An RF input is applied to the primary of Tl, which applies equal amplitude, opposite phase RF drive for output FETs Ql and Q2. With no AF modulation at points A and B, the opposite phase RF signals cancel each other and no output appears at the 50 V output connector. When AF modulation is applied to points A and B, a modulated RF output is obtained. The dc stability and low frequency gain are improved by source resistors R18 and R19.
A phase inverter consisting of a dual op amp (Ula and Ulb) produces the out-of-phase, equal amplitude AF modulation signals.