A simple AM transmitter of this type is illustrated in Figure 3-5. The transmitter consists of oscillator 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 feedback to 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 the base emitter junction of Q2. C6 is an RF and AF bypass capacitor. C9, C10, and L2 form the tank circuit for the collector of Q2. RF is taken from the junction of C9 and C10 and fed to a shortwire antenna. Audio is fed to modulator Q2 via C8 and isolation resistor R5 and mixes with the RF signal in the collector circuit of Q2, producing a signal that has sum and difference frequencies if the RF carrier and AF input (upper and lower sidebands) along with the carrier signal. An AM signal appears at the collector of Q2. Audio with an RMS voltage equal to about 0.7 times the collector voltage of Q2 is needed for full modulation of the output.Because of the high level of audio needed, the modulation obtained from this circuit is somewhat limited with conventional audio sources because several volts of audio into a few hundred ohms is needed. The circuit demonstrates the principle of an AM transmitter, however, and with a suitable audio drive level, produces a wellmodulated AM signal.
L1 and L2 are chosen to resonate with the circuit capacitances of about 150 and 165 pf, respectively, and in the test circuit, inductances of 400–500 microhenries were used to obtain the desired 600-kHz output frequency, but this circuit can be operated anywhere in the AM band with suitable inductors. For operation at 1500–1700 kHz, for example, adjustable inductors having a range of 50–100 microhenries would be suitable.