Transistors

What keeps the typical emitter follower from oscillating?

How often to people like me get into trouble because they don't do whatever it takes to prevent oscillation?

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When was that?

What are they using now and when did they switch from phenolic?

I have a couple of IR photodiodes and IR LEDs in clear plastic packages. I assume modern packaging blocks IR or they wouldn't bother to use a different plastic.

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Somebody used to make, maybe still does, photofets. They were used, among other things, to discharge the capacitors in super-low noise integrating-type radiation and photo detectors.

John

Usually the fact that the base drive isn't RF stiff to ground. A few

10's of ohms equivalent resistance will do it.

Dunno, depends on what you do. If you don't see the RF itself, it can show up as dc offset and noise.

John

The opamp doesn't seem to matter. It's the transistor that oscillates, usually far above the opamp's gbw. I had the last problem with an LM7301 feeding a BCX71, neither especially fast. There was a ferrite bead in the collector, but that didn't help. A base 47 ohm resistor did.

John

At least into the mid '70's

Mid '80's, a circuit designed for photo detection (data), but other parts of the circuit were undesirably photo-sensitive as well.

Another poster, Hobbs I believe, mentioned a plastic I don't recognize.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Small-signal spice tests (frequency sweeps) of power- amplifier circuits like that, at high-frequencies, are close to meaningless - useful for no-signal loop stability perhaps, but certainly not for evaluating how an amplifier can do at delivering power at high frequencies. You can try transient full-power tests with spice, but I'd want to verify the component models on the bench before giving it much credence. And of course, when the failure mode is thermal, and your spice model isn't set up to handle self-heating, well... What you can do with spice, after you've got models you can trust, is to evaluate the continuing emitter current in a class AB transistor after it's supposed to have gone off or nearly off each cycle. Doing this will help teach you what's badly wrong with that circuit for high-frequency high-power use.

On a sunny day (Mon, 19 Nov 2007 08:42:09 -0800 (PST)) it happened Winfield Hill wrote in :

OK, you do have a point, and I have not build this one with these transistors (that would be the real test, I trust spice more then in the past, but reality rules), but this begs the question to *you*: How would you do it? You mentioned class A, and this was an attempt to go towards class A from my side (decreasing the emittor resistors decreases the phase shift). I am curious how you would solve the 20V pp 10MHz say 50 Ohm load and open drive?

50 ohms isn't so hard, with the sourcing resistor it's really 100 ohms or only +/-100mA for +/-10V --> +/-5V at the terminated destination.

Anyway, there's always brute-force class A ---

But first let me say, I'm not an IC designer and I don't have access to a fab, etc., at least not past the usual student stuff, so I don't spend time worrying about that scene. So the low- capacitance low-inductance parts and connections that are needed for efficient 10 to 20MHz power opamp circuits are a past my interest. However, I do use basic class-AB design techniques for my discrete circuits up to say 5MHz.

Looking at your classic output stage, with the class-AB bias voltage, driving output transistors: Imagine instead a design where you have such an output stage running at high class-AB currents, for high fT and slew rates, but with relatively small BJTs to keep the capacitance down, imagine two sets of those, one for the high side and one for the low side of the class-AB bias voltage. Now imaging using these two fast medium-power stages to drive two final output transistors. This way you are providing serious turn-on and turn-off currents separately to the two bases, out of phase with each other. That's the type of circuit, with extra internal stuff to drive each output component, that has possibilities to work to higher frequencies and slew rates.

I'm not Win, but offhand, a 2N3904 cascode running a moderately beefy emitter follower (biased ~ class B with two diodes) should work just fine. For the follower, complementary transistors with fT > 200MHz, Ic > 200mA, Vce > 30V and Pc > 2W should suffice. Something in a TO-5 can with heatsink would do. 2N3766 I think pops up in my head, but I may be mistaken. There's a complementary pair with fT ~ 500MHz, 35Vceo and 500mA Ic that would work excellently.

Tim

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The fact that the driving source isn't bypassed for RF to the center of the earth. I prefer a 100r resistor in series with the base lead; I used to use

33r but had a particularly "hot" RF device make like a canary one time and a hundred ohms doesn't seem to make a hell of a lot of difference when beta generally is in the 200 range or so.

Dunno...depends on the application.

Jim

That would be Helge Granberg & Company? Did Georges Luttgeneau (sp?) ever work for Moto RF?

Jim

The diodes wouldn't give anywhere near the low output impedance or tiny offset voltage of the saturated transistor. The transistor is a bidirectional conductor, too.

John

And speaking of strange semiconductor effects (were we?) I've discovered that the c-b junctions of certain power transistors make nice 20-kilowatt drift step-recovery diodes.

John

Come again? Care to share what power transistors and how the hell you cool them to do kilowatt level work?

Were you aware that SRDs make very nice and linear phase shifters?

Jim

And you accuse ME of having weird tastes in fun. Humph. ;)

Cheers,

Phil Hobbs

On a sunny day (Mon, 19 Nov 2007 09:54:43 -0800 (PST)) it happened Winfield Hill wrote in :

OK, get the idea.

Interesting. My original idea was using BFY90 in the pre stages, but could not find a spice model for it (it is very old, but 1.3GHz ft). In some Dutch magazine a 300MHz wide scope Y amp was published by somebody at Tek, many years ago, it used the BFY90. I do not remember what they used in the final stage, driving the CRT defection plates, though. BFY90 has a Vcb max of 30V, but only a Vce of 15 V. Could be used in common base perhaps :-) Maybe I am a bit stuck in old times here, I am sure there are better ones now.

Accuse? Never.

I was in the pet store in the village, Critter Fritters, and there was a guy ahead of me at the checkstand buying $140 worth of dog food. He was describing some football game in excriciating detail, every move that the team made on some play, some guy in motion, some move the quarterback made, on and on. I couldn't check out my little bag of cat kibble while he kept yapping. Now *that* guy was weird.

John

And twice as ornery! ;-)

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Michael A. Terrell
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