The Transfer Function of Amplitude Modulation Circuits Using Varactor Diode

Amplitude modulation (AM) is widely used for not only wireless communications but also sensing and monitoring biomaterials as well as scientific data. Recently analog multipliers are attracting much attention to mitigate modulation distortion and insufficient dynamic range in the traditional way of AM. However, as the circuit of analog multiplier is complicated and contains many transistors, its use may encounter difficulty in IC implementation. Varactor diode is a capacitor of which capacitance varies according to the change of reverse bias voltage and has advantages of small size (easily integrated), high speed operation, low thermal noise, and low power consumption. Based on this property, varactor diode can work as multiplier so that it can be employed for AM in place of analog multiplier. This brief presents AM circuit using varactor diode of simple structure and shows for the first time a theoretical way to determine its transfer function. Theoretical analysis is validated by using hardware simulation on an example circuit.