Radio Frequency Modulator Circuit

This is a circuit design diagram of Radio Frequency (RF) Modulator. Due to the power MOSFETs offer high power gain at both audio and radio frequencies, they are will be also very useful in many areas of radio-circuit design. For BF applications, a MOSFET's large safe operating area, VDS vs ID, 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 T1, whose secondaries provide equal-amplitude, opposite-phase rf drive signals to output FETs Q1 and Q2.



Output V0 is zero when no audio-frequency signals are present, because the opposite-phase RF signals from Q1 and Q2 cancel. When audio-frequency signals appear at nodes A and B, you obtain a modulated RF output (V0). Source resistors R18 and R19 improve the dc stability and low-frequency gain. A phase inverter, based on the dual op amp IC1A and IC1B, generates the out-of-phase, equal-amplitude, audio-frequency modulation signals.