Because power MOSFETs offer high power gain at both audio and radio frequencies, they are useful in many areas of radio-circuit design. For rf applications, a MOSFET`s large safe operating area, V0 s vs ! 0, protects it against damage from reflected rf energy. As a modulator, a MOSFET`s transfer linearity aids fidelity. In the suppressed-carrier modulator, an rf signal is applied to the primary of transformer Tl, whose secondaries provide equal-amplitude, opposite-phase rf drive signals to output FETs Ql and Q2.
Output V. is zero when no audio-frequency signals are present, because the opposite-phase rf signals from Ql and Q2 cancel. When audio-frequency signals appear at nodes A and B, you obtain a modulated rf output (I";,). Source resistors Rl8 and Rl9 improve the de stability and low-frequency gain. A phase inverter, based on the dual op amp IClA and IClB, generates the out-of-phase, equal-amplitude, audio-frequency modulation signals.