TAILIEUCHUNG - Lecture Electrical Engineering: Lecture 26 - Dr. Nasim Zafar

The main contents of the chapter consist of the following: Introduction, the BJT internal capacitances, high-frequency BJT model, the high-frequency hybrid. | Dr. Nasim Zafar Electronics 1 - EEE 231 Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad The BJT Internal Capacitance and High Frequency Model Lecture No. 26 Contents: Introduction The BJT Internal Capacitances High-Frequency BJT Model The High-Frequency Hybrid-Model Frequency Response of the CE Amplifier Nasim Zafar 2 Lecture No. 26 Reference: The BJT Internal Capacitance and High-Frequency Model Microelectronic Circuits Adel S. Sedra and Kenneth C. Smith. Nasim Zafar 3 The BJT Internal Capacitances Nasim Zafar 4 Introduction So far, we have assumed transistor action to be instantaneous. The models we have developed, do not include any elements like capacitors or inductors, that would cause time or frequency dependence. Nasim Zafar 5 Introduction Actual transistors, however, exhibit charge storage phenomena that limit the speed and frequency of their operation. In this lecture, we study the charge-storage effects that take place in the BJT and take them into account by adding capacitances to the hybrid-π model. Nasim Zafar 6 We now again, define some quantities: BJT: Small Signal Model Nasim Zafar 7 So The output resistance is: BJT: Small Signal Model High-Frequency BJT Model Nasim Zafar 9 High-Frequency BJT Model The BJT inherently has junction capacitances which affect its performance at high frequencies. Cb represents the base charge. Collector Junction: depletion capacitance, Cμ Emitter Junction: depletion capacitance, Cje, and also diffusion capacitance, Cb. Nasim Zafar 10 BJT High-Frequency BJT Model (cont’d) In an integrated circuit, the BJTs are fabricated in the surface region of a Si wafer substrate; another junction exists between the collector and substrate, resulting in substrate junction capacitance, CCS. BJT Cross-Section BJT Small-Signal Model The PN Junction Capacitance The following expressions apply for a PN junction diode: How do we apply this to BJTs? Nasim Zafar 12 The Base-Charging or

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