In this chapter, you will learn about: DC current and voltage analysis, DC analysis of transistor circuits, modes of operation of a BJT Transistor, transistor characteristics.
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 CircuitsI Lecture No: 16 Nasim Zafar NPN Transistor Nasim Zafar Operation of an NPN Transistor: Large current Nasim Zafar Modes of Operation of a BJT Transistor: Mode: BE Junction: BC Junction: cutoff reverse biased reverse biased Active(linear) saturation Nasim Zafar Forward Biased forward biased Reverse Biased forward biased Transistor Characteristics: IC IB Output Circuit Input Circui t IE Nasim Zafar DC Current and Voltage Analysis: NPN IC IB base collector Large current + VBE emitter IE Small Current Nasim Zafar DC Current and Voltage Analysis: PNP IC IB base collector Large current + VBE emitter IE Small Current Nasim Zafar DC Analysis of Transistor Circuits Nasim Zafar DC Analysis of Transistor Circuits: Ø The DC Analysis of the transistor circuits involves solving for all (or most of) the currents and voltages in the circuit. Ø Ø Ø The most important DC parameters to solve are IC and VCE. The “Qpoint” 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 Ø Ø 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: Ø Ø Ø 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: Ø Ø Ø 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 BJTCurrent and Voltage Analysis: Ø When the baseemitter junction, in an NPN transistor is forward biased, it is like a forward biased diode and has a forwardvoltage drop of: VBE = 0.7 V Ø 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 BJTCurrent and Voltage Analysis: Using Ohm’s law: VRS = IB RS NPN 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 = β IB β is the current gain of the transistor 100200 VBE = 0.7V(NPN) VBE = 0.7V(PNP) Nasim Zafar 17 Example 41: CommonEmitter 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 41 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 42 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 BJTOutput Characteristics: Common Emitter Configuration Graphical construction for determining the dc collector current IC and the collectortoemitter voltage VCE Nasim Zafar 21 BJTOutput Characteristics: Common Emitter Configuration Ø Ø We must operate the transistor in the linear region A transistor’s operating point (Qpoint) 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: Ø Bias the transistor so that it operates in the linear region Ø BE junction forward biased, CE junction reversed biased Ø Use VBE = 0.7 (NPN), IC IE, IC = βIB Ø Write BE, and CE voltage loops Ø For DC analysis, solve for IC, and VCE Ø For design, solve for the resistor values (I C and VCE specified) Nasim Zafar 24 Nasim Zafar 25 ...DC Analysis of Transistor CircuitsI Lecture No: 16 Nasim Zafar NPN Transistor Nasim Zafar Operation of an NPN Transistor: Large current Nasim Zafar Modes of Operation of a BJT Transistor:... Small Current Nasim Zafar DC Current and Voltage Analysis: PNP IC IB base collector Large current + VBE emitter IE Small Current Nasim Zafar DC Analysis of Transistor Circuits Nasim Zafar. .. 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 = β IB