Twin T circuit wanted

You demonstrate arrogant ignorance at its highest level.

The current waveform doesn't look even quasi-sinusoidal... it actually looks rectified... not surprising the emitter junction characteristics. I suppose you'd call a full-wave rectifier power supply "class-A"??

But that's what I've come to expect from the BU Pimp :-) ...Jim Thompson

You distracted me... Where's the "AGC" ?:-) ...Jim Thompson

Right; the schottly is a more pure AGC mode. The reverse emitter conduction depends on the inverse beta of the transistor. If the model includes inverse beta, when the collector dips down to close to ground, and the c-b junction forward biases, it essentially flips ends: collector becomes emitter, emitter becomes collector, emitter current flows upwards.

ftp://jjlarkin.lmi.net/LC_YDx.gif

Either way, the base cap gets discharged.

You can force this to work class-C if you jack around with the transformer ratio, and use a small base cap, but that's not the way I originally described the circuit.

Maybe I'll try moving the feedback into the base circuit, one of these days.

Cute circuit.

John

If Q1 conducts in reverse what _class_ is the oscillator ?:-) ...Jim Thompson

It's interesting that Jim's PSpice doesn't show the same spike. I put the schottky clamp in on theoretical grounds though--without it the collector _has_ to swing a diode-drop below the base to discharge the base cap, which puts Q1 at least on the verge if not frankly in reverse mode conduction.

With the schottky, damping (via the bias) kicks in as the collector swing approaches 0.4v below Vb, preventing Vce ever going negative.

I did that first, with the savage versions. Theoretically I think that's more stable, letting the LC ring as it will, but with the effect of the reverse spike...I'm still not sure.

Leaving the transistor on all the time gives a cleaner waveform, but manhandling the tank full-time slightly out-of-phase pulls the tank off its natural frequency.

Hmmm. I guess it doesn't really matter--I was thinking RF oscillator here, but the feedback phase delays at audio are trivial.

I did that too. Not much different really.

I think so too.

-- Cheers, James Arthur

He blames it on defects in LT Spice. That's crazy. I suspect it's actually different transistor models. Note that in my sim, the emitter current reverses exactly at the points in time when Vc swings through Ve.

John

I didn't say that at all. I said I don't see it in PSpice, AND it's an advertised feature of LTspice that models are tweaked for speed.

However I suspect it's that I assigned a resistance to the feedback winding proportionate to the Q assigned to the primary. You two did not.

PSpice does model BR, so it's not that. It could also be that I'm looking way out at 1 second, where the loop is steady, and loop "replenishment current" is very small. It's quite possible that, during loop closure, you have some inverse transistor action. It's certainly not there at 1 second... and the spectral analysis does not show it either.

However, it is dead clear, there is no AGC action controlling TRANSCONDUCTANCE :-) ...Jim Thompson

I re-measured: IE is _never_ less than 376uA OUT of the emitter... no reverse transistor action. ...Jim Thompson

On closer examination I _am_ seeing some kind of burble on the emitter current. Applying a Schottky _does_ reduce the burble, but not completely, since you load the bias cap. No real change in spectrum. Perhaps tie base of Q1 to the juncture of R1/D1? Or a Baker clamp would certainly provide the isolation. But I doubt the ROI :-) ...Jim Thompson

But it's weird, and counterproductive. The "burble" smoothes some, but at the price of peak emitter current going up to 20mA. Weird :-( ...Jim Thompson

Baker clamp does the trick without as much rise in peak IE.

But, if your definition of class-A is current not passing thru zero, it doesn't matter... it already was "class-A" :-)

And your definition of "is" :-) ...Jim Thompson

ohm

_continuously_

The definition i have always heard is that it is conducting through the whole cycle. Nothing about linearity. And that is one fougly non-linear circuit.

Only if you want it to be. It can be designed to be a nice smooth class A oscillator with precise automatic gain control that servoes oscillation amplitude to almost exactly 2*Vcc, with a low TC. Not bad for one transistor.

John

So show us. I maintain there is no variable _linear_gain_ element there. As Win says, "Discuss it" :-) ...Jim Thompson

non-linear

So post that version instead.

I did, several days ago.

ftp://jjlarkin.lmi.net/LC_YDx.gif

And I described it long before that.

ftp://jjlarkin.lmi.net/Ships_Bell.JPG

I can't help it if people keep over-driving it, and then complaining that it's over-driven.

John

Hear yee, hear yee... one time only announcement... it's not a linear AGC'd oscillator... believe Larkin at your own peril :-) ...Jim Thompson

the

non-linear

No way sport, that is running deep class C.

The output is reasonable like a proper TC oscillator, but it is running class C.