In parametric amplifier varactor diode is used. Capacitance parameter of the diode is varied periodically using a suitable pump signal so the name of the amplifier is parametric amplifier.

As shown in the following figure the signal frequency fs which is required to be amplified and the pump frequency fp used to vary capacitive parameter of the varactor are mixed in the nonlinear capacitor. Accordingly, a voltage of the fundamental frequency fs and fp as well as sum and the difference frequencies mfp ± nfs appear across C. Output circuit which is called idler circuit when loaded with a resistive load an output voltage can be generated across the load at the output frequency fo.

$$fo = mfp ± nfs$$

where m and n are integers from zero to infinity.

If fo > fs the device is called a parametric up-converter and for fo˂fs it is called down-converter.

Gain of the circuit is given as

$Gain= \dfrac{fp+fs}{fs}=\dfrac{fo}{fs} \ \ \ \ \ \ \ \ \ \ \ \text{for up-converter} \\ Gain= \dfrac{fo}{fs+fp} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{for down-converter}$

Application: This amplifier is used in the application of low noise amplifier.