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From: Mike Monett
X-Mailer: Mozilla 2.02 (Win16; I)
Subject: Re: Amplifying stage with *negative* Voltage gain??
Date: Thu, 05 Dec 2002 04:34:56 -0500
NNTP-Posting-Date: Thu, 05 Dec 2002 04:34:57 EST
Organization: Bell Sympatico
Paul Burridge wrote:
> I've just put together a single-stage buffer amp for an oscillator
> stage. It's in emitter follower configuration. The input signal is 8v
> p--p; the output, OTOH, is only 2v p-p! The signal frequency is
> 3.57mhz and the active device I'm using for the buffer (a BC337) is
> supposed to be good for 150mhz. So where are all me volts going? I'd
> expect to get 99% of 8v out. The general formula for V gain in C/E
> amps is RE/RE+re so typically comes out at just under 1. The 8v signal
> is present on the base pin; only 2v sig out on the emitter pin.
I just modeled your buffer in SPICE, using a 2N2222 which is close
to a BC337. I assumed 10pF for the scope probe capacitance.
Here's the schematic.
The input impedance is a bit less than 1.5k, mostly due to the
parallel impedance of the 3k bias resistors. You say you are getting
a solid 8V p-p at the input, so I assume the source impedance of the
oscillator is around 150 ohms or less. I picked 150 ohms.
The frequency response shows a 1 dB drop due to the source
impedance. The -3dB bandwidth is over 100MHz, so it can handle
3.57MHz with no problem. I ignored the stray cap at the input.
The 1.08k emitter resistance can easily drive the 10pF scope probe
capacitance. Here's the waveforms.
I hope this give you confidence your circuit should work.
But I have a question. If you can drive the buffer with a solid 8V,
the output impedance of the oscillator must be pretty low.
How are you getting 8V p-p from the oscillator, and why do you need
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