|
|
The varactor diode is another of the active two-terminal devices that operates
in the microwave range. It is a semiconductor diode with the properties of a
voltage-dependent capacitor. Specifically, it is a variable-capacitance,
pn-junction diode that makes good use of the voltage dependency of the
depletion-area capacitance of the diode.
In figure 1, view (A), two materials are brought together to form a pn-junction
diode. The different energy levels in the two materials cause a diffusion of the
holes and electrons through both materials which tends to balance their energy
levels. When this diffusion process stops, the diode is left with a small area
on either side of the junction, called the depletion area, which contains no
free electrons or holes. The movement of electrons through the materials creates
an electric field across the depletion area that is described as a barrier
potential and has the electrical characteristics of a charged capacitor.
Figure 1.
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 conventional amplifier is essentially a variable resistance that uses energy
from a DC source to increase ac energy. On the other hand, 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.
Figure 2.
Conventional Amplifier
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.
Principle of operation of a parametric amplifier
The basic theory of parametric amplification centers around a capacitance that
varies with time. Consider the simple series circuit shown in Figure 3. 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.
Figure 3.
Conventional Amplifier
An increase in the charge Q or a decrease in the capacitance C causes an
increase in the voltage across the plates. Thus, a voltage increase, or
amplification, can be obtained by mechanically or electronically varying the
amount of capacitance in the circuit. In practice a voltage-variable
capacitance, such as a varactor, is used. The energy required to vary the
capacitance is obtained from an electrical source called a PUMP.
Figure 4, view (A), shows a circuit application using a voltage-variable
capacitor and a pump circuit. The pump circuit decreases the capacitance each
time the input signal reaches maximum. The decreased capacitance causes a
voltage build-up as shown by the dotted line in view (B). Therefore, each time
the pump decreases capacitance (view (C)), energy transfers from the pump
circuit to the input signal. The step-by-step build-up of the input-signal
energy level is shown in view (D).
Figure 4.
Energy transfer from pump signal to input signal in a parametric amplifier
Proper phasing between the pump and the input signal is crucial in this circuit.
The electrical pump action is simply a sine-wave voltage applied to a varactor
located in a resonant cavity. For proper operation, the capacitance must be
decreased when the input voltage is maximum and increased when the input voltage
is minimum. In other words, the pump signal frequency must be exactly double the
frequency of the input signal. This relationship can be seen when you compare
views (B) and (C). A parametric amplifier of the type shown in figure 2-44 is
quite phase-sensitive. The input signal and the capacitor variation are often in
the wrong phase for long periods of time.
A parametric amplifier that is not phase-sensitive, referred to as a
nondegenerative parametric amplifier, uses a pump circuit with a frequency
higher than twice the input signal. The higher-frequency pump signal mixes with
the input signal and produces additional frequencies that represent both the sum
and difference of the input signal and pump frequencies.
Figure 5, view (A), is a diagram of a typical nondegenerative parametric
amplifier with the equivalent circuit shown in view (B). The pump signal (fp) is
applied to the varactor. The cavity on the left is resonant at the input
frequency (fs), and the cavity on the right is resonant at the difference
frequency (fp-fs). The difference frequency is called the IDLER- or
LOWER-SIDEBAND frequency. The varactor is located at the high-voltage points of
the two cavities and is reverse biased by a small battery. The pump signal
varies the bias above and below the fixed-bias level.
Figure 5.
Nondegenerative parametric amplifier
The pump signal causes the capacitor in view (A) to vary at a 12-gigahertz rate.
The 3-gigahertz input signal enters via a four-port ferrite circulator, is
developed in the signal cavity, and applied across the varactor. The nonlinear
action of the varactor produces a 9-gigahertz difference frequency (fp-fs) with
an energy-level higher than the original input signal.
The difference (idler) frequency is reapplied to the varactor to increase the
gain and to produce an output signal of the correct frequency. The 9-gigahertz
idler frequency recombines with the 12-gigahertz pump signal and produces a
3-gigahertz difference signal that has a much larger amplitude than the original
3-gigahertz input signal. The amplified signal is sent to the ferrite circulator
for transfer to the next stage.
As with tunnel-diode amplifiers, the circulator improves stability by preventing
reflection of the signal back into the amplifier. Reflections would be amplified
and cause uncontrollable oscillations. The ferrite circulator also serves as an
isolator to prevent source and load impedance changes from affecting gain.
Typically, the gain of a parametric amplifier is about 20 dB. The gain can be
controlled with a variable attenuator that changes the amount of pump power
applied to the varactor.
Parametric amplifiers are relatively simple in construction. The only component
is a varactor diode placed in an arrangement of cavities and waveguides.
PARAMETRIC FREQUENCY CONVERTERS
Parametric frequency converters, using varactors, are of three basic types. the
upper-sideband parametric up-converter produces an output frequency that is the
sum of the input frequency and the pump frequency. The lower-sideband parametric
down-converter produces an output frequency that is the difference between the
pump frequency and the input frequency. The double-sideband parametric
up-converter produces an output in which both the sum and the difference of the
pump and input frequencies are available.
Parametric frequency converters are very similar to parametric amplifiers in
both construction and operation. Figure 2-47 is a functional diagram of a
parametric down-converter.
The parametric frequency converter operates in the same manner as the parametric
amplifier except that the sideband frequencies are not reapplied to the
varactor. Therefore, the output is one or both of the sideband frequencies and
is not the same as the input frequency. The output frequency is determined by
the cavity used as an output. For example, the idler cavity in figure 6 could be
replaced by a cavity that is resonant at the upper-sideband frequency (22
gigahertz) to produce an upper-sideband parametric up-converter. Since input and
output signals are at different frequencies, the parametric frequency converter
does not require a ferrite circulator. However, a ferrite isolator is used to
isolate the converter from changes in source impedance.
Figure 6.
Lower-sideband parametric down-converter.
REVIEW QUESTIONS AND ANSWERS
Q.6.
The varactor is a pn junction that acts as what type of electronic device?
A.6.
Variable capacitor.
Q.7.
The underlying principle of operation of the parametric amplifier is based on
what property?
A.7.
Reactance.
Q.8.
What is the most important feature of the parametric amplifier?
A.8.
The low-noise characteristic.
Q.9.
How is amplification achieved in the circuit shown in figure 2-43?
A9.
By varying the amount of capacitance in the circuit
Q.10.
What is the purpose of the pump in a parametric amplifier?
A.10.
Supplies the electrical energy required to vary the capacitance.
Q.11.
The pump signal frequency must be of what value when compared to the input
signal of a simple parametric amplifier?
A.11.
Exactly double the input frequency.
Q.12.
What is the primary difference between the pump signal of a simple parametric
amplifier and the pump signal of a nondegenerative parametric amplifier?
A.12.
The pump signal of a nondegenerative parametric amplifier is higher than twice
the input signal.
Q.13.
In a nondegenerative parametric amplifier the difference between the input
frequency and the pump frequency is called what?
A.13.
Idler- or lower-sideband frequency.
