From: John Popelish
Organization: This space not available for advertising.
X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: how does this circuit work?
Date: Mon, 28 Oct 2002 05:10:22 GMT
NNTP-Posting-Date: Mon, 28 Oct 2002 00:10:22 EST
David Jones wrote:
> I can't figure out how this oscillator works. I've tried and tried. A
> picture of the schematic is attached to this post. How do the two
> transistors and capacitor work together to oscillate? Could someone please
> explain this step by step? Thanks
Oscillators are always a little hard to get started on, because there
is no input an output, just an echo going around. Lets start with an
assumption of some starting state and see what happens form there. R1
is a pretty high resistance, so lets assume that it brings the base of
Q1 to just slightly forward biased and its collector current brings Q2
also into a modest forward bias. This lets us consider what a tiny
change in voltage anywhere in the circuit does as it propagates
through the rest of the circuit.
If there is a tiny positive excursion on the base of Q1, Q1 conducts
more collector current, pulling down a bit more at the base of Q2.
This causes Q2 to conduct a bit more collector current, pulling up on
the speaker. But C1 couples this positive change back to the base of
Q1, adding to the assumed positive change we started with. This
positive feedback rapidly accelerates, till both Q1 and Q2 are
saturated on, with R3 limiting the total current and the base emitter
junction of the keeping the left side of C1 from ever going more than
.7 volts above the emitter voltage. Once the output voltage stops
rising, there can be no more current driven through C1, so Q1 drops
out of saturation, which eliminates the base drive for Q2.
The voltage on the speaker heads negative and that is also coupled
back to the base of Q1, driving it well into reverse bias. But in
this direction, the base diode does not clamp the capacitor voltage,
so it is recharged back to the threshold of forward bias only by
current through R1. Once both transistors begin to conduct, we are
back where we started.
There is a sharp blast of current followed by a time-out set by the
R1, C1 time constant.