Figure 1.  A simple amplitude modulator using an op-amp

Figure 1 shows a simple amplitude modulator that uses an operational amplifier (op-amp).

The op-amp is configured to amplify the carrier signal that's applied at its non-inverting input.  The gain of the amplification is given by: Gain = 1 + (Rf / Ri), wherein Ri is the resistance exhibited by the FET.  The FET acts as a variable resistor whose source-to-drain resistance depends on the input signal applied at its gate through capacitor C. Note the negative bias (-Vbias) applied at the gate of the FET which is used to keep the gate-source junction reverse-biased.

The input signal to the FET's gate is the modulating signal.  An increase in the input signal will cause a decrease in the FET's resistance, causing the gain of the op-amp to increase. This results in a corresponding increase in the output voltage. On the other hand, a decrease in the input signal will cause an increase in the FET's resistance, causing the gain of the op-amp to decrease. This results in a corresponding decrease in the output voltage.

The FET in this circuit must be properly biased so that its resistance will behave linearly over a wide range of input signal.

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