1-31.
NOISE REDUCTION
a. Parametric amplifiers are considered members of a large family of amplifiers known as reactance
amplifiers. A resistor, even at room temperature, has a noise voltage developed across it. Any device that
dissipates energy acts like a resistor and has a corresponding noise voltage. An ordinary electron-tube amplifier
contains many dissipative components, and the electron tube itself presents an equivalent resistance to the circuit.
These dissipative components all contribute noise to the amplifier. The amplification of a parametric amplifier,
however, depends on the use of reactive components. A purely reactive component (capacitor or inductor) does
not dissipate energy. Consequently, a parametric amplifier does not have the noise-producing, dissipative
components of an electron-tube amplifier.
b. Since no component is purely reactive, some noise will be generated in a parametric amplifier. This
noise, as in any dissipative element, arises from thermal agitation of the electrons within the element. By cooling
a parametric amplifier to a very low temperature, it is possible to substantially reduce the thermal agitation of the
electrons and the noise created thereby. Obviously, this technique would be impractical with an electron-tube
amplifier, because an electron tube depends on a relatively hot cathode for its operation.
c. It should not be assumed that all parametric amplifiers are cooled. The improvement to be realized is
often not worth the problems that arise with cooling a parametric amplifier. Generally liquid nitrogen, which is at
78 Kelvin (K) (-320 F), or liquid helium, which is at 4.2 K (-452 F), is used for cooling. Some of the
representative noise figures for parametric amplifiers in the 8-gigahertz range are:
(1) Uncooled, 3.0 db.
(2) Mechanical refrigerator with liquid nitrogen cooled, 2.0 db.
(3) Argon-nitrogen refrigerator cooled, 1.7 db.
about 1.3 db.
1-32.
OPERATION
a. Essentially, a parametric amplifier consists of a variable reactance device, an ac power source which
supplies a pump frequency, and a signal source. The pump frequency is generally supplied by a reflex klystron
oscillator. When a klystron is used for this purpose it is commonly called a pump klystron. The variable
reactance converts the pump frequency into signal power, and thus produces amplification. This can be done by a
variable inductance or capacitance. Parametric amplifiers that use a variable inductance are more commonly
known as magnetic amplifiers. Our interest, however, is in parametric amplifiers that employ reverse-biased
semiconductor diodes, which are variable-capacitance devices. Varactors and tunnel diodes are commonly used
in parametric amplifiers.
344 L1
30
Previous Page
