8/4/2019 Am Transmitter Working

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YashveerTakoory

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AM TRANSMITTER

All general purpose transistors should work in this circuit, I used three BC549 transistors in

my prototype. The tuned circuit is designed for medium wave, but the circuit will work up to

much higher frequencies if a different tuning coil and capacitor are used. I used a ferrite rod

and tuning capacitor from an old radio which tuned from approximately 550 - 1600 kHz. Q1

and Q2 form a compound transistor pair featuring high gain and very high input impedance.

This is necessary so as not to unduly load the tank circuit. Q1 operates in emitter follower,

Q2 common emitter, self stabilizing bias is via the 120k resistor and the tuning coil. As Q2

operates in common emitter its base voltage will be a Vbe drop higher than ground or about

0.71V in my test sample. The voltage at Q1 base will be this Vbe drop plus the voltage drop

across the 1k resistor and Q1's own Vbedrop, this amounted to 1.34V from base to ground in

my test circuit. For audio amplifiers, Q2 collector would be biased near half supply voltage,

however the input signal levels at RF are tiny, typically 50uV appearing across the coil being

amplified by Q2 and being about 5mV RF across the 2k2 load resistor.

The 120k resistor provides regenerative feedback, between Q2 output and the tank circuit

input and its value affects the overall performance of the whole circuit. Too much feedback

and the circuit will become unstable producing a "howling sound". Insufficient feedback and

8/4/2019 Am Transmitter Working

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8/4/2019 Am Transmitter Working

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Yas veerTakoory

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W t vanes are open a small capacitance is still present(about 40pF) The coil

connections add a sli ht amount of stray capacitance which may be 7 or8pF. With 48pF

capacitance and a 200uH coil the hi hest frequency will be about 1624kHz. Some variable

capacitors, have builtin trimmers to adjustthe hi hest frequency. For any coil and capacitor

thattunes too hi h, a 50pF trimmer may be added in parallel with VC1 to controlthe hi hest

tuneable frequency.