C-multiplier again
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From: Phil Hobbs on 24 May 2010 21:15 On 5/24/2010 6:50 PM, Mike wrote: > Mike<spam(a)me.not> wrote: > > [...] > >> 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. > > I looked at this some more, and added a cap directly across the > transistor from collector to emitter. > > The attenuation curve is like a bathtub. The capacitor in the emitter > affects the low frequency. The C-E cap and ESL change the high frequency > side. These are rational effects, and tend to say SPICE is telling the > truth. But this says the ratio of the C-E cap and the emitter cap have > no bearing on the attenuation, since they affect the opposite ends of > the spectrum. > > One thing that had a dramatic affect on the attenuation is the emitter > current. Going one order of magnitude up reduced the attenuation a great > deal, and dropping the current to 1 mA finally gave close to -140dB. > This says the low noise supply should be used only for the sensitive > parts of a circuit, and as soon as the design permits, run the rest of > the circuit on noisier supply voltages. > > It's not clear what the emitter current is changing, but there has to be > a measurable parameter for the transistor that indicates how well it > will perform in this application. But if this analysis holds water, it > means I have learned something I didn't know before. > > Thanks, > > Mike The Early effect causes problems at higher currents, because it adds a conductance that's more or less proportional to the emitter current. That makes a difference, especially if the base resistor is too big, so that the base current change causes an output voltage change. Lots of Spice models don't handle it well IME. Slower, bigger, higher voltage transistors have higher Early voltages. Big fat ceramics (10 uF) are good for the output cap. I've used MPSA14s to cut 50 mV of 10 kHz ripple down to a nanovolt or two in real circuits. I couldn't do a lot of what I do without cap multipliers. 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: John Larkin on 24 May 2010 22:59 On Mon, 24 May 2010 21:04:58 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >On 5/24/2010 6:56 PM, John Larkin wrote: >> On Mon, 24 May 2010 14:35:59 -0400, Phil Hobbs >> <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> >>> On 5/24/2010 2:12 AM, John Larkin wrote: >>>> On Sun, 23 May 2010 21:45:06 -0700 (PDT), dagmargoodboat(a)yahoo.com >>>> wrote: >>>> >>>>> On May 23, 9:44 pm, Phil Hobbs >>>>> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >>>>>> Winfield Hill wrote: >>>>>>> dagmargoodb...(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 >>>>>>> | | | >>>>>>> --+------+---------------+---- >>>>>> >>>>>> The Kanner Kap uses an audio power amp to do this, applying a small >>>>>> amount of positive feedback to multiply the value of a BFC. Works OK, >>>>>> but it isn't worth paying royalties on. >>>>>> >>>>>> 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. >>>>> >>>>> >>>>> Yep, two-pole--that's the stuff I was fiddling with whilst you guys >>>>> were posting...(ASCII takes time!) >>>>> >>>>> Fig. 3 >>>>> ====== >>>>> Q1 Q2 >>>>> 2n3904 2n3904 >>>>> +15V>--+--------. .----+---. .--+---> +13.3V >>>>> | \ ^ | \ ^ | >>>>> R1 ----- R3 ----- | >>>>> 100R | 100R | | >>>>> | R2 | | | --- C4 >>>>> +--/\/\/----+ +------' --- 100uF >>>>> | 100R | | | >>>>> C1 --- C2 --- --- C3 | >>>>> 100uF --- 100uF--- --- 100uF | >>>>> | | | | >>>>> === === === === >>>>> GND GND GND GND >>>>> >>>>> >>>>> Output ripple is LT-Spice undetectable. Zout ~= 2ohms. >>>>> >>>>> >>>>> Fig. 4 >>>>> ====== Q1 >>>>> 2n3904 >>>>> +15V>--+----------------+----. .----+-----> +13.3V >>>>> | | \ ^ | >>>>> R1 | ----- | >>>>> 3.3k | | --- C3 >>>>> | R2 |/ Q2 | --- 100uF >>>>> +--/\/\/---+---| 2n3904 | | >>>>> | 3.3K | |>. | | >>>>> | | | | === >>>>> C1 --- C2 --- +-------' GND >>>>> 10uF --- 10uF --- | >>>>> | | R3 >>>>> === === 10k >>>>> GND GND | >>>>> === >>>>> GND >>>>> >>>>> Buffer Q2 eliminates loading on filter R1C1-R2C2, greatly improving >>>>> transient response& recovery. >>>>> >>>>> The output at Q2(e) is super-clean, but changes in load current >>>>> modulate Re(Q1) and the drop across it, so output ripple is somewhat >>>>> worse than the reference. Zout is the same as Fig. 3. >>>>> >>>>> Early effect isn't as noticeable as Re, so far. >>>>> >>>>> I did a version following Fig. 4 with a one-pole C-mult stage, biased >>>>> off a divider from Q2(e), and a Sziklai PNP across the whole thing. >>>>> That means the single-pole stage operates as a cascode and sees no >>>>> d(Vce) to speak of. 1KHz ripple disappears, and Rout drops to about >>>>> 0.25 ohms. >>>>> >>>>> This version is silly with parts. There needs to be an op amp in >>>>> there somewhere to greatly simplify things, but it's time for me to >>>>> turn in. Hopefully these musings will inspire John to continue the >>>>> fight. >>>> >>>> >>>> This is what I have so far: >>>> >>>> ftp://jjlarkin.lmi.net/P14_reg.gif >>>> >>>> The wall wart is prefiltered by a C-L-C filter that should buy me >>>> about 40 dB at the switcher frequency. Then this thing should be good >>>> for maybe 100 more. Then I have some more RCs before the photodiodes >>>> and a couple of other critical things. >>>> >>>> This regulates to 13.4 to allow some headroom here and there. >>>> >>>> The LM8261 has about 10 nv/rthz noise, which is a whole nother story. >>>> >>>> John >>>> >>> >>> That's limited by the CMR of the op amp, though, which blows the whole >>> thing out of the water--no? That's the point of using the BJT in the >>> first place. >> >> CMRR and PSRR matter, certainly. But the transistor doesn't regulate, >> and doesn't seem to ripple reject super well at low frequencies, if >> the LT Spice 2N3904 model is to be believed. At high frequencies, >> ripple rejection of the transistor thing improves as the load cap and >> Re start making a lowpass. My 15 ohms is better than 2 ohms of Re, and >> drops less DC too. So the circuits really aren't all that different, >> but I'll have better low-frequency PSRR... clear down to DC. >> >> John >> >> > >But for photodiodes, you don't care too much about the low frequency >stuff, since it's in series with the photodiode capacitance. True, but this circuit has other things going on. And I sure want to zap line frequency and the switcher stuff. And we do need low-dropout regulation. John
From: Tim Williams on 25 May 2010 00:47 More ideas. Try JFETs. No Vbe offset = arbitrarily low dropout. In fact it's negative a lot of the time: even better. Easy to cascade/cascode. Use P or N channel, however you want. Source terminal is squishy (low Gm). Solution: servo with op-amp, or if you want to be quirky, add a shunt regulator so the current draw is constant. You're only drawing like 15mA, right? Which reminds me of another novel, useless circuit I invented, the shunt current source. On my website, http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Shunt_Current_Source.png "This revolutionary (and impressively useless) circuit is the completion of an analogy. Consider: voltage sources are available in two flavors, shunt (e.g., TL431) and series-pass (e.g., LM7805). But current sources are only available in one style, series-pass. These simple circuits complete the analogy, providing a shunt current source. In both cases, a resistor provides a current greater than or equal to the desired output current over the rated range; a current sense resistor, voltage reference and voltage amplifier (VBE and a BJT in the left example; a TL431 and differential pair in the right example) adjust a shunt current to keep the output current constant." Man, this whole thing smacks of audiophoolery. Sometimes, they'll put a CCS into a shunt regulator (even a rather noisy one like a glow discharge tube) just because they feel like it. Difference being, you can actually measure nanovolts. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:da4hv5pumif114u33dau1pqoh93sc11m8b(a)4ax.com... > > > > 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 >
From: Bitrex on 25 May 2010 06:55 Tim Williams wrote: > More ideas. > > Try JFETs. No Vbe offset = arbitrarily low dropout. In fact it's negative > a lot of the time: even better. Easy to cascade/cascode. Use P or N > channel, however you want. > > Source terminal is squishy (low Gm). Solution: servo with op-amp, or if you > want to be quirky, add a shunt regulator so the current draw is constant. > You're only drawing like 15mA, right? > > Which reminds me of another novel, useless circuit I invented, the shunt > current source. > > On my website, > > http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Shunt_Current_Source.png > "This revolutionary (and impressively useless) circuit is the completion of > an analogy. Consider: voltage sources are available in two flavors, shunt > (e.g., TL431) and series-pass (e.g., LM7805). But current sources are only > available in one style, series-pass. These simple circuits complete the > analogy, providing a shunt current source. In both cases, a resistor > provides a current greater than or equal to the desired output current over > the rated range; a current sense resistor, voltage reference and voltage > amplifier (VBE and a BJT in the left example; a TL431 and differential pair > in the right example) adjust a shunt current to keep the output current > constant." > > Man, this whole thing smacks of audiophoolery. Sometimes, they'll put a CCS > into a shunt regulator (even a rather noisy one like a glow discharge tube) > just because they feel like it. Difference being, you can actually measure > nanovolts. > > Tim > Here's a circuit that lurking in this discussion about voltage regulators inspired me to come up with over the weekend, speaking of useless circuits and audiophools. It's a voltage regulator that appears to have decent line regulation without any negative feedback. Cuz negative feedback is bad, right? It's also expensive! http://i227.photobucket.com/albums/dd240/bitrex2007/voltagereg-1.jpg Can you see how it works? Or how I think it is intended to work? :) It doesn't really need a split supply, that's just for messing around. The PSRR is only as good as the output opamp, unfortunately I haven't found a way to get rid of it yet!
From: George Herold on 25 May 2010 08:58
On May 24, 5:04 pm, Mike <s...(a)me.not> wrote: > Phil Hobbs <pcdhSpamMeSensel...(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 1000µf > 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 Hi Mike, Would you mind telling me how I dump this into LT spice. Im not a spice virgin.... but I am still a newlywed. Thanks, George H. |