Technical Feature: First-Time-Right Design Of RF/Microwave Class A Power Amplifiers Using Only S-Parameters
By Ivan Boshnakov
Senior Principal Engineer
Aerial Facilities Limited
The sequel to "Tandem RF software programs streamline the design of power amplifiers"
This article describes and discusses a procedure of how to design RF/microwave Class A power amplifiers in a very efficient and highly accurate manner when the only initial data available are the S-parameters of the transistors. As in the prequel, two software programs are used in conjunction and interaction: a specialized RF/microwave amplifier design software tool and a general-purpose simulator (nodal analysis program). This time the simulator is not used for its nonlinear analysis capabilities but mostly for its integrated layout EM simulation capabilities.
The Power Parameters Design Method
S-parameters can be used to design Class A amplifiers for optimum gain and input/output return loss at the biasing point at which the transistor S-parameters have been measured. If noise parameters are available and they are combined with the S-parameters, it also becomes possible to design for optimum noise figure (NF) and the associated available power gain (Ganopt). The S-parameters by themselves do not allow for controlling the output power obtained from each stage of the amplifier to be designed. The power of interest in a Class A amplifier is usually the maximum linear output power which is universally accepted to be the power at the 1dB compression point of the gain, that is, P1dB. As with the noise parameters, which are needed to control the noise performance of an amplifier, some kinds of power parameters are needed to design for P1dB.
One method of designing and analyzing for P1dB is to use non-linear transistor models and non-linear (harmonic-balanced) simulators. The biggest problem here is that non-linear models are often not readily available. The manufactures of transistors rarely provide them, and the equipment and software that can be used to extract the non-linear models are very expensive and few companies can afford them. The same applies to the method of using tuners to extract the optimum input and output impedances, or the load-pull constant power contours of the transistors. Of course these methods are unavoidable when very heavy non-linear modes of operation are used and information for the signal distortion is needed.
Cripps, in his usual manner of defying the "conventional wisdom," introduced and developed a simple approach for estimating and designing for the maximum output power of mildly non-linear (Class A) power stages. In this approach the transistor is approximated by a very simple equivalent model consisting of the intrinsic voltage controlled current source (generator) and the parasitic output parallel capacitor and series inductor. The weakly non-linear effects are ignored and the transconductance is considered to be linear until the voltage across it and/or the current supplied by it clips strongly when voltage pinchoff and/or current saturation is reached. Under these assumptions, Cripps developed linear mathematical expressions, which tie together the load-line and the voltage and current limits across the intrinsic generator with the external load and the output power delivered to this load. He showed how to present the relation between the intrinsic load-line and the external impedance on a Smith Chart as constant output power (load-pull) contours.
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