# The Colpitts Oscillator

** 20 ** Figure 9.15 shows a Colpitts oscillator circuit, the simplest of the LC oscillators to build.

The feedback signal is taken from the capacitive voltage divider and fed to the emitter. This connection provides a feedback signal to the emitter in the phase required to provide positive feedback.

In this circuit, the reactance of capacitor C_{B} is low enough for the AC signal to pass through it, rather than passing through R_{2}. Capacitor C_{B} should have a reactance, X_{CB}, of less than 160 ohms at the oscillation frequency. If R_{2} happens to be smaller than 1.6 kΩ, choose a value of X_{CB} that is less than one-tenth of R2.

Question

For the circuit shown in Figure 9.15, what is your first estimate for C_{B}? Assume that f_{r} is equal to 1 kHz, and that X_{c} equals 160 ohms._____

Therefore, C_{B} = 1 μF; larger values of C_{B} also work.

** 21 ** Use the Colpitts oscillator component values shown in Figure 9.15 to answer the following questions.

_{T}= _____

_{r}= _____

_{f}= _____

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