Electronics II Lecture 3(a): Transistor Bias Circuits

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458 • All rights reserved.

Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

A/Lectr. Khalid ShakirDept. Of Electrical Engineering

College of EngineeringMaysan University

Copyright @2013 by Dept. of Electrical Engineering, Electronics II- Lecture 3(a)/1st Semester 013/014College of Engineering, Maysan University

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Electronics IILecture 3(a): Transistor Bias Circuits



Copyright @2013 by Dept. of Electrical Engineering,College of Engineering, Maysan University

Introduction

• As you learned in Lecture 2, a transistor must be properly biased inorder to operate as an amplifier.

• DC biasing is used to establish fixed dc values for the transistor currentsand 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 othercircuits that require proper biasing.


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Purpose of the DC Biasing Circuit


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• To turn the device “ON”• To place it in operation in the region of its

characteristic where the device operatesmost linearly, i.e. to set up the initial dcvalues of IB, IC, and VCE




MATHMAICALANALYSIS

GRAPHICALANALYSIS

DC equivalent circuit

Given circuit


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Saturationoperation region

Cut off operationregion

Linear activeoperation region

Various operating points within the limits of operation of a transistor.


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GRAPHICALANALYSIS





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The Transistor as an Amplifier

Amplification of a relatively small ac voltage can be had by placing the ac signal source inthe base circuit.

Recall that small changes in the base current circuit causes large changes in collector currentcircuit.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





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Example: Determine the voltage gain and the ac outputvoltage in Figure if r’e = 50 Ω.

The voltage gain :Av ≈ Rc/r’e = 1.0 kΩ/50 Ω = 20The ac output voltage :AvVb = (20)(100 mV) = 2 V

The Transistor as an Amplifier





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The Transistor as a Switch

A transistor when used as a switch is simply being biased so that itis in cutoff (switched off) or saturation (switched on). Rememberthat the VCE in cutoff is VCC and 0V in saturation.





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The Transistor as a SwitchExample:(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.

kA

V

I

VR

B

RB

B 8650

3.4

(min)(max)

(a) When VIN = 0 VVCE = VCC = 10 V

(b) Since VCE(sat) is neglected,

(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

AmAI

I

mAk

V

R

VI

DC

satCB

C

CCsatC

50200

10

100.1

10

)((min)

)(





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The DC Operating Point - Q





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1.2 3.4 5.6


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Electronics II Lecture 3(a): Transistor Bias Circuits