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Electronic Devices and Circuit Theory Boylestad DC Biasing - BJTs Chapter 4 Ch.4 Summary Biasing Biasing: Applying DC voltages to a transistor in order to turn it on so that it can amplify AC signals. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Operating Point The DC input establishes an operating or quiescent point called the Q-point. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary The Three Operating Regions Active or Linear Region Operation • Base–Emitter junction is forward biased • Base–Collector junction is reverse biased Cutoff Region Operation • Base–Emitter junction is reverse biased Saturation Region Operation • Base–Emitter junction is forward biased • Base–Collector junction is forward biased Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary DC Biasing Circuits Fixed-bias circuit Emitter-stabilized bias circuit Collector-emitter loop Voltage divider bias circuit DC bias with voltage feedback Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Fixed Bias Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary The Base-Emitter Loop From Kirchhoff’s voltage law: +VCC – IBRB – VBE = 0 Solving for base current: VCC VBE IB RB Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Collector-Emitter Loop Collector current: IC IB From Kirchhoff’s voltage law: VCE VCC IC RC Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Saturation When the transistor is operating in saturation, current through the transistor is at its maximum possible value. V ICsat CC R C VCE 0 V Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Load Line Analysis The load line end points are: ICsat IC = VCC / RC VCE = 0 V VCEcutoff VCE = VCC IC = 0 mA The Q-point is the operating point where the value of RB sets the value of IB that controls the values of VCE and IC . Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary The Effect of VCC on the Q-Point Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary The Effect of RC on the Q-Point Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary The Effect of IB on the Q-Point Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Emitter-Stabilized Bias Circuit Adding a resistor (RE) to the emitter circuit stabilizes the bias circuit. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Base-Emitter Loop From Kirchhoff’s voltage law: VCC IE RE VBE IE RE 0 Since IE = ( + 1)IB: VCC IBRB (β 1)IBRE 0 Solving for IB: VCC VBE IB RB (β 1)RE Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Collector-Emitter Loop From Kirchhoff’s voltage law: I E RE VCE IC RC VCC 0 Since IE IC: VCE VCC – I C(RC RE ) Also: VE I E RE VC VCE VE VCC IC RC VB VCC – I R RB VBE VE Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Improved Biased Stability Stability refers to a condition in which the currents and voltages remain fairly constant over a wide range of temperatures and transistor Beta () values. Adding RE to the emitter improves the stability of a transistor. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Saturation Level The endpoints can be determined from the load line. VCEcutoff: VCE VCC IC 0 mA Electronic Devices and Circuit Theory Boylestad ICsat: VCE 0 V VCC IC RC RE © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Voltage Divider Bias This is a very stable bias circuit. The currents and voltages are nearly independent of any variations in . Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Approximate Analysis Where IB << I1 and I1 I2 : VB R2VCC R1 R2 Where RE > 10R2: IE VE RE VE VB VBE From Kirchhoff’s voltage law: VCE VCC IC RC IE RE IE IC VCE V CC IC (RC RE ) Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Voltage Divider Bias Analysis Transistor Saturation Level I Csat ICmax V CC RC RE Load Line Analysis Cutoff: VCE VCC IC 0 mA Electronic Devices and Circuit Theory Boylestad Saturation: V CC I C R R C E VCE 0 V © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary DC Bias With Voltage Feedback Another way to improve the stability of a bias circuit is to add a feedback path from collector to base. In this bias circuit the Q-point is only slightly dependent on the transistor beta, . Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Base-Emitter Loop From Kirchhoff’s voltage law: VCC – IC RC –IBRB –VBE –IE RE 0 I' C IC IB IC Where IB << IC: Knowing IC = IB and IE IC, the loop equation becomes: VCC – β–BRC IBRB VBE βIBRE 0 Solving for IB: IB Electronic Devices and Circuit Theory Boylestad VCC VBE RB β(RC RE ) © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Collector-Emitter Loop Applying Kirchoff’s voltage law: IE + VCE + I’CRC – VCC = 0 Since IC IC and IC = IB: IC(RC + RE) + VCE – VCC =0 Solving for VCE: VCE = VCC – IC(RC + RE) Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Base-Emitter Bias Analysis Transistor Saturation Level I Csat ICmax V CC RC RE Load Line Analysis Cutoff Saturation VCE VCC V CC I C R R C E IC 0 mA Electronic Devices and Circuit Theory Boylestad VCE 0 V © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Transistor Switching Networks Transistors with only the DC source applied can be used as electronic switches. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Switching Circuit Calculations Saturation current: ICsat VCC RC To ensure saturation: IB ICsat βdc Emitter-collector resistance at saturation and cutoff: Electronic Devices and Circuit Theory Boylestad Rsat VCEsat ICsat Rcutoff VCC ICEO © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Switching Time Transistor switching times: t on t r t d t off t s t f Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary Troubleshooting Hints Approximate voltages VBE .7 V for silicon transistors VCE 25% to 75% of VCC Test for opens and shorts with an ohmmeter. Test the solder joints. Test the transistor with a transistor tester or a curve tracer. Note that the load or the next stage affects the transistor operation. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved Ch.4 Summary PNP Transistors The analysis for pnp transistor biasing circuits is the same as that for npn transistor circuits. The only difference is that the currents are flowing in the opposite direction. Electronic Devices and Circuit Theory Boylestad © 2013 by Pearson Higher Education, Inc Upper Saddle River, New Jersey 07458 • All Rights Reserved