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Although the use of parametric amplifiers for high frequency, low noise amplification is relatively recent, the principles themselves are not at all new. They were first propounded by Lord Reyleigh in the 1880s for Mechanical Systems and by R. Hartley in 1930s for electrical applications. The need for extremely low noise amplification , as in radio telescopes, space probe tracking and communications, and tropospheric scatter receivers, spurred the design of useful parametric amplifiers in the late 1950s. The low noise properties of such amplifiers and the advent of suitable VARACTOR diodes were the greatest stimuli.
BASIC PRINCIPLES:-
The parametric amplifier uses a device whose reactance is varied in such a manner that amplification results . It is a low noise apparatus because no resistance need to be involved in the amplifying process . A VARACTOR diode is now always used as the active element . Amplification is obtained when the reactance (capacitive here) is varied electronically in some pre determined fashion at some frequency higher than the frequency of signal being amplified. The name of the amplifier stems from the fact that Capacitance is a parameter of tuned circuit.
FUNDAMENTALS:-
To understand the operation of one of the forms of the parametric amplifier, consider an LC Circuit oscillating at its natural frequency . If the capacitor plates are physically pulled apart at the instant of time when the voltage between them is at its positive maximum, the work is done on the capacitor since a force must be applied to separate the plates. This work, or energy addition, appears as an increase in voltage across the capacitor. Since,
V= (q/C) ,
And the charge ‘q’ remains constant, voltage is inversely proportional to capacitance. Since the capacitance has been reduced by pulling apart of the plates, voltage across them has just increased proportionately. The plates are now returned to their initial separation just as the voltage between them passes through zero, which involves no work . As the voltage passes through the negative maximum, the plates are pushed apart, and voltage increases once again. The process is repeated regularly, so that energy is taken from the “pump” source and added to the signal, at signal frequency; amplification will take place if an input circuit and load are connected. In practice, the capacitance is varied electronically (as could be the inductance). Thus, the reactance variation can be made at a much faster rate than by mechanical means, and it is also sinusoidal rather than a square wave.
Comparing the principles of the parametric amplifiers with those of more conventional amplifiers, we see that the basic difference lies in the use of variable reactance ( and an AC power supply) by the former, and a variable resistance (and a DC power source) by the latter. As an example , in an ordinary transistor amplifier, changes in base current causes changes in collector current when the collector supply voltage is constant; it may be said that the collector resistance is being changed.
The basic amplifier just described requires a capacitance variation to occur at a pump frequency that is exactly twice the resonant frequency of the tuned circuit, and hence, twice the signal frequency . It is thus , phase sensitive ; this is property that sometimes limits its usefulness. This mode of operation is called degenerate mode and may also be shown that the amplifier is a negative resistance one.
AMPLIFICATION MECHANISM:-
The introduction laid down the parametric amplifiers and fig (i) illustrates the process graphically. It will be seen that (as outlined), the voltage across the capacitor is increased by pumping at its signal voltage peak. Furthermore, the energy, thus, given to the circuit is not removed when the plates are restored to their initial position ( i.e. when the capacitance of diode is restored to its initial value) because this is done when the voltage across the capacitor is zero.
The process of signal build up is shown in fig(i) c. Note that it requires more energy in each successive step to increase the voltage across the capacitance, because the peak charge is greater each time.The capacitor voltage would tend to increase indefinitely, except that the driving power is finite. Thus, in practice the build up process until the energy added at each peak equals the maximum energy available for the pump source.