Fundamentals of Switching RF Power Amplifiers

How to determine if an amplifier is actually switching, or is just operating in compression, is developed here. While compressed amplifier operation is any deviation from linear operation toward lower output at the highest amplifier output powers, switching has additional requirements that the transistor achieve two steady states within each RF cycle: OFF and ON. Conditions necessary for transistor switching operation to occur at RF signal frequencies are derived, and related to the transistor transition frequency metric fT. The degradation of available amplifier efficiency as the operating frequency approaches fT is evaluated. Testing methods to verify that transistor switching operation is achieved are proposed.