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Author Topic: Resonance impedance matching Colpitts oscillator  (Read 1210 times)
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dotm
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« on: February 05, 2013, 05:09:48 17:09 »

Hi.
This is not about any particular project, i just need to understand that.
Given a colpitts oscillator in emitter-configuration like:
http://www.electronics-tutorials.ws/oscillator/osc13.gif
You clearly see that the collector is AC-coupled by that choke, so the Inductor gets a zeropointsymmetrical Signal, thus the phase shift in C2 will occur.

Now imagine that oscillator as quadrupole view, having an amplifier-quadrupole without any particular specifications and the resonance circuit as feedback-quadrupole.

So now in resonant condition, what is the input impedance of the feedback-quadrupole? Is it just the (pure real) resonance resistance  of the tank? Or is it transformed by the capacitive divider somehow? I see that the resonance resistance is transformed towards the output impedance to match the transconductance Gm.

Lets say you'd have an output impedance of your amplifier quadrupole Z1 which will of course be way smaller than the the resonant resistance, wouldn't it be useful to add a matching network so that the input impedance of the feedback-quadrupole Z1'  matches  Z1  ?

I hope i could express myself clearly with my clumsy english...
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