Twin T circuit wanted
in [Basics]
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From: John Larkin on 10 Jun 2010 18:53 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. John
From: YD on 10 Jun 2010 19:58 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. Hey, why don't you three join up in a skiffle or jug band, "Two Johns And A Jim"? -YD. -- File corruption detected. Select option: 1 - Call the cops 2 - Call the press 3 - Bribe it Remove HAT if replying by mail.
From: John Fields on 10 Jun 2010 20:03 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?
From: John Fields on 10 Jun 2010 20:28 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? --- >Hey, why don't you three join up in a skiffle or jug band, "Two Johns >And A Jim"? --- Maybe because, so far, we like to play in different keys. JF
From: MooseFET on 10 Jun 2010 20:37
On Jun 10, 11:06 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: [....] > Can you think of other ways to make a very frequency and amplitude > stable sine wave using early-70s technology? I suppose that a square > wave generator and bandpass filter would work, but that's more parts. A tuning fork "self hummer" circuit using inductive drive and inductive pick-up could be quite frequency stable. Amplitude stability comes from making the cathode current on the 6SN7 nearly constant with a really high voltage supply and a large resistor. |