Collector AM circuit

The figure above is a collector amplitude modulation circuit. The equal-amplitude carrier generated by the high-frequency carrier oscillator is added to the transistor base through T1. The low-frequency modulation signal is coupled to the collector through T3. C1, C2, and C3 are high-frequency bypass capacitors, and R1 and R2 are bias resistors. The LC parallel circuit of the collector resonates at the carrier frequency. If the static operating point of the transistor is selected in the curved part of the characteristic curve, the transistor is a nonlinear device. Because the collector current of the transistor changes with the modulation voltage, the two signals in the collector are amplitude modulated due to nonlinear effects. Since the LC resonant circuit is tuned to the base frequency of the carrier, an amplitude modulated wave output can be obtained at the secondary of T2.