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LC Clapp oscillator - Capacitive divider - How to decide on values?


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Given the frequency for a Clapp oscillator and having already designed the amplifier stage, how should I decide on the values of C1 and C2, forming the capacitive divider?

I understand that the value of C3 defines de frequency of resonance if, compared vis-a-vis C1 and C2, it is definitely smaller.

While the ratio between C1 and C2 establish the level of feedback, in this case to the emitter, is it that you are destined to get them defined by trial and error?

Clapp capacitive divider.png


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Rewinding for a minute, back to the Colpitts oscillator, you have seen me post variations of this circuit before:


Basic rules of thumb for the frequency determining components come from the book Solid State Design for the Radio Amateur, Page 34 fig 2,
where is suggests that the reactances are set as follows:

Xcfeedback = 45 Ohm
Xccoupling = 100 Ohm
XL = 140 Ohm

It leaves Cparallel as a bit of a mystery, but if you make the value such that its reactance in parallel with the series chain of Ccoupling + C feedback + Cfeedback is 140 Ohm, it all works out nicely.
(Ouch that is a very bad description!)

Just to add a bit of confusion, the same author (Wes Hayward W7ZOI) in his book Introduction to Radio Frequency Design describes this variation of the Colpitts oscillator with the Coupling capacitor between the parallel tuned circuit and the transistor as a Seiler Oscillator.

At which point we wander back to the subject of this thread, the Clapp oscillator.
Back in the book Solid State Design for the Radio Amateur, Page 34 fig 2, there are rules of thumb for a Clapp oscillator.

Again, component reactances are suggested as follows:

Xcfeedback = 45 Ohm
Xcclapp = 200 Ohm
XLclapp = 260 Ohm

Important note - in the book, the Clapp oscillator was drawn with an FET rather than a BJT, the Clapp circuit as drawn above is untested.

For a more mathematical treatment of Clapp and Colpits oscillators see the book Introduction to Radio Frequency Design, Chapter 7



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Thanks to you both for replying. After revising again the so many "successful" circuits out there, I will start with the rules of thumb to see what the outcome is.


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