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Robert C Monsen
- Jan 1, 1970
- 0
fred said:I agree, also in Colpitts, the center tap of the two capacitors
is connected to ground whereas here it's connected to the
emitter (which is not bypassed to ground).
Kevin pointed out that colpitts is really just a 'mode of oscillation', as
opposed to any particular configuration that produces it. I'll buy that. So
Kevin's assertion that this is a 'colpitts oscillator' is probably true,
although its unlike any of the colpitts oscillators I've seen described.
I agree, this is a common base. I don't think all the capacitors
are in parallel though, C4 and C5 are in parallel? unless you are
ignoring R7, are you ?
Actually, I believe that the capacitance is Ct = C4 + C5||Cbe + Cbc. The
capacitance out of the junction is the important thing, I think, at least in
this application. Whether it goes to the transistor's base, or to Vcc (both
of which's voltages are stationary in relation to the other due to C3)
doesn't really matter all that much, at least as far as how the junction
sees the capacitors. Both have a nearly constant voltage in this system in
relation to ground.
I want to see Kevin's response to my question, since he clearly knows alot
more about the internal capacitances of a transistor than I do, due to his
work on Spice. With that additional help, we may be able to predict where
the modulation will go based on changes to the base voltage, which would be
useful. Without that, one can't tell how wide the band of the transmitter
is, given an input signal specification. However, since the oscillation is
based on fairly unpredictable properties of the transistor, it may not be
possible to predict this in any useful way. I hope Kevin has the answer to
this.
One useful thing I've discovered with this analysis is that if you stiffen
up the voltage at Q2's base by adding a 0.04uF cap to ground, you can get
alot more power into the antenna. The new cap value is designed so that the
reactance is 1/4 the reactance of C3. Biasing the transistor properly helps
as well; changing R6 to 47k, and adding a 10k resistor to ground helps.
I also think that moving the antenna to the emitter of Q2 will allow it to
produce quite a bit more power, because a common base amplifier won't
produce much power at the collector. I don't know how to quantify this
effect, though.
I've learned alot about common base amplifiers with this exercise that I
didn't know before. One interesting this is that I was able to simulate Paul
Burridge's 40.5Mhz to 41Mhz VCO with this circuit pretty easily. By moving
around the base voltage, one gets to mess with the output frequency. Its
not, unfortunately, a linear effect (which also probably means that the
output of the transmitter will be distorted unless this is compensated.
Another reason for knowing the exact relation between Cbe and Vb.)
Regards,
Bob Monsen