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Capacitance divider of Colpitts Oscillator

promach

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For the following Colpitts Oscillator, Why does employing the capacitance divider up-converts the source impedance of transistor by a factor of (1+C2/C1)2 ?

 

BradtheRad

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A capacitor by itself creates current lead (90 degrees). By adding a second series capacitor, and tapping between them, these effects result:

* modifies the amount of phase change (possibly cancelling it if C values are equal)

* changes reactive impedance

* changes resonant frequency

* changes frequency response

* changes filter rolloff curve

* can be adjusted to provide feedback signal at correct time so as to sustain oscillations

Etc.

The same principle applies to inductors.
I'm not certain this answers the initial question but it might lead to an answer.
 

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