Variable Capacitor

Variable capacitors use the property of the reverse biased p-n junction to change the capacity when the bias is changed. The greater the bias is, the smaller the capacity. Practically everyone has to deal with capacitors. Everyone who tunes his radio set to the station he wants to get, changes the capacitance of the variable capacitor. Without variable capacitors no TV transmission would be possible, espe­cially that of color TV. The semiconductor capacitances, controlled by bias voltage, underlie the basis of supersensitive transformers changing the direct voltage into alternat­ing voltage. Due to them the amplification of superweak signals is realized, including the signals from cosmic space. They are used to generate signals with a frequency of hundreds of milliards of Hertz. All these applications of the variable semiconductor capacitance are based on the fact that the value of this capacitance can be changed very rapidly by means of bias. The capacitance of the variable capacitor changes, depending on its construction, in thousandth, millionth and sometimes milliardth fractions of a second, when the bias is changed. One of the most important parameters of a variable capacitor is the capa­citance change coefficient K_c, defined as the ratio of the maximal capacitance of the variable capacitor C_max to its minimal capacitance C_min.

Since the capacitance of the variable capacitor depends on bias, it is clear that the value K_c depends on the possible range of the reverse bias applied to the variable capacitor. The main condition, limiting this range, is formulated quite simply: within the operational bias range of the variable capacitor the current across it must not be large.

The minimal reverse bias ‘U_0’ at the variable capacitor should as a rule is equal to zero. Then the voltage drop at the p-n junction is minimal and is equal to U_pn, while the capacitance of the variable capacitor is maximal.

Questionnaire:

• What Variable capacitors?