Twin T circuit wanted
in [Basics]
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From: dagmargoodboat on 11 Jun 2010 00:45 John Larkin wrote: > dagmargoodb...(a)yahoo.com wrote: > >John Larkin wrote: > >> In my oscillator, a c-b schottky diode would keep the transistor c-b > >> junction from conducting, and keep the transistor out of saturation. > >> Tempco would still be low. That simplifies things considerably. Not > >> bad. > > >Good idea. > > >> >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. > > >> I seem to recall the DC base voltage being about +.6. So the collector > >> swings to just about zero, and the AC output is 2*Vcc p-p. Somebody > >> could Spice this, if they were interested, and see exactly what > >> happens. > > >I Spice'd all the circuits I posted. > > >> The transformer ratio gets involved some, too. > > >Yep, but to a 1rst order: average emitter voltage = 0, ignore the > >swing 'cause it's small, and that gets you pretty close. V(b) = 120mV > >in my 5KHz example. > > > How much p-p voltage on the emitter? 1 volt. That might be a bit hot, as I noted in the post. I did that on purpose, thinking a smaller conduction angle would give better frequency stability. You know, let the L-C ring unmolested as much as possible? Might not be helpful though--if the drive is sine-ish, the tendency to pull might not apply. Not sure. > That low a DC base voltage suggests more like class-C action. With > less turns on the emitter winding, the thing gets more class A-ish, > and I'd expect the DC base voltage to go up some. I think. Yes, class-C. > I wonder what happens to the DC base voltage as the base bias resistor > changes. I'm not even sure which direction things will go. > > Complicated, for 5 parts. I like the schottky. Takes trr(b-c) out of the equation. James
From: John Larkin on 11 Jun 2010 01:16 On Thu, 10 Jun 2010 21:45:08 -0700 (PDT), dagmargoodboat(a)yahoo.com wrote: >John Larkin wrote: >> dagmargoodb...(a)yahoo.com wrote: >> >John Larkin wrote: > >> >> In my oscillator, a c-b schottky diode would keep the transistor c-b >> >> junction from conducting, and keep the transistor out of saturation. >> >> Tempco would still be low. That simplifies things considerably. Not >> >> bad. >> >> >Good idea. >> >> >> >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. >> >> >> I seem to recall the DC base voltage being about +.6. So the collector >> >> swings to just about zero, and the AC output is 2*Vcc p-p. Somebody >> >> could Spice this, if they were interested, and see exactly what >> >> happens. >> >> >I Spice'd all the circuits I posted. >> >> >> The transformer ratio gets involved some, too. >> >> >Yep, but to a 1rst order: average emitter voltage = 0, ignore the >> >swing 'cause it's small, and that gets you pretty close. V(b) = 120mV >> >in my 5KHz example. >> >> >> How much p-p voltage on the emitter? > >1 volt. That might be a bit hot, as I noted in the post. I did that >on purpose, thinking a smaller conduction angle would give better >frequency stability. You know, let the L-C ring unmolested as much as >possible? Might not be helpful though--if the drive is sine-ish, the >tendency to pull might not apply. Not sure. I prefer smaller drive, a tenth or so p-p on the emitter. Maybe even less, basically class A. The whole thing behaves differently if the secondary drive is large and the transistor conduction angle is small: the emitter voltage will swing down, way below ground, and pull the base down with it before the collector voltage gets down to ground... blasting a spike of collector current into the tank. Then it will swing way up and turn the base off. Brutality! Chaos! I prefer a more delicate touch: the collector dips down elegantly, like a swan landing on a pond. It just barely touches the water, err, emitter, and together they remove a bit of charge from the base cap. And then it flies away. Did I mention the sunset in the background? John
From: dagmargoodboat on 11 Jun 2010 01:34 On Jun 10, 11:45 pm, dagmargoodb...(a)yahoo.com wrote: > John Larkin wrote: > > dagmargoodb...(a)yahoo.com wrote: > > >John Larkin wrote: > > >> The transformer ratio gets involved some, too. > > > >Yep, but to a 1rst order: average emitter voltage = 0, ignore the > > >swing 'cause it's small, and that gets you pretty close. V(b) = 120mV > > >in my 5KHz example. > > > How much p-p voltage on the emitter? > > 1 volt. That might be a bit hot, as I noted in the post. I did that --^^^^^^ Ooops. That was for another sim, which uses 1mH and 10uH. The posted 5 KHz ckt used 1mH / 25uH, so the emitter swing was about 1.8v p-p. James Arthur
From: Jasen Betts on 11 Jun 2010 07:46 On 2010-06-11, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>I use spice as a sanity check. Sometimes it even finds some. >> >>LTSpice is also nice for making a schematic to email to someone. > > It is just about the only portable schematic format the industry has > ever seen. Not a bad editor, but the circuits seem to wander all over > the screen as you zoom. I have to keep selecting my whole circuit and > dragging it back into sight. Point away from the bit you want to see and zoom out. (or use the scroll bars) --- news://freenews.netfront.net/ - complaints: news(a)netfront.net ---
From: John Fields on 11 Jun 2010 08:41
On Thu, 10 Jun 2010 17:44:54 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 10 Jun 2010 19:03:28 -0500, John Fields ><jfields(a)austininstruments.com> wrote: > >>On Thu, 10 Jun 2010 15:53:53 -0700, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Thu, 10 Jun 2010 12:50:00 -0700 (PDT), dagmargoodboat(a)yahoo.com >>>wrote: >>> >>>>John Larkin wrote: >>>>> dagmargoodb...(a)yahoo.com wrote: >>>>> >>>>> >On Jun 9, 9:18 pm, Winfield Hill wrote: >>>> >>>>> >> 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. >>>>> >>>>> In my oscillator, a c-b schottky diode would keep the transistor c-b >>>>> junction from conducting, and keep the transistor out of saturation. >>>>> Tempco would still be low. That simplifies things considerably. Not >>>>> bad. >>>> >>>>Good idea. >>>> >>>>> >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. >>>>> >>>>> I seem to recall the DC base voltage being about +.6. So the collector >>>>> swings to just about zero, and the AC output is 2*Vcc p-p. Somebody >>>>> could Spice this, if they were interested, and see exactly what >>>>> happens. >>>> >>>>I Spice'd all the circuits I posted. >>>> >>>>> The transformer ratio gets involved some, too. >>>> >>>>Yep, but to a 1rst order: average emitter voltage = 0, ignore the >>>>swing 'cause it's small, and that gets you pretty close. V(b) = 120mV >>>>in my 5KHz example. >>>> >>>>James >>> >>>How much p-p voltage on the emitter? >>> >>>That low a DC base voltage suggests more like class-C action. With >>>less turns on the emitter winding, the thing gets more class A-ish, >>>and I'd expect the DC base voltage to go up some. I think. >>> >>>I wonder what happens to the DC base voltage as the base bias resistor >>>changes. I'm not even sure which direction things will go. >>> >>>Complicated, for 5 parts. >> >>--- >>So, _there's_ a "circuit designer" who can't even figure out how a >>circuit which he's put into the world works, > >It works fine the way I designed it to work. I admit I don't >understand all the possible variations, and the entire possible >operating envelope, because it didn't matter 35 years ago, and it >doesn't matter now. It's just sort of interesting to discuss. > >Discussion sort of requires that you don't assume you know everything. > > > and yet wants to elevate >>himself into the position of a judge of circuit designs? > >I have never called myself a "judge" --- Perhaps not in so many words, but your neverending patting yourself on the back while demeaning the work of others speaks volumes about what you perceive yourself to be. --- >, and Win has never called himself >a "master." You and JT call us that, so you can then abuse us for >saying things we never said. --- I've never called you a judge and I've never called Win a master, so it seems _you're_ the one putting words in my mouth so you can create a straw man, you insidious trash. --- >How lame. --- PKB, cheater. --- >Tell us more about tuning fork oscillators. --- Why, when all you're interested in is generating another row? Forget it; I'm done with you for the time being. |