Basic Low Power AM transmitter Circuit

Published:2013/11/12 19:59:00 Author:lynne | Keyword: Basic Low Power AM transmitter | From:SeekIC

This transmitteris basic but allows transmission of audio to an AM radio. It consists of an RFoscillator operating in the AM broadcast band, together with a modulator stage,which mixes the incoming audio and the RF. A signal appears on the output, whichhas an AM component that can be picked up on a nearby AM radio receiver. The transmitter consists ofoscillator stage Q1 and modulator/buffer stage Q2. Q1 is biased via R1, R2, and R3.L1, C3, and C4 form the tank circuit with feedback network C3-C4 providing feedbackto the emitter of Q1. RF voltage at the junction of C3 and L1 drives buffer/modulator stage Q2. Q2 is biased by base current produced by RF rectification in thebase emitter junction of Q2. C6 is an RF and AF bypass capacitor. C9, C10, and L2form the tank circuit for the collector of Q2. RF is taken from the junction of C9 andC10 and fed to ashort-wireantenna. Audio is fed to modulator Q2 via C8 and isolationresistor R5 and mixes with the RF signal in the collector circuit of Q2, producinga signal that has sum and difference frequencies if the RF carrier and AF input along with the carrier signal. An AM signal appears atthe collector of Q2. Audio with an RMS voltage equal to about 0.7 times the collectorvoltage of Q2 is needed for full modulation of the output.Because of the high level of audio needed, the modulation obtained from this circuitis somewhat limited with conventional audio sources because several volts ofaudio into a few hundred ohms is needed. The circuit demonstrates the principle ofan AM transmitter, however, and with a suitable audio drive level, produces a well modulatedAM signal.



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