Analysis of SSB Power Amplifiers

It is well known that one of the critical items in a single sideband (SSB) transmitter is the power amplifier with which are associated the basic problems of distortion, output power and plate dissipation. Tubes are available which may be used either as quasi-linear RF amplifiers operated class AB or B or as gridmodulated amplifiers operated class C. In the class-AB case, the SSB signal is introduced directly on the grid in modulated form. In the class-C case, the low-frequency envelope and high-frequency phase-modulated carrier of the SSB signal are initially separated and subsequently recombined through remodulation at the grid of the power tube. The performance of the power amplifier in these two modes of operation is compared theoretically for idealized tube characteristics. The analysis assumes in particular a quasi-linear plate current-grid voltage characteristic and is based on a twotone SSB wave. The results of the analysis are the following: 1) Class-B operation yields more average output ac power for a given plate supply voltage or peak envelope RF power and less for a given average plate dissipation. Efficiency and power tables are given. Typical normalized figures for average ac power output and average plate dissipation for two-tone SSB are 0.39 and 0.24 for class B and 0.26 and 0.11 for gridmodulated class C if plate voltage is fixed. The class-C figures change to 0.32 and 0.14 for equal peak envelope powers and to 0.57 and 0.24 for equal amounts of average plate dissipation. 2) Class-B operation is distortionless for two-tone SSB because of the idealized quasi-linear plate current-grid voltage characteristic. 3) The maximum distortion in class-AB operation is slightly less than that in grid-modulated class-C operation for twotone SSB. Distortion tables are provided for the class-AB and class-C cases which are applicable when these two cases correspond to equal peak envelope powers. Typical spurious tones in the two-tone SSB output have maximum voltage amplitudes of around 0.08 for class-AB operation and 0.11 for grid-modulated class-C operation when normalized with respect to the output voltage amplitude of either equal tone.