Here it is described how a BJT transistor based AM modulator is designed and how it works. Any non-linear active component like BJT transistor, FET, diode can be used for making a modulator. It is because the output is non-linearly related to the input. Here a general purpose 2N2222 transistor is used.
Circuit Diagram of AM modulator with BJT Transistor
The following shows the circuit diagram of AM modulator based on BJT transistor.
In the above circuit diagram, the BJT transistor 2N222A along with biasing resistors R1,R2,RC and RE forms the BJT modulator. Here the BJT transistor is biased using the voltage divider biasing technique. The values of the biasing resistors were calculated using the BJT amplifier calculator. The modulating signal or the message signal(Vm) is fed into the BJT modulator collector via the coupling capacitor 10uF. The modulating signal is 1kHz and the amplitude is 130mV. The carrier signal(Vc) is fed into the BJT modulator base via the C1 coupling capacitor 10uF. The frequency and amplitude of the carrier signal are 20kHz and 420mV. The C1 coupling capacitor value must be appropriately adjusted according frequency of the carrier signal. The The AM signal appears at the collector which goes into the LC tank circuit that is tuned to the carrier signal frequency of 20kHz. The LC tank is used for filtering and also for oscillating the output AM signal. For 20KHz carrier signal the value of capacitor C is 0.1uF and the value of inductor L is 634uH.
The value of the capacitor and inductor of the LC tank was calculated using the LC Parallel Resonant Circuit Online Calculator as shown below.
The following graph shows the modulating signal, the carrier signal and the AM signal from the BJT modulator.
Video of AM modulator with BJT Transistor Design and Simulation
The following video shows how to design AM modulator with BJT Transistor and simulate the circuit in Proteus software.
In the video, the AM modulator circuit is modified several times to see the effect of the coupling capacitors and the bypass capacitor. First the waveform shows incorrect AM waveform. This was due to the LC tank resonant frequency mismatch. After changing the value of the C and L to match the carrier signal the AM signal was obtained. Then the existing coupling capacitor from the capacitor to the LC tank was removed. Also the by-pass capacitor was removed as this had no effect on the modulator.
