A varicap is a diode with a high capacitance in its reverse-biased p-n junction, where the capacitance depends on the applied reverse voltage. A varicap changes its capacitance proportionally to the magnitude of the reverse bias voltage, typically ranging from a few to hundreds of picofarads.
Varicaps are used as voltage-controlled variable capacitors. They serve as elements with electrically adjustable capacitance in circuits for tuning the frequency of resonant circuits in frequency-selective networks, frequency division and multiplication, frequency modulation, voltage-controlled phase shifters, and more.
In addition to single varicaps, double and triple varicaps with a common cathode are frequently used. These are typically employed in radio receivers where both the input circuit and the local oscillator must be tuned simultaneously using a single potentiometer.
Main Parameters of a Varicap
Although a varicap is technically a semiconductor diode, in practice it is used as a variable capacitor. Therefore, parameters related to capacitance are the most significant.
Urev max — maximum reverse DC bias voltage.
This is the highest voltage that can be applied to the p-n junction of the varicap.
Cnom — nominal capacitance of the varicap.
This is the capacitance at the nominal (specified) reverse voltage.
Cmin and Cmax — minimum and maximum capacitance of the varicap.
Electronics reference materials often specify the minimum and maximum possible capacitance at fixed reverse voltages.
Cd — capacitance of the varicap at a reverse voltage close to the maximum rated value.
Kc — capacitance tuning ratio.
This is the ratio of the maximum capacitance of the varicap to its minimum capacitance.
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