in the really flat part. I suppose I should breadboard, grumble. John
Reply by ●May 23, 20102010-05-23
On Sun, 23 May 2010 11:29:24 -0500, Vladimir Vassilevsky <nospam@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
Reply by ●May 23, 20102010-05-23
On Sun, 23 May 2010 12:07:44 -0700, John Larkin <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:>On Sun, 23 May 2010 11:29:24 -0500, Vladimir Vassilevsky ><nospam@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". ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
Reply by ●May 23, 20102010-05-23
John Larkin wrote:> On Sun, 23 May 2010 11:29:24 -0500, Vladimir Vassilevsky > <nospam@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
Reply by ●May 23, 20102010-05-23
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 ?
Reply by ●May 23, 20102010-05-23
On May 23, 11:29=A0am, John Larkin <jjlar...@highNOTlandTHIStechnologyPART.com> wrote:> On 23 May 2010 04:28:01 -0700, Winfield Hill > > > > <Winfield_mem...@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... =A0ftp://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? =A0There 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). =A0The 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. > > JohnWin's idea looks pretty decent to me, IIUIC: FIG. 1 (View in fixed font) =3D=3D=3D=3D=3D=3D Q1 2n3906 Vin >--+----. .-------+---+------+--> +13.3v | V / | | | R1 ------ | R2 --- C1 470 | Q2 | 1k --- 15uF | | 2n3904 | | | '------+---. / =3D=3D=3D =3D=3D=3D \ ^ ----- | 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) =3D 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 =3D=3D=3D=3D=3D=3D R1 +15V >--+------------------/\/\/\--------+--> Vout =3D 14.8v | 5 | | | | | | | | .-------+------+--------+ | | | | | | | | R6 | | | | 1k | | R3 R5 | |<' Q3 | 2.7M 10K +------| 2n3906 | | | | |\ | | | |/ Q2 | | | +----| 2n3904 | | | | |>. | | C1 | |<' | | '---||---+----| Q1 '--------+ 10uF |\ 2n3906 | | R4 | 4.7R | | =3D=3D=3D =3D=3D=3D 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
Reply by ●May 23, 20102010-05-23
Jim Thompson wrote:> On Sun, 23 May 2010 12:07:44 -0700, John Larkin > <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote: > > >>On Sun, 23 May 2010 11:29:24 -0500, Vladimir Vassilevsky >><nospam@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
Reply by ●May 23, 20102010-05-23
Vladimir Vassilevsky wrote:> > > John Larkin wrote: > >> On Sun, 23 May 2010 11:29:24 -0500, Vladimir Vassilevsky >> <nospam@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!.
