An AM modulator circuit is a mixer circuit wherein modulating signal amplitude varies the carrier signal amplitude to produce modulated AM signal. There are different ways of building AM modulator such as AM modulator design using BJT transistor, AM modulator design using JFET transistor, AM modulator design with single diode, AM modulator design with two diodes, AM modulator design using ring diodes. Here an AM modulator circuit design using two JFET transistor is illustrated.
The circuit diagram of an AM modulator build with two JFET transistor is shown below.
Here the modulating signal enters the two JFET AM modulator via the center tapped transformer TR1. The two transistors Q1 and Q2 are 2N3819 N-channel JFET transistors. The carrier signal turns on and off the two JFET transistors via the two center tapped transformer TR1 and TR2. The output is AM signal with intermodulation products which appears at the transformer TR2. The AM signal along with the harmonic byproducts are passed through the LC resonant band pass filter. The AM signal appears at the load resistor RL.
In the above AM modulator circuit, a modulating signal of amplitude 200mV and frequency of 1KHz and carrier signal of amplitude 800mV and frequency of 10KHz is applied to the JFET mixer.
The following shows animation of signal flow in the two JFET transistor based AM modulator circuit.
The following shows signal waveform on the oscilloscope.
The following shows the spectrum of the resulting AM signal.
As can be seen from the above frequency spectrum of AM signal, the spectra is located at the carrier frequency(10KHz), at the difference 9KHz between the carrier(10KHz) and modulating signal(1KHz) and at the sum 11KHz between the carrier(10KHz) and modulating signal(1KHz) which proves that the output from the circuit is an AM signal.
