How does a Colpitts oscillator work?
A Colpitts oscillator is the electrical dual of a Hartley
oscillator. In the Colpitts circuit, two capacitors and one
inductor determine the frequency of oscillation. The feedback
needed for oscillation is taken from a voltage divider made by the
two capacitors, where in the Hartley circuit the feedback is taken
from a voltage divider made by two inductors (or a tapped single
inductor). (Note: the capacitor can be a variable device by using a
varactor). Oscillation frequency The ideal frequency of oscillation
for the circuit is given by the equation: where the series
combination of C1 and C2 creates the effective capacitance of the
LC tank. Real circuits will oscillate at a slightly lower frequency
due to junction capacitances of the transistor and possibly other
stray capacitances