** 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._____

Answer

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.

Questions

A. What is the effective total capacitance of the two series capacitors in the tuned circuit?

C_{T} = _____

B. What is the oscillator frequency?

f_{r} = _____

C. What is the impedance of the tuned circuit at this frequency?

Z = _____

D. What fraction of the output voltage is fed back?

V_{f} = _____

E. What ...

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