From: John Woodgate
Subject: Re: Amplifying stage with negative Vgain??
Date: Thu, 5 Dec 2002 17:31:37 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Thu, 5 Dec 2002 20:43:07 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Mike Monett wrote
(in <3DEF5D0B.2EB3@Spam.Bots>) about 'Amplifying stage with negative
Vgain??', on Thu, 5 Dec 2002:
>John Woodgate wrote:
>> IIRC, that's 'sort of' true but misleading. Weird things happen, AIUI,
>> to the input and output impedances as the frequency goes above fbeta.
>> Regards, John Woodgate
>An emitter follower has pretty high bandwidth. That's why parasitic
>oscillations are so hard to find - they can be above the scope bandwidth.
That's exactly what the impedance problems cause.
>In the example I posted, the -3dB point was above 100MHz with 150 ohms
>source impedance. Not bad for a nickle:)
Did you include any inductance in the emitter circuit? It's bound to
have some, except in a simulation of course. At what frequency did the
input impedance go negative real?
If someone has emitter-follower problems and is operating above fbeta,
it's a good move to suggest a faster device. Agreed, that MIGHT not be
the problem. My head-scratching on this dates back to germanium days.
OC71 - oscillation (I don't remember the frequency, but above 20 kHz),
OC44 - no oscillation.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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