High efficiency and circuit linearity cannot coexist, as demanded by Ohm’s Law. Improving efficiency therefore requires finding ways to tolerate circuit nonlinearities (including time varying circuitry) while achieving output signal accuracy. I-V plane analysis pulls together the first-order impacts of transistor technology and multiple PA architectures. Evaluation of basic transistor linearity using the Booth Chart is presented. Selection of signal modulations also has a large impact on achievable PA efficiency. Additional complications exist for operation above 20GHz.