TAILIEUCHUNG - Lecture Electrical Engineering: Lecture 22 - Dr. Nasim Zafar

The main contents of the chapter consist of the following: Graphical analysis of the BJT amplifier circuits, biasing the BJT for discrete-circuit design, small-signal operation and models. | Dr. Nasim Zafar Electronics 1 - EEE 231 Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad Biasing in BJT Amplifier Circuits and Small-Signal Operation and Models Lecture No. 22 Contents: Graphical Analysis of the BJT Amplifier Circuits. Biasing the BJT for Discrete-Circuit Design. Small-Signal Operation and Models. Nasim Zafar 2 Graphical Analysis of the BJT Amplifier Circuits Reference: Microelectronic Circuits Adel S. Sedra and Kenneth C. Smith. The BJT Circuits The DC Biased BJT Circuit The BJT Amplifier Circuit The Transistor Amplifier Circuit (a) Conceptual circuit to illustrate the operation of the transistor as an amplifier. (b) The circuit of (a) with the signal source vbe eliminated for dc (bias) analysis. 7 Transistor Biasing as an Amplifier Circuit: The purpose of dc biasing is to establish the Q-point for operation. The collector curves and load lines help us to relate the Q-point and its proximity to cutoff and saturation. The Q-point is best established where the signal variations do not cause the transistor to go into saturation or cutoff. What we are most interested in is, the ac signal itself. Since the dc part of the overall signal is filtered out in most cases, we can view a transistor circuit in terms of just its ac component. 8 Review: Sinusoidal Analysis Any voltage or current in a linear circuit with a sinusoidal source is a sinusoid of the same frequency (w). We only need to keep track of the amplitude and phase, when determining the response of a linear circuit to a sinusoidal source. Any time-varying signal can be expressed as a sum of sinusoids of various frequencies (and phases). Applying the principle of superposition: The current or voltage response in a linear circuit due to a time-varying input signal can be calculated as the sum of the sinusoidal responses for each sinusoidal component of the input signal. Graphical Analysis of the BJT Amplifier Let us consider the .

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