Twin T circuit wanted
in [Basics]
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From: John Larkin on 10 Jun 2010 20:37 On Thu, 10 Jun 2010 19:28:30 -0500, John Fields <jfields(a)austininstruments.com> wrote: >On Thu, 10 Jun 2010 20:58:49 -0300, YD <ydtechHAT(a)techie.com> wrote: > >>Late at night, by candle light, Jim Thompson >><To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> penned this >>immortal opus: >> >>>On Thu, 10 Jun 2010 07:03:49 -0500, John Fields >>><jfields(a)austininstruments.com> wrote: >>> >>>>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. ;) >>> >>>Larkin copies circuits from others, Rohde in this case, then totally >>>blows the explanation. >>> >>>Then, unfortunately, Win has backed up the BAD explanation :-( >>> >>> ...Jim Thompson >> >>So cough up the GOOD explation, or shut up. > >--- >Why not let Win defend himself instead of adding to the fray? Win has no reason to defend himself; he's discussing a circuit intelligently. All you and JT have done in this thread is is whine. JT has added nothing of substance. He seldom does. Like Sloman, all he does is say how smart he is, and how dumb everybody else is, with no actual content. So how *does* a tuning fork oscillator provide amplitude stability? John
From: MooseFET on 10 Jun 2010 20:21 On Jun 11, 12:01 am, John Larkin [....] > I don't think so. People designed radar, magnetrons and klystrons and > waveguides and servos and all that, without computers. They did the > math. Early computers were obviously designed without help from > computers. <PITA> Make that "electronic computers". At one time, a computer was a person who computed. Companies had rooms full of people grinding through the numbers to make sure that the sums were right. </PITA> There were also some analog computers and mechanical computers. Each generation has used the tools made by the previous. Just try to imagine designing with Roman numerals and not even a slide rule. > I don't use Spice a lot, and could certainly get along without it. It > is helpful when evaluating nonlinear systems, where math solutions >get messy. I use spice as a sanity check. Sometimes it even finds some. LTSpice is also nice for making a schematic to email to someone.
From: John Larkin on 10 Jun 2010 20:44 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", 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. How lame. Tell us more about tuning fork oscillators. John
From: Winfield Hill on 10 Jun 2010 20:55 John Larkin wrote... > > I have never called myself a "judge", 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. How lame. That's correct. I work hard at what I do, but I'm always on the lookout for mistakes I may make, or more often, things I don't yet understand. Hopefully I'll not pipe up about something I don't yet understand, but oops, oops, sometimes one doesn't yet know that they don't understand something, or they may just make a silly thoughtless mistake. -- Thanks, - Win
From: BlindBaby on 10 Jun 2010 21:07
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, and yet wants to elevate >himself into the position of a judge of circuit designs? And circuit design tools as well. |