Evaluation of Junction Diode Capacitance

The capacitance associated with P-N junction is a parameter of importance in many circuit applications. Its variation with reverse bias finds an important use for junction diodes as varicaps. If one analyses the equivalent circuit of a junction diode, it would become obvious that measurements have to be conducted within certain frequency ranges only to obtain the junction capacitance value accurately. Such an analysis has been done for a typical tuning varactor diode (varicap) and measurements have been done on practical diodes to evaluate the junction capacitance and confirm the analysis.Another feature of importance in case of tuning varactor is the percentage variation of capacitance with reverse bias. A continuous plot of capacitance versus bias voltage will have the greatest utility as it provides a quickest means of selecting matched diodes. The paper describes a circuit which provides the C versus V plot on oscilloscope.