Applications of varactor diodes

A varactor diode is an electronically variable capacitor. In other words, the capacitance exhibited by a varactor diode is a function of the reverse bias voltage. This phenomenon leads to several common applications of varactor diodes in situations where capacitance needs to be considered. Figure 1 shows a typical varactor diode-tuned LC oscillation circuit. The circuit coupling inductor L2 serves to input the RF signal into the oscillation circuit when the oscillator is used as an RF amplifier. The main LC oscillation circuit includes the main inductor L1 and the series capacitance of C1 and Cr1. In addition, the stray capacitance Cs, which is widely present in electronic circuits, should also be considered. The functions of the DC-blocking capacitor and series resistor have been introduced previously. The role of capacitor C2 is to filter the tuning voltage Vin.  diode
Figure: Varactor diode tuning circuit  
Since the resonant frequency of an LC-tuned oscillation circuit is a function of LC, we find that the ratio of the maximum to minimum resonant frequencies of the oscillation circuit varies with the square root of the capacitance ratio. Here, the capacitance ratio refers to the ratio of the capacitance at the minimum reverse bias voltage to the capacitance at the maximum reverse bias voltage. Therefore, the tuning characteristic curve of the circuit (bias voltage vs. resonant frequency) is basically a parabola.diode