Twin T circuit wanted
in [Basics]
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From: dagmargoodboat on 10 Jun 2010 00:23 On Jun 9, 9:18 pm, Winfield Hill <Winfield_mem...(a)newsguy.com> wrote: > Jim Thompson wrote... > > >>> I know John won't respond, but could someone, perhaps Win, tell me > >>> how the "AGC" works? > > >> I'm sure he would, but why should I, over the years you've insulted > >> me at least as much as him, and perhaps more aggressively? > > >> Anyway, he did explain it, SFAICT. > > >> Note the BJT is over-biased - plenty of base current, that if left > >> unchecked would charge the base-to-ground capacitor and over-current > >> the transistor. So the oscillator runs and examining cycle-by-cycle, > >> the collector swings higher and higher until it goes negative with > >> respect to the base voltage, close to saturating the transistor, > >> and turning on the base-collector diode a bit, robbing current from > >> the base capacitor. This process servos the BJT current to just the > >> right level to sustain an oscillation collector-voltage level where > >> just the right amount of current is robbed each cycle to control the > >> base voltage. Thereby insuring that the collector goes close to the > >> emitter on each cycle, establishing a tightly-controlled amplitude, > >> which as John pointed out, is temperature independent to first order > >> since Vce(sat) is relatively temperature independent. > > >> John said Vcc peak, but actually it must be closer to Vcc - Vce(sat). > > > I'll buy that the collector forwards biases, and you enter a limit > > cycle. Thus I'd call it ALC. I don't see any _gain_ variation that > > "AGC" would imply. > > Picky, picky. To my mind, the base current robbed by the > collector starves the base, lowering the CE stage's gain, > until the exact equilibrium is achieved. ALC, AGC, pick > your name as you like. Either way it gets the job done > rather nicely, and is a bit different from what we've seen > elsewhere, such as in old radio circuits. I see that it > has been analyzed as a possible RF oscillator technique. > But it seems to me that, working as we imagine, Vce(sat) > and all, this trick would be limited to far far below fT. Just to clarify, the RF versions I posted are similar to, but not the same as John's. They're standard UHF designs, Class A, without John's precision AGC. I don't think they can use John's AGC method directly-- if saturated, the transistors would be too slow--but maybe a Baker-ish clamp thing would do the job. Oh, and John's oscillator really swings ~ 2* (Vcc + Vbe), not 2* (Vcc - Vbe). Reason being, the AGC operates as the average base voltage gets sucked down to near 0v, killing the gain. The collector has to swing downward from Vcc to ~Vbc _below_ GND to forward bias the b-c junction on the negative peaks, and swings up a symmetrical amount on the positive peaks. -- Cheers, James Arthur
From: Tim Williams on 10 Jun 2010 02:27 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:ord016d4r36bbakqba007ir05p4b03n69m(a)4ax.com... > I did a constant-voltage/constant-current crossover power supply sort > of thing recently, analog ORing two loop error signals, with a wide > range of possible customer loads, and it was great for tweaking. The > actual implementation will be firmware. I did one of those the other day. Works perfectly. 5V, 100A variable output. Jam a screwdriver on the output... all you'll get is a warm screwdriver. ;-) Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: John Fields on 10 Jun 2010 08:03 On Wed, 09 Jun 2010 20:46:42 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On 9 Jun 2010 19:18:40 -0700, Winfield Hill ><Winfield_member(a)newsguy.com> wrote: > >>Jim Thompson wrote... >>> >>>>> I know John won't respond, but could someone, perhaps Win, tell me >>>>> how the "AGC" works? >>>> >>>> I'm sure he would, but why should I, over the years you've insulted >>>> me at least as much as him, and perhaps more aggressively? >>>> >>>> Anyway, he did explain it, SFAICT. >>>> >>>> Note the BJT is over-biased - plenty of base current, that if left >>>> unchecked would charge the base-to-ground capacitor and over-current >>>> the transistor. So the oscillator runs and examining cycle-by-cycle, >>>> the collector swings higher and higher until it goes negative with >>>> respect to the base voltage, close to saturating the transistor, >>>> and turning on the base-collector diode a bit, robbing current from >>>> the base capacitor. This process servos the BJT current to just the >>>> right level to sustain an oscillation collector-voltage level where >>>> just the right amount of current is robbed each cycle to control the >>>> base voltage. Thereby insuring that the collector goes close to the >>>> emitter on each cycle, establishing a tightly-controlled amplitude, >>>> which as John pointed out, is temperature independent to first order >>>> since Vce(sat) is relatively temperature independent. >>>> >>>> John said Vcc peak, but actually it must be closer to Vcc - Vce(sat). >>> >>> I'll buy that the collector forwards biases, and you enter a limit >>> cycle. Thus I'd call it ALC. I don't see any _gain_ variation that >>> "AGC" would imply. >> >> Picky, picky. To my mind, the base current robbed by the >> collector starves the base, lowering the CE stage's gain, >> until the exact equilibrium is achieved. ALC, AGC, pick >> your name as you like. Either way it gets the job done >> rather nicely, and is a bit different from what we've seen >> elsewhere, such as in old radio circuits. I see that it >> has been analyzed as a possible RF oscillator technique. >> But it seems to me that, working as we imagine, Vce(sat) >> and all, this trick would be limited to far far below fT. > >When I post circuits, JT and JF pull out dictionaries and start >arguing about what words mean. Circuits don't spend much time reading >dictionaries. --- Neither do you, obviously. ;)
From: John Fields on 10 Jun 2010 08:13 On Wed, 09 Jun 2010 20:42:20 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On 9 Jun 2010 18:54:01 -0700, Winfield Hill ><Winfield_member(a)newsguy.com> wrote: > >>Jim Thompson wrote... >>> >>> John Larkin wrote: >>> >>> [snip] >>>> >>>> The cool thing is that the collector swing is almost exactly 2xVcc >>>> peak-to-peak. As the amplitude builds up, at the negative swing peak >>>> the emitter goes a little bit negative, to get out of the way, and the >>>> collector swings to just about ground. That forward-biases the c-b >>>> junction and discharges the base cap, reducing transistor base current >>>> hence gain. So it has a built-in peak detecting AGC amplitude >>>> leveling loop with close to zero TC. All from 5 parts. Or sometimes >>>> six. >> >> What's the 6th part, I wonder? > >The peak detection can put a tiny flat on the negative swing of the >sine wave. A small resistor in series with the collector fixes that >and doesn't seem to do a lot of harm otherwise. > >> >>> I know John won't respond, but could someone, perhaps Win, tell me >>> how the "AGC" works? >> >> I'm sure he would, but why should I, over the years you've insulted >> me at least as much as him, and perhaps more aggressively? > > >But he didn't insult your wife, too. > > >> Anyway, he did explain it, SFAICT. >> >> Note the BJT is over-biased - plenty of base current, that if left >> unchecked would charge the base-to-ground capacitor and over-current >> the transistor. So the oscillator runs and examining cycle-by-cycle, >> the collector swings higher and higher until it goes negative with >> respect to the base voltage, close to saturating the transistor, >> and turning on the base-collector diode a bit, robbing current from >> the base capacitor. This process servos the BJT current to just the >> right level to sustain an oscillation collector-voltage level where >> just the right amount of current is robbed each cycle to control the >> base voltage. Thereby insuring that the collector goes close to the >> emitter on each cycle, establishing a tightly-controlled amplitude, >> which as John pointed out, is temperature independent to first order >> since Vce(sat) is relatively temperature independent. >> >> John said Vcc peak, but actually it must be closer to Vcc - Vce(sat). > > >Oh, picky picky. It's actually, probably, a bit more complex, since >the emitter is a little negative at the instant of collision, and the >transistor is almost saturating, so some of the stolen base current is >going into the collector and some is going into the emitter. It would >be interesting to simulate, just to see where the currents really go. >Adding the collector resistor changes things, too. --- So simulate it; I posted a working circuit list earlier if you can bear to use it without rancor getting in the way. --- >But the AGC thing does work, and the TC is close to zero. --- Serendipitous, no doubt, unless you intentionally designed it in. Did you?
From: John Fields on 10 Jun 2010 08:27
On Wed, 09 Jun 2010 21:01:19 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Wed, 09 Jun 2010 16:02:57 -0500, John Fields ><jfields(a)austininstruments.com> wrote: > >>On Wed, 09 Jun 2010 10:30:37 -0700, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Wed, 09 Jun 2010 11:31:31 -0500, John Fields >>><jfields(a)austininstruments.com> wrote: >>> >>>>On Tue, 08 Jun 2010 15:09:51 -0700, John Larkin >>>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>>>If I claimed that there was nitrogen in the air, he and >>>>>JF would hack a Spice simulation and prove me wrong. >>>> >>>>--- >>>>You're being absurd, as usual, but it seems you lucked out this time >>>>and your oscillator works in LTspice. >>>> >>>> >>> >>>Since we manufactured and sold lots of them before Spice was >>>available, and they worked just fine, the luck is on Spice's part. Or >>>yours. >>> >>>This will shock the kiddies, but it *is* possible to design circuits >>>without using Spice. Usually it's faster and better. >> >>--- >>You're preaching to the choir, bucko. >> >>In your world, maybe, but when you're talking circuits with hundreds >>of thousands or millions of transistors, it's not possible. >> >>This may come as a surprise to you, but many (if not most) of the >>circuits which you buy and incorporate into your products were >>designed using SPICE, so the fact that you assemble them into working >>product that you don't simulate doesn't mean it's free of SPICE. > >SPICE 1 was introduced at a conference in 1973. It wasn't very good, >and SPICE 2, 1975, was better. DRAMS were introduced in 1970. --- Red herring, cheater, or, at the very least, ignoratio elenchi. |