C-multiplier again
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From: Tim Williams on 25 May 2010 12:31 "Bitrex" <bitrex(a)de.lete.earthlink.net> wrote in message news:J_-dnXww97pUMGbWnZ2dnUVZ_sadnZ2d(a)earthlink.com... > 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! Yech, could've saved on a lot of mess by adding an OTA or two. Considering the multiplier, I would hazard a guess at some overly complicated truncated-Taylor-series correction. It's worth noting that, if Vo is the output, then all the other nodes supplied by it inherently have feedback. In particular, Vref will vary a small amount; Vbias will vary proportionally; I_R5 will vary proportionally; and there's early effect on all transistors, and PSRR in the multiplier. With R15 and R19 so excessively large compared to the impedances on the other sides (R20 is shunted by D1, and R29 by R18), the OTA offsets will be huge, and proportional to I_R6 (hence, OTA). The first LT1014 sections seem to be doing I-to-V conversion, relative to Vbias (a "safe" value, given the OTA outputs will work somewhere between Vref and Vo, assuming Vo/2 > ~Vref). The LHS OTA output is subtracted from Vbias, to which it is relative, so the multiplier gets something centered around 0. This is superfluous, as it has differential inputs to begin with. The RHS OTA gets the same treatment, and this zero-referenced value goes to the mult's add input. Output goes to inverting amp to Vo. Now, LHS OTA has Vref on one side (assuming the zener is actually biased in breakdown), and <0V on the other, so it can be simplified as taking Q4 through a mirror to the IC input. RHS OTA has a squishy side, and a nearly zero (~6.2mV) not-squishy side. I_R6 ~ 0.5mA, so if Q7/Q9 are balanced, they'll see 0.25mA each, and R19 will see about 2.5uA, dropping 0.25V. But then it'll be cut off, so it won't actually be that far down. The actual point is somewhere inbetween, with Q7 taking more current than Q9. At any rate, it's still a ratio of I_R6. This is converted to V and added as offset to the product. So, it looks like milivolt level signals go into and out of the multiplier block, R21/R27 and the op-amp pushes it up to useful levels. Vref has little effect, and might be a divider like Vbias, just lower (so the first OTA doesn't run out of output range). There isn't any particular reason for the voltage to be anything at all, it's just that, if everything is just so, variation in whatever parameter can be made to fall on the vertex of the parabolic transfer curve, having zero apparent gain for small values of gain. This is akin to approximating e.g. cos(x) with 1 - x^2/2, which works for small x. So I was right, it's essentially a truncated Taylor series correction. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: Phil Hobbs on 25 May 2010 17:40 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. Just because V_GS < 0 doesn't mean V_DS < 0. JFETs fall off a cliff at low V_DS, typically well before they get to 0.7V. BF862s fall off starting above 1 V. They also have far worse Early effect (or the FET equivalent, whatever it's called). > > 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 Fairly horrible tempco, though, IIUC. > "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. You've never designed a high performance photoreceiver, have you? Noise on the bias supply comes in exactly like TIA voltage noise--they're on opposite ends of the same capacitor (the photodiode capacitance). Achieving nanovolt noise on the supply is very frequently the difference between success and failure. 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: Mike on 25 May 2010 17:43 George Herold <gherold(a)teachspin.com> wrote: [...] > Thanks Mike that was easy.... pretty soon I�ll be ready for the spice > �Karma Sutra�. > > George H. I forgot to mention - if there is a plot file, it needs the same prefix as the asc file. Mike
From: Mike on 25 May 2010 17:48 > I did about the same, similar results. > > ftp://jjlarkin.lmi.net/C-multiplier.gif Phil mentioned many spice programs don't handle this very well. Using the data from his later post showed the results with LTspice are not usable. Mike
From: John Larkin on 25 May 2010 18:49
On Tue, 25 May 2010 21:48:13 GMT, Mike <spam(a)me.not> wrote: >> I did about the same, similar results. >> >> ftp://jjlarkin.lmi.net/C-multiplier.gif > >Phil mentioned many spice programs don't handle this very well. Using the >data from his later post showed the results with LTspice are not usable. > >Mike The question is whether the Early voltage slope is realistic. I don't know. I suppose I should breadboard some parts but... the Gerbers are gone! John |