Q.What is the purpose of a buffer amplifier stage in a transmitter?

An AM signal is generated by combining the carrier and the baseband signal in a non-linear device; the circuit that does this is an amplitude modulator. Modulators are classed by where the power amplification happens:

• Low-level modulation takes place before the output element of the final stage, so less modulating power is needed to reach a high percentage of modulation; a class A amplifier is typical.
• High-level modulation takes place in the final element of the final stage, where the carrier is already at full amplitude, so a much larger modulating signal is required; a class C amplifier is used.

The collector modulator is a common high-level circuit: the modulating signal is injected (through a transformer) in series with the collector supply of a class C RF amplifier whose tuned LC tank resonates at the carrier frequency. As the audio changes the effective collector supply, the amplitude of the RF current pulses — and hence the tank's RF output — follows the message, producing the AM wave. It offers good linearity, high efficiency and high power output, but needs a large modulating power. …