Twin T circuit wanted
in [Design]
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From: Winfield Hill on 10 Jun 2010 16:30 Jim Thompson wrote... > > What was with, "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"? Where in the world did that come > from... Larkin's _incorrect_ description? Yep, CE was my mistake. Jim, I'm going to continue the conversation over in the part of the thread where the "John Larkin's LC oscillator" heading is, instead of "Twin T circuit wanted" - we don't want to get flamed again for being off topic. Meanwhile, you might think about the way the circuit works at low supply voltages near the end of a bell ring, just before it stops oscillating. -- Thanks, - Win
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 |