Chapter 5: BJT AC Analysis Islamic University of Gaza Dr. Talal Skaik BJT Transistor Modeling • A model is an equivalent circuit that represents the AC characteristics of the transistor. • A model uses circuit elements that approximate the behavior of the transistor. • There are two models commonly used in small signal AC analysis of a transistor: – re model – Hybrid equivalent model Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 2 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. BJT Transistor Modeling Capacitors chosen with very small reactance at the frequency of application → replaced by low-resistance or short circuit. Removal of the dc supply and insertion of the short-circuit equivalent for the capacitors. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 3 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. BJT Transistor Modeling Circuit redrawn for smallsignal ac analysis Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 4 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. The re Transistor Model Zi Common Emitter Configuration V i V be Ib Ib V be I e re I c I b re I b I b re 1 I b re V be 1 I b re Zi 1 re Ib Ib Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky re Dr. Talal Skaik 2014 5 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. The re Transistor Model Common Emitter Configuration 26 mV re IE Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 6 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. The re Transistor Model Common Emitter Configuration I C 1 slope V CE r0 V CE r0 I C The output resistance r is typically in the range of 40 kΩ to 50 kΩ Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 7 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Common-Base Configuration Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 8 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Common-Base Configuration The output resistance r0 is quite high. typically extend into the megaohm range. Common Base re equivalent circuit Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 9 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Common Emitter Fixed Bias Configuration Common-emitter fixed-bias configuration. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Network after the removal of the effects of VCC, C1 and C2. Dr. Talal Skaik 2014 10 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Common Emitter Fixed Bias Configuration Substituting the re model into the network. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 11 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Common Emitter Fixed Bias Configuration Input impedance: Z i R B || re Z i re R EB 10 re Output impedance: Z o R C || rO Z o R C ro 10R C Voltage gain: Vi Vi Vo I b (R C ||ro ) , I b , Vo (R C ||ro ) re re Vo (R C ||ro ) Av Vi re Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky RC , Av re Dr. Talal Skaik 2014 12 ro 10R C Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Common Emitter Fixed Bias Configuration Av Vo (R ||r ) C o Vi re Demonstrating the 180°phase shift between input and output waveforms. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 13 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Example 5.1 Determine re, Zi (with ro=∞), Zo (with ro=∞), Av (with ro=∞). Repeat with ro=50 kΩ. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 14 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Example 5.1 - Solution Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 15 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Common-Emitter Voltage-Divider Bias re model requires you to determine , re, and ro. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 16 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Common-Emitter Voltage-Divider Bias Input impedance: R R 1 || R 2 Z i R || re Output impedance: Z o R C || ro Z o R C ro 10R C Voltage gain: Vi Vi Vo I b (R C ||ro ) , I b , Vo (R C ||ro ) re re Av Vo (R ||r ) C o Vi re Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky , Av Dr. Talal Skaik 2014 17 RC re ro 10R C Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Example 5.2 Determine re, Zi , Zo (with ro=∞), Av (with ro=∞). Repeat with ro=50 kΩ. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 18 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Example 5.2 - Solution Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 19 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Common-Emitter Emitter-Bias Configuration Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 20 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Impedance Calculations Input impedance: V i I b re I e R E V i I b re 1 I b R E Zb Vi re 1 R E Ib Z b re R E re R E Z b RE for R E re Output impedance: Zi R B ||Zb Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Zo R C Dr. Talal Skaik 2014 21 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Gain Calculations Voltage gain: Vo I o RC I b RC Vi Vo RC Zb Vo RC Av Vi Zb substituting Zb (re R E ) Av Vo RC Vi re R E and for the approximation Zb R E Av Vo R C Vi RE Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 22 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Example 5.3 Without CE (unbypassed): Determine re, Zi , Zo , Av . ignore ro for ro ≥ 10(RC+RE) Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 23 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Emitter-Follower Configuration • This is also known as the common-collector configuration. • The input is applied to the base and the output is taken from the emitter. • There is no phase shift between input and output. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 24 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Impedance Calculations Input impedance: Zi R B ||Zb Zb re ( 1)R E Zb (re R E ) Zb R E (for R E >>re ) Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 25 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Impedance Calculations Output impedance: Vi Ib , Ie =( +1)I b Zb Vi ( +1) Zb ( +1)V i Ie re ( +1)R E sin ce ( +1) Vi Ie re R E To determine Zo , V i is set to zero Zo R E ||re , Zo re Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky R E re Dr. Talal Skaik 2014 26 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Gain Calculations Voltage gain: RE Vo Vi R E re Vo RE Av Vi R E re Vo Av 1 Vi R E re , R E re R E Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 27 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Example 5.7 Determine re, Zi , Zo , Av . Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 28 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Example 5.7 - solution Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Dr. Talal Skaik 2014 29 Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.
