VARACTOR devices are another of the active two-terminal diodes that operates in the microwave range. Since the basic theory of varactor operation was presented in the tutorials on the Introduction to Solid- State Devices and Power Supplies, only a brief review of the basic principles is presented here.
The varactor 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 the figure below, 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.
View A: PN-junction diode as a variable capacitor.
External bias, applied in either the forward or reverse direction, as shown in the next figure view B and C, affects the magnitude, barrier potential, and width of the depletion area. Enough forward or reverse bias will overcome the barrier potential and cause current to flow through the diode. The width of the depletion region can be controlled by keeping the bias voltage at levels that do not allow current flow.
Since the depletion area acts as a capacitor, the diode will perform as a variable capacitor that changes with the applied bias voltage. The capacitance of a typical varactor can vary from 2 to 50 picofarads for a bias variation of just 2 volts.
View B: Pn-junction diode as a variable capacitor.
View C: 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.
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 the figure below. 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.
Voltage amplification from a varying capacitor.
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.
In the next figure below, 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 (E) across the capacitor reaches maximum. The decreased capacitance causes a voltage buildup 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 buildup of the input-signal energy level is shown in view (D).
Energy transfer from pump signal to input signal.
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 the figure above 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.
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