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Chapter 4 DC Biasing–BJTs Biasing Biasing: The DC voltages applied to a transistor in order to turn it on so that it can amplify the AC signal. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Operating Point The DC input establishes an operating or quiescent point called the Q-point. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. The Three States of Operation • 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, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. 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, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Fixed Bias Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. The Base-Emitter Loop From Kirchhoff’s voltage law: +VCC – IBRB – VBE = 0 Solving for base current: IB VCC VBE RB Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Collector-Emitter Loop Collector current: I C I B From Kirchhoff’s voltage law: VCE VCC I C R C Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Saturation When the transistor is operating in saturation, current through the transistor is at its maximum possible value. VCC I Csat RC VCE 0 V Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Load Line Analysis The end points of the load line 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 sets the values of VCE and IC Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Circuit Values Affect the Q-Point more … Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Circuit Values Affect the Q-Point more … Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Circuit Values Affect the Q-Point Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Emitter-Stabilized Bias Circuit Adding a resistor (RE) to the emitter circuit stabilizes the bias circuit. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Base-Emitter Loop From Kirchhoff’s voltage law: VCC - I E R E - VBE - I E R E 0 Since IE = ( + 1)IB: VCC - I B R B - ( 1)I B R E 0 Solving for IB: IB VCC - VBE R B ( 1)R E Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Collector-Emitter Loop From Kirchhoff’s voltage law: I R V I R V 0 E E CE C C CC Since IE IC: VCE VCC – I C (R C R E ) Also: VE I E R E VC VCE VE VCC - I C R C VB VCC – I R R B VBE VE Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Improved Biased Stability Stability refers to a circuit condition in which the currents and voltages will 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, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Saturation Level The endpoints can be determined from the load line. VCEcutoff: ICsat: VCE VCC I C 0 mA Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky VCE 0 V IC VCC RC RE Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. 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, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Approximate Analysis Where IB << I1 and I1 I2 : VB R 2 VCC R1 R 2 Where RE > 10R2: VE RE VE VB VBE IE From Kirchhoff’s voltage law: VCE VCC I C R C I E R E IE IC VCE V CC I C (R C R E ) Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Voltage Divider Bias Analysis Transistor Saturation Level I Csat I Cmax V CC RC RE Load Line Analysis Cutoff: Saturation: VCE VCC I C 0mA Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky VCC IC RC RE VCE 0V Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. 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, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Base-Emitter Loop From Kirchhoff’s voltage law: VCC – I C R C – I B R B – VBE – I E R E 0 Where IB << IC: I' I I I C C B C Knowing IC = IB and IE IC, the loop equation becomes: VCC – I B R C I B R B VBE I B R E 0 Solving for IB: IB VCC VBE R B (R C R E ) Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. 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, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Base-Emitter Bias Analysis Transistor Saturation Level I Csat I Cmax V CC RC RE Load Line Analysis Cutoff: VCE VCC I C 0 mA Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Saturation: V CC I C R R C E VCE 0 V Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Transistor Switching Networks Transistors with only the DC source applied can be used as electronic switches. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Switching Circuit Calculations Saturation current: I Csat VCC RC To ensure saturation: I I B Csat dc Emitter-collector resistance at saturation and cutoff: R sat VCEsat I Csat R cutoff VCC I CEO Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. Switching Time Transistor switching times: t on t r t d t off t s t f Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. 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, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved. 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, 10/e Robert L. Boylestad and Louis Nashelsky Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.