Connection of a varactor diode in a tuned circuit.
When a diode is reverse-biased, there is a region at the P-N junction with dielectric (insulating) properties. As you know from previous sections, this is called the depletion region, because it has a shortage of majority charge carriers. The width of this zone depends on several things, including the reverse-bias voltage.
As long as the reverse bias is less than the avalanche voltage, varying the bias affects the width of the depletion region. This in turn varies the junction capacitance. This capacitance, which is always small (on the order of picofarads), varies inversely with the square root of the reverse-bias voltage, as long as the reverse bias remains less than the avalanche voltage. Thus, for example, if the reverse-bias voltage is quadrupled, the junction capacitance drops to one-half; if the reverse-bias voltage is decreased by a factor of 9, then the junction capacitance increases by a factor of 3.
Some diodes are manufactured especially for use as variable capacitors. Such a device is known as varactor diode, as you learned in previous sections. Varactors are used in a special type of circuit called a voltage-controlled oscillator (VCO). Above figure is a simple example of the LC circuit in a VCO, using a coil, a fixed capacitor, and a varactor. This is a parallel-tuned circuit. The fixed capacitor, whose value is large compared with that of the varactor, serves to keep the coil from short-circuiting the control voltage across the varactor. Notice that the symbol for the varactor has two lines on the cathode side.