Question 5.18: What does the circuit of Fig. 5-39a do?
What does the circuit of Fig. 5-39a do?
A transistor is a semiconductor device that acts like a current source. In Fig. 5-39a, the transistor pumps a fixed number of milliamperes into the resonant LC tank circuit. A negative dc voltage reverse-biases the varactor. By varying this dc control voltage, we can vary the resonant frequency of the LC circuit.
As far as the ac signal is concerned, we can use the equivalent circuit shown in Fig. 5-39b. The coupling capacitor acts like a short circuit. An ac current source drives a resonant LC tank circuit. The varactor acts like variable capacitance, which means that we can change the resonant frequency by changing the dc control voltage. This is the basic idea behind the tuning of some radio receivers.