Principle of Transistor as Frequency Modulator

Audio signals can be used to achieve frequency modulation in a simple manner by connecting a capacitor microphone to a tuned oscillating circuit. However, for practical purposes, a better level of linearity and control over the center frequency is required. One solution to this is by using a varactor diode as a modulator, with two diodes connected across the frequency-determining circuit. The speech signal is applied to one diode and a control signal is applied to the other to maintain the center frequency stability. This control signal is obtained by comparing the frequency-modulated output with a stable frequency source, serving as a measure of frequency error. 

A transistor can also be used as a frequency modulator, as its junction capacitances can be varied with the modulating signal. A simple circuit diagram of using transistor as frequency modulator is shown below.

circuit diagram of transistor based frequency modulator

Feedback from the collector to the base of the common-emitter stage is applied through R1 and C1 path, with C1 being small to ensure its reactance at the operating frequency is much larger than R1. This leads to the current, ifb, in R1 and C1 path, lagging the collector alternating signal voltage vc by 90 degrees. The signal voltage generated across R1 by ifb is given by vcjwC1R1 and is applied between the base and emitter, producing the collector current ic. This results in a collector impedance given by 1/gmjwC1R1 which is a capacitive reactance corresponding to a capacitance of gmR1C1. 

By altering the base-emitter voltage, the capacitance can be varied, enabling frequency modulation by applying the audio signal between the base and emitter and connecting the collector and emitter to an oscillating tuned circuit.

Reversing the positions of R1 and C1 with C1 having a small reactance compared to R1, results in the collector circuit having an effective inductive reactance corresponding to an inductance value of R1C1/gm, which is also controllable by altering gm. These circuits are known as reactance modulators and are used in FM transmitters. To achieve the necessary level of linearity in modulation, two such circuits are used in a balanced push-pull configuration, with negative feedback applied from the output of a high-grade demodulator fed from the transmitter's output. A second feedback loop is used to stabilize the center frequency.

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