Final stage class 'B' modulation

A transmitter using this system comprises a master oscillator followed by a chain of h.f. amplifiers terminating in a high-efficiency plate-modulated amplifier, and a chain of l.f. amplifiers terminating in a Class B push-pull power amplifier. Equivalent circuits of the output transformers are given, and they are shown to constitute low-pass filters having cut-off frequencies well beyond the highest to be transmitted. The l.f. amplifier fidelity depends much on the regulation of the penultimate stage. Amplitude distortion considerations require this stage to be Class A push-pull. Calculations and curves are given for harmonics introduced (for single-tone modulation) and intermodulation components (for two-tone modulation). In its application to short-wave transmitters, the Class B system is advantageous in providing wide flexibility in the h.f. adjustments, facilitating rapid wave change.