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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Amplifying stage with *negative* Voltage gain??
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
Date: Wed, 04 Dec 2002 09:32:01 -0500
NNTP-Posting-Date: Wed, 04 Dec 2002 09:32:02 EST
Organization: Bell Sympatico
Tony Williams wrote:
> In article ,
> Paul Burridge wrote:
> > There's no load attached at present (see my other post to Kevin A)
> The circuit you described is so simple that at this
> stage I wouldn't be looking for obtuse technical
> reasons..... I'd be looking for a silly perp.
> C-E swapped, C not connected, an unseen load on
> that output 1nF, power supply not decoupled, even
> checking the x1/x10 switches on the scope probes.
> Tony Williams.
Also check for parasitic oscillations. An emitter follower loves to form
a Colpitts oscillator with the stray capacity between the base-emitter
and from emitter to ground. This can resonate with just a short length of
wire on the base and oscillate at a frequency above your scope bandwidth
so you can't see it. The oscillations can start or stop with the added
capacitance of the scope probe, which makes the symptoms difficult to
A quick check is a hand-waving exercise. Wave your hand near the
suspected circuit. If the behavior changes, suspect a parasitic
One cure is to add a 50 to 100 ohm resistor right at the base. This kills
the Q of the tank and stops the oscillation. It also reduces the high
frequency bandwidth of the circuit, but won't have much effect at 3.57
The biggest problem is oscillations that come and go with changes in
temperature, supply voltage, or variation from one device to the next.
Since the oscillations often occur at the most inopportune times, I find
it a good practise to always add parasitic suppression resistors to
emitter followers and any other circuit that has a chance of oscillating.
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