VHF bipolar transistor power amplifiers: measurement, modeling, and design

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Virginia Polytechnic Institute and State University

Widely used design techniques for radio frequency power amplifiers yield results which are approximate; the initial design is usually refined by applying trial-and-error procedures in the laboratory. More accurate design techniques are complicated in their application and have not gained acceptance by practicing engineers. A new design technique for VHF linear power amplifiers using bipolar junction transistors is presented in this report. This design technique is simple in its application but yields accurate results.

The design technique is based upon a transistor model which is simple enough to be useful for design, but which is sufficiently accurate to predict performance at high frequencies. Additionally, the model yields insight into many of the processes which take place within the typical RF power transistor. The fundamental aspect of the model is the inclusion of charge storage within the transistor base. This charge storage effect gives rise to a nearly sinusoidal collector current waveform, even in a transistor which ostensibly is biased for class B or nonsaturating class C operation.

Methods of predicting transistor input and output impedances are presented. A number of other topics related to power amplifier measurement and design are also included. A unique measurement approach which is ideally suited for use with power amplifiers is discussed. This measurement approach is a hybrid of the common S-parameter measurement technique and the "load-pull" procedure. Practical considerations such as amplifier stability, bias network design, and matching network topology are also included in the report.