Varactor Diode Symbol:

A specially processed junction diode that assumes the properties of a variable capacitor (about 5 – 250 pf) when its dc reverse voltage is varied is called a Varactor Diode shown in Fig. 14.37.

This unique behavior can be used in special modulator circuits designed to turn a low voltage dc signal input into an ac output of proportional magnitude. A representative circuit is shown in Fig. 14.38.

The modulator is essentially a ‘pumped’ parametric amplifier which can provide power gain. Assuming that the voltage variable capacitor (diodes) have the form C₁ = C_o + Kf(ν),

where

• C_o = leakage capacitance,
• K = constant, and
• f(ν) = variable voltage function

The magnitude of the output voltage at R_L can be shown to be proportional to the product of the magnitude of the carrier signal and the dc input signal supplied ahead of R_g. The system’s pump sources (carrier insertion) are provided by V₁ and V₂. These modulating units are usually composed in bridge configuration and the drift voltage of the modulator is limited by the precise balance of the two varicaps (varactor diode).