b. Note in figure 28 that the pump voltage is applied across the diode. The diode is reverse biased so as
to behave as a capacitor that changes with voltage. Thus, as the pump voltage varies at a high-frequency rate, so
also does the circuit capacitance. The nonlinear variation of the capacitance with voltage causes a mixing
(heterodyning) of the signal and the pump frequency. As a result, sum and difference frequencies, called idler
frequencies, are produced.
Figure 28. Parametric amplifier.
c. As a special case, we will consider only the idler difference frequency of the pump and signal
frequency and stipulate that the pump frequency is twice the signal frequency. This makes the difference idler
frequency equal to the signal frequency. Since the idler frequency receives power from the pump frequency and
the idler has the same frequency as the signal, signal power is boosted. More specifically, we will assume a signal
frequency of 100 megahertz (MHz) and a pump frequency of 200 MHz. When heterodyned, there is produced an
idler frequency 200 MHz - 100 MHz = 100 MHz. This 100-MHz idler frequency can have far greater power than
the 100-MHz signal input power because it obtains its power from the pump source. This means that it is possible
to obtain an output power at 100 MHz, which is considerably more than the input signal power. Therefore, the
signal is amplified.
d. Another viewpoint to explain the pump action is to regard the diode as a negative resistance (in other
words, a generator source) to the signal. This viewpoint is justified inasmuch as a mathematical analysis of the
heterodyning action shows that the variable-capacitance diode presents a positive resistance to the pump
frequency and a negative resistance to the signal frequency. A gain is obtained when a device acts as a negative
resistance.
e. The simple amplifier shown in figure 28 illustrates the principle of the parametric amplifier.
However, this particular circuit is not practical because it requires that the proper phase and frequency
relationship between the signal and pump frequencies be maintained. This relationship holds that the pump
frequency must coincide with the positive and negative peaks of the input signal. While this phase and frequency
relationship is required for maximum transfer of energy, amplification can still be obtained when the relationship
is not maintained. If the pump frequency is not twice the signal frequency, sum and difference frequencies result
from the heterodyning of the two frequencies.
344 L1
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