Parametric Amplifier Basics: Advantages and Disadvantages

This page covers Parametric Amplifier basics, Parametric Amplifier advantages and Parametric Amplifier disadvantages. It is the device in which periodic variation of its parameters e.g. capacitance of the varactor diode under the influence of a suitable pump signal.

Due to this, it is known as a parametric amplifier. If a small input signal with frequency $f_g$ and AC power with frequency $f_p$ are applied to the varactor diode, linear amplification of the small input signal occurs.

This happens due to the time-varying capacitance of the varactor diode. Here, the pump signal ($f_p$) provides the power needed for amplification. The power output is either at the input frequency ($f_g$) or at the idler frequency ($f_i = f_p - f_g$).

A set of relations were obtained by Manley-Rowe in order to determine the maximum gain of the parametric amplifier.

The power gain of the signal is expressed as follows:

$Power\,Gain = f_o/f_g = (f_g+f_p)/f_g = 1 + f_p/f_g$

Manley-Rowe relations provide the maximum gain theoretically. But practically, the gain is less than $1 + (f_p/f_g)$ due to losses. This type of parametric amplifier is referred to as an up-converter.

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

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

The figure-part (a) depicts the equivalent circuit of a parametric up-converter. As shown, $f_p$ and $f_g$ are applied through tuned circuits to the varactor diode. Output is taken at frequency $f_i$ equal to $f_p + f_g$.

The series tuned circuits will allow only currents with respective frequencies ($f_g$, $f_p$, and $f_i$) in each of the loops.

Parametric Amplifier Advantages

Following are the advantages of Parametric amplifiers:

• Noise Figure: Because of minimum resistive elements, thermal noise in the parametric amplifier is very less in comparison to a transistor amplifier. Hence, the noise figure is less and will be in the range of 1-2 dB.
• Frequency Range: The upper-frequency limit (about 40 to 200GHz) is set by the difficulty of obtaining a source power at the 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 the circuit.
• Because of its low noise, parametric amplifiers are used in space communication systems, tropo-receivers, and radio telescopes.

Parametric Amplifier Disadvantages

Following are the disadvantages of Parametric amplifiers:

• Bandwidth: The parametric amplifier 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 the pump source and the time-varying capacitance. It is usually in the range of 20 to 80 dB.