C-multiplier again
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From: John Larkin on 25 May 2010 18:52 On Mon, 24 May 2010 23:47:29 -0500, "Tim Williams" <tmoranwms(a)charter.net> 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." Interesting power dissipation situation. I do this occasionally, with transistors or fets: Isink | | | | | c gnd---------b e | | | R | | | -10V John
From: Bitrex on 25 May 2010 21:17 Tim Williams wrote: > "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. > I just downloaded a model of the LM13700 - I'll test it out. > 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. Shhh! :) > > 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). > You're right, I missed that. Those resistors are too large. Performance improves when I make them a value that's more in line with the impedance on the other side. > 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. I had the differential amplifier there because I was messing around with scaling factors going into the multiplier. It turned out to not be necessary and the bias voltage can just be applied to pins 2 and 4. > 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. > Pretty close - the circuit is in essence an analog computer. My idea was that the transfer function of a differential pair is approximately Iout = (Io/2VT)*tanh(vid), where Io is the LTP current. By selecting the V-I converter resistor and Io appropriately you can cause the multiplying terms to drop out (at only one temperature, though) and you get tanh(vid). This is then squared by the multiplier to get tanh^2. The reference voltage is divided down 1000-1 and applied to a similar differential pair. The idea is that the amplified output of the second differential pair will be approximately the _derivative_ of the tanh function with respect to the reference voltage, or sec^2h(vid), or tanh^2(vid) - 1, which is also inverted to get -tanh^2(vid) + 1. Then it's added in the multiplier to cancel the tanh^2 terms and get 1, or a voltage that's stable with respect to variations in the reference. In practice I'm not getting 100mv out as I expected but the multiplier does seem to put out a stable voltage, with 1V P-P at 1000 Hz bouncing on the supply a FFT on the output shows the first harmonic down -115 dB, which is about the PSRR of the output op amp. Whether this is better than just two TL431s set to different voltages attached to the inputs of the same op amp as a differential amplifier, I do not know. :) > Tim >
From: Bitrex on 25 May 2010 21:22 Bitrex wrote: > The idea is that the amplified output of the second differential pair > will be approximately the _derivative_ of the tanh function with respect > to the reference voltage, or sec^2h(vid), or tanh^2(vid) - 1, which is > also inverted to get -tanh^2(vid) + 1. Then it's added in the > multiplier to cancel the tanh^2 terms and get 1, or a voltage that's > stable with respect to variations in the reference. In practice I'm not > getting 100mv out as I expected but the multiplier does seem to put out > a stable voltage, with 1V P-P at 1000 Hz bouncing on the supply a FFT on > the output shows the first harmonic down -115 dB, which is about the > PSRR of the output op amp. > I meant the derivative is 1 - tanh^2....I think in the circuit however things are added up the correct way. I'll have to check again!
From: Bitrex on 26 May 2010 00:03 Bitrex wrote: > Bitrex wrote: > >> The idea is that the amplified output of the second differential pair >> will be approximately the _derivative_ of the tanh function with >> respect to the reference voltage, or sec^2h(vid), or tanh^2(vid) - 1, >> which is also inverted to get -tanh^2(vid) + 1. Then it's added in >> the multiplier to cancel the tanh^2 terms and get 1, or a voltage >> that's stable with respect to variations in the reference. In >> practice I'm not getting 100mv out as I expected but the multiplier >> does seem to put out a stable voltage, with 1V P-P at 1000 Hz bouncing >> on the supply a FFT on the output shows the first harmonic down -115 >> dB, which is about the PSRR of the output op amp. >> > > I meant the derivative is 1 - tanh^2....I think in the circuit however > things are added up the correct way. I'll have to check again! Eh, the idea doesn't seem to be working out - the output is actually better without the multiplier in the cicuit! It looked good on paper, oh well. :(
From: Tim Williams on 26 May 2010 02:44
"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:90lov5d1lvqrrhqihs9gehltl79h9920at(a)4ax.com... > I do this occasionally, with transistors or fets: > > <snip> Durr... I've even been known to do it with tubes. ;-) Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms |