Chapter 4 DC Biasing–BJTs
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Biasing Biasing: T Biasing: The DC voltages applied to a transistor in order to turn it on so that it can amplify the AC signal.
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Operating Point The DC input establishes an operating or quiescent point Q-point called the Q-point.
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 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 Saturation Region Operation Base–Emitter junction is forward biased Base–Collector junction is forward biased
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky DC Biasing Circuits Fixed-bias circuit Emitter-stabilized bias circuit Collector-emitter loop Voltage divider bias circuit DC bias with voltage feedback
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Fixed Bias
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky The Base-Emitter Loop From Kirchhoff’s voltage law: Solving for base current : +V CC – I B R B – V BE = 0
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Collector-Emitter Loop Collector current: From Kirchhoff’s voltage law:
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Saturation When the transistor is operating in saturation, current through the transistor is at its maximum possible value.
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Load Line Analysis I Csat CCCC I C = V CC / R C CE V CE = 0 V V CEcutoff CECC V CE = V CC C I C = 0 mA B Bwhere the value of R B sets the value of I B CECthat sets the values of V CE and I C The Q-point is the operating point: The end points of the load line are:
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Circuit Values Affect the Q-Point more …
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Circuit Values Affect the Q-Point more …
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Circuit Values Affect the Q-Point
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Emitter-Stabilized Bias Circuit Adding a resistor (R E ) to the emitter circuit stabilizes the bias circuit.
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Base-Emitter Loop From Kirchhoff’s voltage law: Since I E = ( + 1)I B : Solving for I B :
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Collector-Emitter Loop From Kirchhoff’s voltage law: Since I E I C : Also:
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 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.
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Saturation Level : V CEcutoff :I Csat : The endpoints can be determined from the load line.
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage Divider Bias This is a very stable bias circuit. any The currents and voltages are nearly independent of any variations in .
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Approximate Analysis Where I B << I 1 and I 1 I 2 : Where R E > 10R 2 : From Kirchhoff’s voltage law:
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Voltage Divider Bias Analysis Transistor Saturation Level Load Line Analysis Cutoff:Saturation:
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 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, .
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Base-Emitter Loop From Kirchhoff’s voltage law: Where I B << I C : Knowing I C = I B and I E I C, the loop equation becomes: Solving for I B :
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Collector-Emitter Loop Applying Kirchoff’s voltage law: I E + V CE + I ’ C R C – V CC = 0 Since I C I C and I C = I B : I C (R C + RE ) + V CE – V CC =0 Solving for V CE : V CE = V CC – I C (R C + R E )
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Base-Emitter Bias Analysis Transistor Saturation Level Load Line Analysis Cutoff:Saturation:
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Transistor Switching Networks Transistors with only the DC source applied can be used as electronic switches.
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Switching Circuit Calculations Saturation current: To ensure saturation: Emitter-collector resistance at saturation and cutoff:
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Switching Time Transistor switching times:
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky Troubleshooting Hints Approximate voltages –V BE .7 V for silicon transistors –V CE 25% to 75% of V CC 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.
Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey All rights reserved. Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky 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.