2-46
Figure 2-42C.—Pn-junction diode as a variable capacitor.
The variable capacitance property of the varactor allows it to be used in circuit applications, such as
amplifiers, that produce much lower internal noise levels than circuits that depend upon resistance
properties. Since noise is of primary concern in receivers, circuits using varactors are an important
development in the field of low-noise amplification. The most significant use of varactors to date has
been as the basic component in parametric amplifiers.
PARAMETRIC AMPLIFIERS.—The parametric amplifier is named for the time-varying
parameter, or value of capacitance, associated with the operation. Since the underlying principle of
operation is based on reactance, the parametric amplifier is sometimes called a REACTANCE
AMPLIFIER.
The conventional amplifier is essentially a variable resistance that uses energy from a dc source to
increase ac energy. The parametric amplifier uses a nonlinear variable reactance to supply energy from an
ac source to a load. Since reactance does not add thermal noise to a circuit, parametric amplifiers produce
much less noise than most conventional amplifiers.
Because the most important feature of the parametric amplifier is the low-noise characteristic, the
nature of ELECTRONIC NOISE and the effect of this type of noise on receiver operation must first be
discussed. Electronic noise is the primary limitation on receiver sensitivity and is the name given to very
small randomly fluctuating voltages that are always present in electronic circuits. The sensitivity limit of
the receiver is reached when the incoming signal falls below the level of the noise generated by the
receiver circuits. At this point the incoming signal is hidden by the noise, and further amplification has no
effect because the noise is amplified at the same rate as the signal. The effects of noise can be reduced by
careful circuit design and control of operating conditions, but it cannot be entirely eliminated. Therefore,
circuits such as the parametric amplifier are important developments in the fields of communication and
radar.
The basic theory of parametric amplification centers around a capacitance that varies with time.
Consider the simple series circuit shown in figure 2-43. When the switch is closed, the capacitor charges
to value (Q). If the switch is opened, the isolated capacitor has a voltage across the plates determined by
the charge Q divided by the capacitance C.
