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for what this capacitor is ? Answered

I can't find 4.7pF ceramic capacitor

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steveastrouk

Best Answer 8 years ago

High frequency bypass capacitor - Its not a critical value, but it needs to be there.

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RendydevaraRendydevara

Answer 8 years ago

but how to calculate its frequency ?, for example L = 0.2 uH C = 20pF (on Collector - emitter) C = 20pF (beside an inductor, if i use a ceramic capacitor)

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hlee tech

7 years ago

you have shown an oscillator .. an oscillator needs gain (the transistor) and it needs positive feedback (the 4.7 pf capacitor) .. the capacitor can be any type .. not only ceramic but mica or other .. you can even use a gimmic .. thats where you tack a small insulated wire to the emitter and one to the collector then twist them together .. the tighter the twist or the more twists the larger the capacitance .. if the capacitor is too large then RF output distortion will be seen .. if its too small then the oscillator may not start reliably .. the frequency of oscillation is primarily dominated by the inductor and associated variable capacitor .. there is a small effect from miller capacitance .. thats capacitance from collector to base of the transistor .. that will lower the frequency slightly from calculated but can usually be neglected ..