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Abstract
Loadline theory is described as applied to high power microwave amplifiers. An example design with simulation is presented.

Introduction
The design of microwave GaAs high power amplifiers (HPAs) to demanding specifications is quite difficult. Excellent high yielding design is typically the product of an experienced designer who made use of every tool in his kit to patch together the design. Even with application of the best available simulation tools, the final design is usually the result of several carefully structured design iterations each building empirically from the proceeding. This said, it is easy to lose track of the fact that most of the power characteristics of a GaAs MESFET amplifier are elementary consequences of voltage and current clipping, not esoteric non-linear device behavior.

Power amplifier design based upon voltage and current waveform limits is called loadline design. Loadline design has a couple of key advantages as compared to non-linear simulation. First, since loadline simulation is much faster than non-linear simulation, it is better suited to the initial design phase of topology selection and first optimization. For the same reason, we use loadline simulation for circuit thermal simulation and manufacturing tolerance analysis. Second, although loadline design is approximate, it is never far off. If loadline simulation shows a problem, there is a problem.

In this paper we will present loadline from the point of view of a working level GaAs MESFET microwave power MMIC designer. The ideas are equally applicable to power design using any device which has reasonably sharp, non-destructive current limiting operating at fixed bias voltage.

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•Technical Article: Application Of Loadline Simulation To Microwave High Power Amplifiers