Reactance modulator can be designed with FET or BJT devices. They are used in the production of FM signal. A reactance modulator consist of two parts- reactance device(capacitive or inductive) and oscillator circuit. These two are connected together. The input modulating signal is connected to the reactance device. The modulating input signal changes the transconductance of the reactance device which in turn changes its reactance which can be either inductive or capacitive. Because this inductive or capacitive reactance is connected to an oscillator circuit, the frequency of oscillation changes with varying inductive or capacitive reactance.
Following are different ways in which reactance modulator can be constructed.
1. Reactance modulator with BJT based reactance device with BJT based oscillator
BJT reactance modulator alternative is shown below.
Alternative input connection to BJT reactance modulator:
2. Reactance modulator with FET based reactance device with BJT based oscillator
