Introduction to Crystal Diodes 5

The volt ampere characteristic curve of the voltage regulator is shown in Figure 5. When the reverse voltage reaches Vz, even a small increase in voltage will cause a sharp increase in reverse current (the reverse breakdown curve is very straight). At this point, the diode is in a breakdown state. If the breakdown current is limited within a certain range, the transistor can operate stably in the reverse breakdown state for a long time.

5. Varactor diode

Varactor diode is a nonlinear capacitive element made by utilizing the characteristic that the capacitance of PN junction changes with the applied bias voltage. It is widely used in microwave circuits such as parametric amplifiers, electronic tuning, and frequency multipliers. Varactor diode mainly highlights the nonlinear relationship between capacitance and voltage through a series of paths such as structural design and process, and improves the Q value to be suitable for application.

The structure of varactor diode is similar to that of ordinary diode, and its symbol is shown in Figure 6. The model parameters of several commonly used varactor diodes

It is to use the variable capacitance characteristics of the varactor to tune the frequency of local oscillation (referred to as local oscillation in TV receiver tuner). Figure 7 (b) shows a tuned signal source consisting of a sawtooth oscillator composed of a varactor, a single junction transistor, and a constant current diode. The output signal is used for frequency modulation. Since most varactor transistors operate under reverse bias, constant current protection should be added to prevent breakdown.