Twin T circuit wanted
in [Basics]
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From: dagmargoodboat on 10 Jun 2010 11:51 On Jun 10, 10:06 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 10 Jun 2010 07:13:42 -0500, John Fields > > > > <jfie...(a)austininstruments.com> wrote: > >On Wed, 09 Jun 2010 20:42:20 -0700, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > >>On 9 Jun 2010 18:54:01 -0700, Winfield Hill > >><Winfield_mem...(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? > > Of course I did. As I noted, I needed a very amplitude-stable sinewave > source for the C5A inclinometer thing. This was a long time before DDS > and DAC versions were feasible. > > 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. > > Got any ideas? > > John Use a PIC !
From: Jim Thompson on 10 Jun 2010 11:52 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 -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: Jim Thompson on 10 Jun 2010 11:53 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. > [snip] I didn't insult your wife... you did it yourself. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | The only thing bipartisan in this country is hypocrisy
From: John Larkin on 10 Jun 2010 12:01 On Thu, 10 Jun 2010 09:09:55 -0500, John Fields <jfields(a)austininstruments.com> wrote: >On Thu, 10 Jun 2010 06:56:15 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Thu, 10 Jun 2010 07:27:47 -0500, John Fields >><jfields(a)austininstruments.com> wrote: >> >>>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. >> >> >>The first few generations of RAM were designed before Spice existed, >>which you declared to be "not possible." >> >>I don't know how to say that in Latin. > >--- >Pity, as is your lack of Englishy reading comprehension, since the >point which was being made was that even though you pooh-pooh SPICE, >your livelihood, today, depends on it. 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. 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 think that most board-level designers use, and trust, Spice too much. Given the cost and time for IC fab, it makes sense for IC designers to use it to check their work before baking silicon. I don't think that Widlar used Spice. John
From: John Fields on 10 Jun 2010 12:02
On Thu, 10 Jun 2010 08:06:23 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 10 Jun 2010 07:13:42 -0500, John Fields ><jfields(a)austininstruments.com> wrote: > >>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? > > >Of course I did. As I noted, I needed a very amplitude-stable sinewave >source for the C5A inclinometer thing. This was a long time before DDS >and DAC versions were feasible. > >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. > >Got any ideas? --- Tuning fork oscillator. |