Re: OT - Archie - I Dare You

I suspect that you are just another sheep that wants to look as though you are on the side with the most favor.

I do, f*****ad. What is it to you?

It could be that all the bullshit all you retarded twits have been making up is coming around to bite you stupid fucktards in the ass.

You're an idiot.

Only because you are too stupid to read the group. Otherwise you would know. I think it was just last week that I was describing my 4X layout days.

I think your powers of assessment are shadowed only by the true retards that mouth off in here. You should really re-think your position of attacking me, asswipe.

It would, if they were looking for me, idiot.

Oh WAIT! My IP address is not that IP address. Looks like the Terrell tard doesn't know his ass from a hole in the ground.

Sure they're there. Your problem didn't state the real world though. Sorta a SPICE thing. ;-)

Actually, I've done a *lot* of emitter followers and have never had one oscillate (a ton of ECL thingies and followers out of op-amps). I guess I've lead a charmed life.

Or never owned a fast scope. ;)

Cheers

Phil Hobbs

ium)

=A0That

Hmm I slapped this together on some white protoboard. 2n3904. I didn't see any oscillations. (X10 'scope probe and 200MHz 'scope) But maybe the proto-board is 'saving' me. Do I need to air wire it?

George H.

If the base is RF-stiff to ground (ie, well bypassed) emitter followers often oscillate, typically at 100 MHz or so. It's somewhat erratic but risky enough that it should be considered. If the transistor is driven from a resistive source, it's usually OK. 33 ohms or so will do.

Faster RF-type transistors will usually oscillate in emitter followers or diff pairs, lacking reasonable base resistance.

Oscillation can produce weird DC offsets even if you can't see them on a scope. If, say, touching the base with a pencil changes DC behavior, suspect oscillation.

John

Base-emitter capacitance and emitter/ground strays can resonate with just a short length of wire on the base. Your scope might see it, but they can start/ stop with the added capacitance of the scope probe, so it's hard to know. A common 'cure' is 50-100 ohms to the base (more or less... I've seen upwards of 200 ohms and more.) It softens the 'tank' Q. (And affect the high frequency roll-off.)

Jon

Yup. I was given an audio twin-tone oscillator by a local ham... he'd built it based on a magazine article, and it had never really worked right. Couldn't adjust the tones properly, the tone amplitudes jumped around, etc. - squirreley as you can imagine.

The instability was easily reproduced by the same sort of tests you suggest, by touching ground leads with my finger, etc. A fast o'scope and a spectrum analyzer confirmed that the audio oscillator was "singing" - it would chirp out a burst of RF at the low-voltage portion of each audio waveform. The birdies were up in the 100-130 Mhz range most of the time. Definitely a bad case of snivets.

It was a simple "twin-T" audio oscillator... which has a sizable capacitor-to-ground right on the base of each transistor. The transistors were classic metal-can 2N2222 parts, in plastic sockets - nothing exotic or fast by today's standards, but a good deal of gain and a nontrivial amount of lead inductance.

I stuck a ferrite bead on each base lead (and one on each collector lead for good measure), and the circuit went from shrieking lunatic to well-behaved kitten. No more oscillations, no more sensitivity to touch by probes or fingers.

--
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I'd think more the resonance of Cob with the 20 nh/inch of wire connecting the collector to the power supply.

Jim

Is a 7104 or 7S11 fast enough to see it? I worked for Deep Blue Pockets too. ;-)

My experience is that adding collector impedance (resistor or ferrite bead) usually doesn't help.

Emitter followers can present a negative impedance at the base, and oscillate their own wire bonds and/or external traces and capacitors. Base resistance kills the Q.

Not so lomg ago, I was doing some opamp-pnp-transistor closed-loop positive current sources; there was a ferrite in the collector to improve the current source impedance at higher frequencies. I did the usual loop stabilizing stuff, but reasoned that the open-loop output impedance of the opamp, driving the base, would be sufficient to damp the transistor oscillations. Wrong! Rather than kluging a resistor into the base, I substituted one of those "digital transistors" that has the built-in base resistor. That worked.

John

For your amusement, easily calculated from the classic pi model. Having done many ECL/PECL part designs, I have seen multi-GHz oscillations when outputs are appropriately loaded.

...Jim Thompson

--
| James E.Thompson, P.E.                           |    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  |
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          Obama... Recklessness Cloaked in Righteousness

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ranium)

. =A0That

s:

Opps my error. I made the 5V on the base with two 100 ohm resistors as voltage dividers from the collector. I'll try bypassing these with caps and see if I can get it to 'sing'. (This is going to have to wait till later, I've got a pile of diode lasers to test.)

George Herold

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ranium)

. =A0That

s:

Yup, seen that scope probe loading effect too.

Thanks, George

Cranium)

 That

That would yield, in effect, a 50 ohm resistor. Viola. Murdered Q and no oscillation.

Jon

This'll make most UHF transistors oscillate at...UHF:

+10v -+- |_ _) _) L1 _) | +---. Q1 |/ | +5v ------| C1 |>. | | | +---' | R1 1k | ===

L1 and C1 can be tiny, unintended, even strays, if that gives you any notion of it

--
Cheers,
James Arthur

A couple years ago he was bragging about working for Time Warner Cable TV in Cincinnati, OHio, on their 'Cube TV' system. He uses Cox cable to post his crap, and Cox has a system in San Diego. Do you think that he's still a cable Gypsy, working in their boiler room? It would explain how he flaunts their TOS, because low grade MSO cable companies don't give a damn what their employees do. Too bad I don't know anyone at the local CoX Cable offices. I left that business too long ago to have any contacts. Its been 25 years since I walked out of a head end for the last time.

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You can\'t have a sense of humor, if you have no sense!

Funny that dirty little retarded bastards like you actually make all these untrue petty mouthings, and then you actually begin to believe your own bullshit.

I sure hope that rotted jaw bone does its thing on you soon.

Oh, touchy! DimBulb, I do believe they've hit your nerve.

And its like finding the only nerve in a huge rotting bag of puss.

--
You can\'t have a sense of humor, if you have no sense!