2-47
Figure 2-43.—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.
Figure 2-44, 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).
Figure 2-44.—Energy transfer from pump signal to input signal.
