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## Parametric Amplifier basics

It is the device in which periodic variation of the it's parameters e.g. capacitance of the varactor diode under the influence of suitable pump signal. Due to this. it is known as parametric amplifier.

If a small input signal with frequency fg and AC power with frequency fp are applied to varoctor diode, linear amplification of small input signal occurs. This happens due to time varying capacitance of the varactor diode.

Here pump signal (fp) provides power needed for amplification. The power output is either at the input frequency (fg) or at the idler frequency (fi=fp-fg). A set of relations are obtained by Manley-Rowe in order to determine maximum gain of parametric amplifier. The power gain of signal is expressed as follows:

Power Gain = fo/fg = (fg+fp)/fg = 1 + fp/fg

Manley Rowe relations provides maximum gain theoretically. But practically, gain is less than 1+(fp/fg) due to losses. This type of parametric amplifier is referred as up converter.

When the output frequency is equal to fg-fp, the parametric device is referred as down converter. In this condition it provides loss instead of gain.

The figure-part (b) depicts equivalent circuit of parametric amplifier. The varactor diode represents nagative resistance at signal frequency to provide amplification of the input signal.

The figure-part (a) depicts equivalent circuit of parametric up converter. As shown fp and fg are applied through tuned circuits to the varactor diode. Output is taken at frequency fi equal to fp + fg. The series tuned circuits will allow only currents with respective frequencies (fg,fp and fi) in each of the loops.

Following are advantages of Parametric amplifier:

•  Noise Figure: Because of minimum resistive elements, thermal noise in parametric amplifier is very less in comparison to transistor amplifier. Hence noise figure is less and will be in the range 1-2 dB.

•  Frequency Range: The upper freqency limit (about 40 to 200GHz) is set by the difficulty of obtaining a source power at pump frequency and also by the frequency at which the varactor capacitance can be pumped. The lower frequency limit is set by the cut-off frequency of the microwave components used in circuit

• Bacause of its low noise, parametric smplifiers are used in space communications systems, tropo-receivers and radio telescopes.

Following are disadvantages of Parametric amplifier:

• Bandwidth: Parametric amplyfier bandwidth is small due to the presence of tuned circuits. Bandwidth can be increased by stagger tuning.

• Gain: It is limited by the stabilities of pump source and the time varying capacitance. It is usually in the rane of 20 to 80 dB.