Frank Pacquiao

user Test Engineer

city ISI


  • RF Modulator

  • Created: Nov 26, 2014

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Summary

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, 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 Vo 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 (Vo). Source resistors R18 and R19 improve the dc stability and low-frequency gain. A phase inverter, based on the dual op amp U1A and U1B, generates the out-of-phase, equal-amplitude, audio-frequency modulation signals.

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