Lecture Electrical Engineering: Lecture 16 - Dr. Nasim Zafar

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COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 DC Analysis of Transistor Circuits-I Lecture No: 16 Contents:  DC Current and Voltage Analysis.  Examples and Exercises. Nasim Zafar 2 NPN Transistor Nasim Zafar 3 Operation of an NPN Transistor: Large current Nasim Zafar 4 Modes of Operation of a BJT Transistor: Mode: B-E Junction: B-C Junction: cutoff reverse biased reverse biased Active(linear) saturation Nasim Zafar Forward Biased forward biased Reverse Biased forward biased 5 Transistor Characteristics: IC IB Output Circuit Input Circui t IE Nasim Zafar 6 DC Current and Voltage Analysis: NPN IC base IB collector Large current + VBE Small Current Nasim Zafar - emitter IE 7 DC Current and Voltage Analysis: PNP IC base IB collector Large current + VBE Small Current Nasim Zafar - emitter IE 8 DC Analysis of Transistor Circuits. Nasim Zafar 9 DC Analysis of Transistor Circuits:  The DC Analysis of the transistor circuits involves solving for all (or most of) the currents and voltages in the circuit.  The most important DC parameters to solve are IC and VCE.  The “Q-point” of a transistor, gives the values of IC and VCE that are present in the given transistor circuit.  The first step in the DC analysis, of any transistor circuit, is to solve for one of the unknown currents, IB, IC, or IE. Nasim Zafar 10 DC Analysis of Transistor Circuits:  We need a set of equations – a model of the transistor – to be used in transistor circuit theory.  In a basic transistor circuit, we have three terminals - collector, base and the emitter. Nasim Zafar 11 DC Analysis of Transistor Circuits:  In a basic transistor circuit, we have three terminals - collector, base and emitter; and correspondingly we have:  Three possible DC voltages: VBE, VCB, VCE - but only two are independent due to KVL; and  Three transistor DC currents: IB, IC, IE­ - but only two are independent due to KCL. Nasim Zafar 12 DC Analysis of Transistor Circuits:  If we solve for one of these unknowns, the other two can be found by using the current gain equations and the given value of .  The recommended way of solving for one of the currents is to write a Kirchhoff's Voltage Law (KVL) loop.  The KVL states the sum of all voltages around a closed loop equals zero. Va = V1 + V2 Nasim Zafar 13 BJT-Current and Voltage Analysis:  When the base-emitter junction, in an NPN transistor is forward biased, it is like a forward biased diode and has a forward-voltage drop of: VBE = 0.7 V  Since the emitter is grounded, by Kirchhoff’s voltage law, the voltages in the input circuit are: NPN VBB = VRS + VBE VRS = VBB -- VBE Nasim Zafar 14 BJT-Current and Voltage Analysis: NPN Using Ohm’s law: VRS = IB RS IB RS = VBB -- VBE The drop across RL is: VRL = IC RL The collector voltage is: VCE = VCC -- IC RL Nasim Zafar 15 DC Analysis of Transistor Circuits Examples and Exercises: Nasim Zafar 16 Summary of equations for a BJT: IE  IC IC = b IB b is the current gain of the transistor  100-200 VBE = 0.7V(NPN) VBE = -0.7V(PNP) Nasim Zafar 17 Example 4-1: Common-Emitter Configuration: Given: IB = 50  A , IC = 3.65 mA Determine: IE , dc and dc Solution: IE= IB+ IC= 50  A + 3.65 mA= 3.7 mA dc = IC / IB = 3.65 mA / 0.05 mA = 73 dc = IC / IE = 3.65 mA/ 3.7 mA = 0.986 Nasim Zafar 18 Exercise 4-1 A certain transistor has a dc of 200. When the base current IB = 50  A. Determine the collector current. Also calculate dc . Nasim Zafar 19 Example 4-2 A given NPN transistor has dc = 150. Determine IB, IC , IE , VCB , VCB and VBE in the circuit shown below (ignore ac signal): Nasim Zafar 20 BJT-Output Characteristics: Common Emitter Configuration. Graphical construction for determining the dc collector current IC and the collector-to-emitter voltage VCE. Nasim Zafar 21 BJT-Output Characteristics: Common Emitter Configuration  We must operate the transistor in the linear region.  A transistor’s operating point (Q-point) is defined by IC, VCE, and IB. Nasim Zafar 22 Example 4.3 From the output characteristics of the common emitter configuration shown below, find ac and dc with an Operating point at IB=25 A and VCE =7.5V. Nasim Zafar 23 Summary of DC Analysis:  Bias the transistor so that it operates in the linear region  B-E junction forward biased, C-E junction reversed biased.  Use VBE = 0.7 (NPN), IC  IE, IC = bIB  Write B-E, and C-E voltage loops.  For DC analysis, solve for IC, and VCE.  For design, solve for the resistor values (IC and VCE specified). Nasim Zafar 24 Nasim Zafar 25
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