Design and Study of Voltage-Dependent Capacitor Using BJT and Its Application as A Reactance Modulator

Amplitude and frequency modulation, along with the design of oscillators, and tank circuits, are fundamental components of communication systems. A critical aspect of frequency modulation is a comprehensive understanding of reactance modulators and their operational limitations. In this study, we experimentally investigated the performance of a reactance modulator using a Bipolar Junction Transistor (BJT, 2N3904). We analyse the variation of transconductance as a function of the base-emitter voltage and demonstrate the linearity of the equivalent capacitance across different input base-emitter voltages. Base-emitter voltages varied from 0.5 volts to 0.75 volts, and the corresponding equivalent capacitance of the reactance modulator was observed to be 0.35 pF to 4273.34 pF, respectively. Frequency modulation with a 220 kHz carrier signal was studied using a Hartley oscillator. The experimental results showcase the effectiveness of employing a BJT as a reactance modulator for achieving frequency modulation, highlighting its potential applications in communication systems.