The determination and importance of internal capacitances of transistors in the frequency band of commercial FM broadcasting

Abstract

Using a single-capacitor Colpitts oscillator a method has been proposed for determination of internal capacitances of bipolar junction transistors (BJTs) and junction field effect transistors (JFETs) in the frequency band of commercial frequency modulation (FM) broadcasting. Variation of these capacitances with the operating bias point and the operating frequency of the active device has been considered. Practical implementation of this method does not require any unconventional measuring system. The changing behavior of the base-emitter capacitance of BJTs versus operating bias current has been utilized for designing a simple high-frequency amplitude-modulating (AM) system. This efficiently designed AM signal generator does not have any transformer and is an example showing the importance of internal capacitances of active devices.