Twin T circuit wanted

Tell us about, or better show us, something you're designing.

John

Answer the question.

John

That's what those of us that understand the importance of simulation are trying to tell YOU.

Nice snip of the calculator and clipboard reference, supporting your pathetic claim that I do not know what is going on and do not do math. Sorry, Johnny, but that math on that bright move is pretty evident, loser.

Since you are so against it, it is obvious that it is YOU that "does not know what is going on".

No, you ass, it was an adjective for YOU. Just so you know, the term does have several meanings, and the one you chose is incorrect.

Of what I can tell you about... a motor controller for an electric bike. Up to a 15kW peak burst capacity.

-

Good idea.

I Spice'd all the circuits I posted.

Yep, but to a 1rst order: average emitter voltage =3D 0, ignore the swing 'cause it's small, and that gets you pretty close. V(b) =3D 120mV in my 5KHz example.

James

[snip in favor of the improved ASCII drawing below ]

Jim pointed out elsewhere, in the old Twin-T portion of the thread, that I had misunderstood how the oscillator works. He gave a hint: not CE. Yep, of course, now it becomes a bit more clear. Thought of as an amplifier, it has to be operating in common-base mode.

First, given the transformer winding, the average DC voltage on the emitter always has to be zero volts. Then, if the transformer turns ratio is high enough, high enough to force class A operation, with very little signal on the emitter, the base capacitor voltage will be at roughly Vbe.

Being class A, the gain includes 1/re in its terms, and the gain has some proportionality to emitter/collector current. The LC tank amplitude and collector-voltage swing can also grow, proportional to this current. As oscillation starts, with increasing current and gain, the tank amplitude grows until the collector approaches saturation on each cycle, causing two things to happen.

First, the base-collector diode conducts and robs some base current, controlling the base voltage and DC current at the right value for the class A operation we're observing. Second, this base-collector diode conduction directs some of the emitter signal current to the base, away from the LC tank, reducing the common-base gain, the tank amplitude, and stabilizing the oscillator.

Operating this way, it seems fair to characterize the level control as an AGC common-base amplifier gain type of control, but you can also think of it as a type of current-steering amplitude control.

A high transformer turns ratio may not be a healthy way to run this oscillator, what if it's too high, and stops? So it's likely that our happy turns ratio will be well below a class-A operating limit. If so the oscillator emitter amplitude will increase, and it'll be running in a kind of switching or class C mode.

In class C the resonant tank is excited by short pulses of collector current. At the maximum emitter current, at the bottom of each tank cycle, the emitter voltage will be well below ground. The forward-biased base-collector diode will get serious about stealing collector current from the tank, when it has a chance, thereby regulating the AC RMS current and the LC tank voltage amplitude. It appears Jim wants to call this ALC, not AGC, action. The regulating function may be better thought of as signal current control than as a gain control, although the right term to use seems a bit semantic to me. Unless, of course, there's some guiding literature that we should be following.

I'm wondering if it wouldn't be a good idea to have an emitter resistor to help control the currents and stretch out the pulses. Maybe John had one in his old version.

In a ringing bell application, as the supply voltage sags, and the amplitude drops, I imagine the circuit will move from class C back to class A operation, before stopping.

--
 Thanks,
    - Win

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

--
I included one in the circuit list I posted earlier, and it has a
dramatic effect on the decay time.

I recall a pretty small AC voltage across the emitter winding, like

0.1 or 0.2 volts p-p maybe. Not pure linear class A, but not too radical either.

It is a simplification to say that the c-b conduction, at the bottom of the sine wave, is the only thing that pulls the base voltage down. Some base current goes into the emitter, too. It's sort of academic, since the collector and emitter essentially collide, and *both* help to discharge the base capacitor.

Two interesting variants are James's suggestion of a Baker clamp, and your idea of an emitter resistor. Both get the emitter out of the way as the discharge/AGC path, and make the c-b clamp idea purer. I'm not sure if the DC drop in an emitter resistor is a problem.

As far as the idea that there is *not* an AGC effect, consider:

A lot of current comes in through the base resistor.

The collector voltage is above saturation 98% of the time.

So the collector current should be about the current through the base resistor times beta.

But it isn't. It's much less. The reason is that the AGC effect has stolen most of the base current. Just enough, in fact, to make nice oscillations.

One picks the base resistor to give a healthy margin for starting and running, but not so low that the flat-top (bottom, actually) on the sinewave is gross.

The temperature thing is interesting. Small-signal, the base/capacitor voltage is one jd above the emitter voltage, with the usual tc. When the collector swings down, it has one jd worth of tc in pulling down the base capacitor. So the tc of the c-b junction compensates the remembered tc of the b-e junction.

John

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

Late at night, by candle light, Jim Thompson penned this immortal opus:

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.

--
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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?

On Jun 11, 12:01 am, John Larkin [....]

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.

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 use spice as a sanity check. Sometimes it even finds some.

LTSpice is also nice for making a schematic to email to someone.

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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

[....]

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.

Maybe there's a definition problem with the term AGC. We're looking at the circuit and saying, yes there's a gain-control function going on inside the circuit. But, in the legacy sense, the term AGC has been used to refer to gain changes in amplitude-modulated radio receivers, etc. That is, AGC of a signal pathway.

Here we have an oscillator that needs a level-control functionality, so maybe, ALC is the right term to use, even if AGC is how it's accomplished. Not that I want to grant Jim any space for ragging so severely on us. I mean, WTF, this is a discussion group! Or used to be.

--
 Thanks,
    - Win

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

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