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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Amplifying stage with negative Vgain??
Date: Thu, 05 Dec 2002 16:37:00 -0500
NNTP-Posting-Date: Thu, 05 Dec 2002 16:37:01 EST
Organization: Bell Sympatico
John Woodgate wrote:
> 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:
> >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?
There is not much problem with lead inductance at these low frequencies. Even
10nH has only 6 ohms reactance at 100 MHz. The emitter resistor is 1.08k, so
not much will happen there. A small amount of inductance in the collector has
little effect in a CC. Some inductance in the base can cause problems with
stray capacity - that's where parasitic oscillations love to hide.
As I mentioned, I ignored the stray capacity at the input and replied on SPICE
to figure out the junction capacities. It is good enough to illustrate the OP's
emitter follower has plenty of bandwidth for a 3.57 Mhz signal.
> 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.
A faster device is often easier to work with. I use 8 GHz devices for most of
my discrete work: MMBR941's, which Motorola stopped making. Macom supposedly
picked the line up but I can never figure out how to navigate their site so I
can't find the old Motorola parts. However, Philips has a pretty good
replacement in the PBR941.
I must admit I often have problems with parasitic oscillations in emitter
followers when I forget to add the base resistor. The most recent case was a
zero-lead-length CC with a few 0805 components soldered directly to each other
over a ground plane. No base resistor. Sang like a bird!
> Regards, John Woodgate
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