Page 26 - Radar Technology Encyclopedia

16 amplifier, backward-wave(-tube) amplifier, doppler and range frequencies

A backward-wave(-tube) amplifier uses a backward-wave A class-AB (vacuum-tube) amplifier is one in which “the
tube as its basic element. There are linear-beam and crossed- grid bias and alternating grid voltages are such that anode
field backward wave amplifiers. The linear beam backward current in a specific tube flows for appreciably more than half
wave amplifier operates as a synchronized generator. Stable but less than the entire electrical cycle.”
synchronization is achieved by matching the intrinsic fre-
A class-B (vacuum-tube) amplifier is one in which “the grid
quency of the tube with the frequency of excitation. The
bias is approximately equal to the cutoff value so that the
amplitron is also a backward-wave amplifier, which combines
anode current in a specific tube flows for approximately one
the operating principles of the crossed-field backward wave
half of each cycle when alternating grid voltage is applied.”
amplifier with that of a magnetron.
In linear-beam backward-wave types amplification A class-C (vacuum-tube) amplifier is one “in which the grid
occurs at lower levels of beam current, which is the reason bias is appreciably greater than the cutoff value so that the
why these amplifiers do not achieve high power levels. The anode current in a specific tube is zero when no alternating
amplifier gain and passband width vary widely over the fre- current is applied, and so that anode current flows for appre-
quency tuning range. These amplifiers are used as voltage- ciably less than one half of each cycle when alternating grid
tunable preselectors. The amplifier gain is 3 to 20 dB. voltage is applied.” SAL
Backward wave amplifiers other than amplitrons are sel- Ref.: IEEE (1993), p. 33.
dom used in radar applications. IAM corporate structure [-combined] amplifier (see power
Ref.: Andrushko (1981), p. 66; Skolnik (1990), pp. 4.12–4.14. amplifier).
A balanced amplifier is “an amplifier in which there are two crossed-field amplifier (see CROSSED-FIELD AMPLI-
identical signal branches connected so as to operate in phase FIER).
opposition and with input and output connections each bal-
A difference [differential] amplifier produces an output sig-
anced to ground.” It typically consists of a parallel combina-
nal proportional to the difference between input signals. It can
tion of two single-stage signal branches and two bridge
also be realized as a nonlinear integrated circuit whose func-
circuits for input and output. The balanced circuit is used to
tion is to output logic “1” or logic “0,” depending on which of
increase the output power and reliability. Balanced amplifiers
two inputs is more positive than the other. One of the applica-
have more stable amplitude and phase characteristics and
tions is in EW crystal detector receivers. SAL
exhibit higher stability than unbalanced amplifiers. Short-
Ref.: IEEE (1993), p. 344; Wiegand (1991), p. 140.
comings of balanced amplifiers are, as a rule, lower sensitiv-
ity and the necessity of using active elements with identical A diode amplifier uses a diode as its active element. A circu-
gain and noise figures. lator is customarily required to separate the input and output
In radar applications, balanced amplifiers are typically signals. Based on the type of active element, diode amplifiers
used in low-noise input circuits. The balanced amplifier is are classified into tunnel diode, IMPATT-TRAPATT diode, or
sometimes termed a push-pull amplifier. IAM Gunn diode amplifiers. The fundamental parameters of diode
Ref.: IEEE (1993), p. 33; Gassanov, (1988), p. 171.; Rudenko (1971), p. 85. amplifiers are gain, output power, noise figure, bandwidth,
and efficiency. Another important characteristic of the diode
A bandpass amplifier passes signals in fixed-frequency
amplifier is its stability. Diode amplifiers cover practically the
bands and possesses a constant gain. It is used to separate of
entire microwave band from 1 to 300 GHz. They are used in
signals that have spectral components in the frequency range
various radar subsystems, from input circuits (tunnel diode
of the amplifier. To achieve the desired frequency characteris-
amplifiers) to power amplifiers (IMPATT-TRAPATT diode
tics, bandpass amplifiers use coupled resonator filters or cou-
amplifiers). IAM
pled, tuned tank circuits. IAM
Ref.: Howes (1976).
Ref.: Buda (1986), p. 86; Fink (1982), p. 3.38.
A doppler and range frequencies amplifier is used in con-
A cascode amplifier is a two-stage tube IF amplifier con-
tinuous-wave radars to amplify the (converted) signal at the
nected in a common-cathode or common-grid circuit. As
frequency f + f to a level necessary for the reliable operation
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compared with pentodes, the use of triodes results in lower
of the unit converting the frequency shift to a current or volt-
noise figures (1.3 to 1.4 dB). High amplifier stability is
age level shift (where f is the doppler shift and f is the range
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assured by using the triodes in a common-grid connection. A
frequency increment). The amplifier parameters depend on
high amplifier input resistance is achieved by connecting the
the type of radar. In radars with frequency modulation with
first stage in a common-cathode circuit.
f > f , the amplifier has a linear frequency characteristic with
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In older radar applications cascode amplifiers were used
a gain slope of 12 dB/octave (6 dB/octave for operation
in the first stage of intermediate frequency amplifiers. IAM
against extended targets) over its range of operating frequen-
Ref.: Valley (1948), p. 440; Fradkin (1969), p. 55; Benson (1986), p. 14.92.
cies. In doppler radars and in radars having frequency modu-
A class-A (vacuum-tube) amplifier is one in which “the grid lation with f < f , the amplifier has a flat frequency
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bias and alternating grid voltages are such that anode current characteristic and a nonlinear (usually logarithmic) amplitude
in a specific tube flows all the time.” characteristic. To compensate for the effects of fluctuation of