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From: Paul Burridge
Subject: Re: Amplifying stage with *negative* Voltage gain??
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Date: Mon, 09 Dec 2002 11:42:50 +0000
NNTP-Posting-Date: Mon, 09 Dec 2002 13:11:12 GMT
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On Sat, 07 Dec 2002 13:10:38 -0800, Mike Monett opined
>When the probe is set to X1, the resistor and cap are shorted. The probe
>cable and input capacity of the scope now appear at the probe tip. This
>loads the circuit, and is what Tony referred to.
I see. So what's X1 any good for then? Just low frequency audio
>The effect this has on the signal depends on the source impedance and
>In the case of the emitter follower, there are two different impedances.
>On the positive-going signal, the emitter follower is turned on and
>provides a low impedance. On the negative transition, the resistance in
>the emitter may not supply enough current to the load, and the transistor
>turns off. The output signal becomes distorted, and starts looking like
>the output of a half-wave rectifier driving a capacitor.
My output signal does appear to be quite markedly clipped at the
bottom edge, but I can't see how current starvation should be a
problem since I reduced RE to 400 ohms. There's a pretty healthy level
of Ie running now and this config is *supposed* to be very low-z out
and thus capable of driving such a modest load as a scope!
>I simulated this effect using load capacitances from 0pf to 400pf:
>There are 5 different waveforms superimposed. They represent the value of
>the cable and scope capacitance in 100pf steps. Do any of these waveforms
>resemble what you see on the scope?
Yes, flattened off at the bottom.
"I was going to help him up, but then I thumped him."
Michael Head's plea in mitigation to Weybridge Magistrates, Sept. 1975
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