The evolution of the push-pull RFPA

The evolution of the push-pull power amplifier is summarized. The technique would appear to have originated about 100 years ago, and quickly became the default circuit configuration for early vacuum tube audio amplifiers. Its use has also been widely adopted for implementing power amplifiers at radio frequencies (“RF”), but the key requirement for near-ideal magnetically coupled transformers has halted the progress of the push-pull amplifier into the GHz bands. When transformers are replaced by balun structures, based on the properties of transmission lines, the assumed low frequency advantages of push-pull operation can quickly be lost. In particular, little attention has been paid to the response of balun structures at harmonic frequencies.