Q.Explain with circuit diagram the working of collector modulated Class-C amplifier.

A collector-modulated Class-C amplifier is a high-level AM circuit in which the modulating signal is added in series with the collector supply of a Class-C RF power amplifier. As the total collector voltage rises and falls with the message, the amplitude of the amplified RF output rises and falls with it, producing AM at high power.

[!ANSWER]

The carrier drives the base of a Class-C stage; the audio, applied through a modulation transformer in series with $V_{CC}$, varies the effective collector supply, so the RF output amplitude follows the modulating signal and the collector tank reconstructs the AM wave.

[!TLDR]

The audio is injected in series with the collector supply of a Class-C RF amplifier, so the RF output amplitude varies with the message - high-level, high-efficiency AM.

Circuit arrangement: A transistor is biased for Class-C operation, meaning it conducts for less than half of each RF cycle. The radio-frequency carrier is applied to its base through a coupling capacitor. A parallel LC tank circuit, tuned to the carrier frequency $f_c$, is connected in the collector, and the output is coupled to the antenna. The modulating (audio) signal is applied to the primary of a modulation transformer whose secondary is connected in series between the dc supply $V_{CC}$ and the tank / collector, through a radio-frequency choke.

Working:

1. With no modulation, the Class-C stage amplifies the carrier; the tank circuit, being tuned to $f_c$, converts the pulses of collector current into a clean sinusoidal RF output of constant amplitude.
2. When the audio signal is applied, the secondary of the modulation transformer adds a varying voltage in series with $V_{CC}$. The effective collector supply becomes $V_{CC}+e_m$, rising above and falling below $V_{CC}$ in step with the message.
3. In a Class-C amplifier the amplitude of the RF output is proportional to the collector supply voltage. Therefore the amplitude of the RF output varies in accordance with the modulating signal.
4. The tuned collector tank, acting as a flywheel, smooths the current pulses and reproduces the full amplitude-modulated wave, which is then radiated by the antenna.

Because the modulation is done at the final high-power stage, this is called high-level modulation. Its main advantage is high efficiency (Class-C operation) and the ability to modulate at high power; its drawback is that the modulating amplifier must itself deliver considerable audio power.

[!ANSWER]

In a collector-modulated Class-C amplifier the Class-C stage amplifies the carrier applied to its base, while the audio signal is inserted in series with the collector supply through a modulation transformer. The RF output amplitude, being proportional to the collector voltage, varies with the modulating signal, and the tuned collector tank reconstructs the amplitude-modulated wave. It is a high-level, high-efficiency method of AM generation.