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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.
COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 Fall Semester – 2012 Biasing in BJT Amplifier Circuits and SmallSignal Operation and Models Lecture No. 22 Nasim Zafar Graphical Analysis of the BJT Amplifier Circuits Reference: Ø Chapter5.3.3 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 Transistor Biasing as an Amplifier Circuit: Ø The purpose of dc biasing is to establish the Qpoint for operation Ø Ø The collector curves and load lines help us to relate the Qpoint and its proximity to cutoff and saturation. The Qpoint is best established where the signal variations do not cause the transistor to go into saturation or cutoff. Ø 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 Review: Sinusoidal Analysis Ø Any voltage or current in a linear circuit with a sinusoidal source is a sinusoid of the same frequency (ω) – Ø We only need to keep track of the amplitude and phase, when determining the response of a linear circuit to a sinusoidal source Any timevarying 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 Ø Let us consider the graphical analysis of the operation the BJT amplifier circuit: Small Signal Operation The DC Bias Conditions Ø The DC Bias Conditions: by Setting the Signal Source vbe =0 Ø The dc currents and voltages are given by: Conceptual Amplifier Circuit (contd.) Current Equations: Ø Collector current iC iB Ø I se iC Is VT e v BE VT Base current iE Ø In v BE Emitter current iC Is e v BE VT SmallSignal Operation and Models Ø Ø Ø The Collector current and the Transconductance The Base Current and the Input Resistance at the Base The Emitter Current and the Input Resistance at the Emitter The Collector Current and the Transconductance The Collector Current and the Transconductance The Small Signal Approximation Transconductance Transconductance Ø Expression gm Ø I CQ VT Physical meaning gm is the slope of the iC–vBE curve at the bias point Q Ø At room temperature, g m 40ms Base Current and Input Resistance at the Base Ø Ø Ø To determine the resistance seen by vbe, we first evaluate the total base current iB using Eq. (5.84), as follows: Thus, where IB is equal to IC/β and the signal component ib is given by: Base Current and Input Resistance at the Base Ø Ø Substituting IC /VT for by gm gives The smallsignal input resistance between base and emitter, looking into the base, is denoted by rπ and is defined as Using Eq. (5.91) gives: Base Current and Input Resistance at the Base Ø Thus rπ is directly dependent on ß and is inversely proportional to the bias current IC. Substituting for gm in Eq. (5.93) from Eq. (5.87) and replacing IC / by IB gives an alternative expression for rπ , The Emitter Current and the Input Resistance at the Emitter Ø The total emitter current iE can be determined from where IE is equal to IC ∕ α and the signal current ie is given by The Emitter Current and the Input Resistance at the Emitter Ø If we denote the smallsignal resistance between base and emitter, looking into the emitter, by re, it can be defined as Thus, we find that re, called the emitter resistance, is given by: The Emitter Current and the Input Resistance at the Emitter Ø Thus, Ø Which yields Summary Input Resistance at Base and Emitter Ø Input resistance at base vbe ib r Ø gm Input resistance at emitter v V re Ø VT I BQ be ie T I EQ gm r (1 ) re Relationship between these two resistances ...Biasing in BJT Amplifier Circuits and SmallSignal Operation and Models Lecture? ?No. 22 Nasim Zafar Graphical Analysis of the BJT Amplifier Circuits Reference: Ø Chapter5.3.3 Microelectronic Circuits... Which represents a linear relationship between vCE and iC Transistor Amplifier Basics: ib + IB = iB Nasim Zafar 15 Graphical Analysis(contd.) Graphical determination of the signal components vbe, ib, ic, and vce when a