Upper Saddle River, New Jersey 07458 • All rights reserved. Electronic Devices
and Circuit Theory, 10/e. Robert L. Boylestad and Louis Nashelsky. A/Lectr.
Electronics II Lecture 3(a): Transistor Bias Circuits
A/Lectr. Khalid Shakir Dept. Of Electrical Engineering College of Engineering Maysan University
Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
Electronics II- Lecture 3(a)/1st Semester 013/014
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Introduction • As you learned in Lecture 2, a transistor must be properly biased in order to operate as an amplifier. • DC biasing is used to establish fixed dc values for the transistor currents and voltages called the dc operating point or quiescent point (Q-point). • In this Lecture, several types of bias circuits are discussed. • This material lays the groundwork for the study of amplifiers, and other circuits that require proper biasing.
Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
Electronics II- Lecture 3(a)/1st Semester 013/014
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Copyright @2013 by Dept. of by Electrical Copyright ©2009 PearsonEngineering, Education, Inc. College of Engineering, Maysan University Upper Saddle River, New Jersey 07458 • All rights reserved.
Purpose of the DC Biasing Circuit • To turn the device “ON” • To place it in operation in the region of its characteristic where the device operates most linearly, i.e. to set up the initial dc values of IB, IC, and VCE
Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
Electronics II- Lecture 3(a)/1st Semester 013/014
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Copyright @2013 by Dept. of by Electrical Copyright ©2009 PearsonEngineering, Education, Inc. College of Engineering, Maysan University Upper Saddle River, New Jersey 07458 • All rights reserved.
Given circuit
DC equivalent circuit
MATHMAICAL ANALYSIS Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
GRAPHICAL ANALYSIS Electronics II- Lecture 3(a)/1st Semester 013/014
Page 4-12
Copyright @2013 by Dept. of by Electrical Copyright ©2009 PearsonEngineering, Education, Inc. College of Engineering, Maysan University Upper Saddle River, New Jersey 07458 • All rights reserved.
Various operating points within the limits of operation of a transistor.
Linear active operation region
GRAPHICAL ANALYSIS Saturation operation region
Cut off operation region Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
Electronics II- Lecture 3(a)/1st Semester 013/014
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Copyright @2013 by Dept. of by Electrical Copyright ©2009 PearsonEngineering, Education, Inc. College of Engineering, Maysan University Upper Saddle River, New Jersey 07458 • All rights reserved.
The Transistor as an Amplifier
Amplification of a relatively small ac voltage can be had by placing the ac signal source in the base circuit. Recall that small changes in the base current circuit causes large changes in collector current circuit. The ac emitter current : Ie ≈ Ic = Vb/r’e The ac collector voltage : Vc = IcRc Since Ic ≈ Ie, the ac collector voltage : Vc ≈ IeRc The ratio of Vc to Vb is the ac voltage gain : Av = Vc/Vb Substituting IeRc for Vc and Ier’e for Vb : Av = Vc/Vb ≈ IcRc/Ier’e The Ie terms cancel : Av ≈ Rc/r’e Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
Electronics II- Lecture 3(a)/1st Semester 013/014
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Copyright @2013 by Dept. of by Electrical Copyright ©2009 PearsonEngineering, Education, Inc. College of Engineering, Maysan University Upper Saddle River, New Jersey 07458 • All rights reserved.
The Transistor as an Amplifier Example: Determine the voltage gain and the ac output voltage in Figure if r’e = 50 Ω.
The voltage gain : Av ≈ Rc/r’e = 1.0 kΩ/50 Ω = 20 The ac output voltage : AvVb = (20)(100 mV) = 2 V
Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
Electronics II- Lecture 3(a)/1st Semester 013/014
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Copyright @2013 by Dept. of by Electrical Copyright ©2009 PearsonEngineering, Education, Inc. College of Engineering, Maysan University Upper Saddle River, New Jersey 07458 • All rights reserved.
The Transistor as a Switch
A transistor when used as a switch is simply being biased so that it is in cutoff (switched off) or saturation (switched on). Remember that the VCE in cutoff is VCC and 0V in saturation. Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
Electronics II- Lecture 3(a)/1st Semester 013/014
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Copyright @2013 by Dept. of by Electrical Copyright ©2009 PearsonEngineering, Education, Inc. College of Engineering, Maysan University Upper Saddle River, New Jersey 07458 • All rights reserved.
The Transistor as a Switch Example:(a) For the transistor circuit in Figure, what is VCE when VIN = 0 V? (b) What minimum value of IB is required to saturate this transistor if βDC is 200? Neglect VCE(sat). (c) Calculate the maximum value of RB when VIN = 5 V.
(a) When VIN = 0 V VCE = VCC = 10 V (b) Since VCE(sat) is neglected,
I C ( sat )
VCC 10 V 10 mA RC 1.0 k
I B (min)
I C ( sat ) DC
10 mA 50 A 200
(c) When the transistor is on, VBE ≈ 0.7 V. VRB = VIN – VBE ≈ 5 V – 0.7 V = 4.3 V Calculate the maximum value of RB
RB (max)
VRB I B (min)
4.3V 86 k 50 A
Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
Electronics II- Lecture 3(a)/1st Semester 013/014
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Copyright @2013 by Dept. of by Electrical Copyright ©2009 PearsonEngineering, Education, Inc. College of Engineering, Maysan University Upper Saddle River, New Jersey 07458 • All rights reserved.
The DC Operating Point - Q
Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
Electronics II- Lecture 3(a)/1st Semester 013/014
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Copyright @2013 by Dept. of by Electrical Copyright ©2009 PearsonEngineering, Education, Inc. College of Engineering, Maysan University Upper Saddle River, New Jersey 07458 • All rights reserved.
The DC Operating Point - Q
Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
Electronics II- Lecture 3(a)/1st Semester 013/014
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Copyright @2013 by Dept. of by Electrical Copyright ©2009 PearsonEngineering, Education, Inc. College of Engineering, Maysan University Upper Saddle River, New Jersey 07458 • All rights reserved.
The DC Operating Point - Q
1.2 3.4 Copyright @2013 by Dept. of Electrical Engineering, Electronic Devices and Circuit Theory, 10/e College of Engineering, Maysan University Robert L. Boylestad and Louis Nashelsky
5.6 Electronics II- Lecture 3(a)/1st Semester 013/014
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Copyright @2013 by Dept. of by Electrical Copyright ©2009 PearsonEngineering, Education, Inc. College of Engineering, Maysan University Upper Saddle River, New Jersey 07458 • All rights reserved.
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