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Chapter 6
Basic Electronic Devices and Circuits
EE 111
Electrical Engineering
Majmaah University
2nd Semester 1432/1433 H
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd1
BJT Amplifiers
Introduction
• The things you learned about biasing a transistor in Chapter 5 are now applied in this chapter where BJT circuits are used as small-signal
amplifiers.
• The term small-signal refers to the use of signals that take up a
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd2
relatively small percentage of an amplifier’s operational range.
•Additionally, you will learn how to reduce an amplifier to an equivalentdc and ac circuit for easier analysis.
AC Quantities
AC quantities are indicated with a lowercase italic
subscript;
rms values are assumed unless otherwise stated.
Vce
VceVce
rmsavg
V
rms: 1/√2 ≈ 0.707avg: 2/π ≈ 0.6366
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd3
unless otherwise stated.ce
Vce
vce
VCE
00
t
Vce
The figure shows an example of a specific waveform for the collector-emitter voltage. Notice the DC component is VCE and the ac component is Vce.
Resistance is also identified with a lowercase subscript when analyzed from an ac standpoint.
Linear Amplifier
A linear amplifier produces a replica of the input signal at the output.
+VCC
Vb
Ic
ICQ
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd4
For the amplifier shown, notice that the voltage waveform is inverted between the input and output but has the same shape.
RCR1
RER2 RL
C2
b
IbRs
ICQ
Vce
VCEQ
Vs
C1IBQ
VBQ
AC Load Line
Operation of the linearamplifier can be illustrated using an ac load line.
0 VCE
I b
IC
Ic
Q
Vce
ICQ
I BQ
VCC/(RC||RL +RE)
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd5
The ac load line is differentthan the dc load line because a capacitor looks open to dcbut effectively acts as a short to ac.
Thus the collector resistor RC appears to be in parallelwith the load resistor RL.
VCEQ
ac short; ZC = 1/( j ω C )
ac ground
Solution:
Projections on the graph of Figure 6–4 show the collector current Ic varying from 6 mA to 4 mA for a peak-to-peak
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd6
c
to 4 mA for a peak-to-peak value of 6 – 4 = 2 mAand the collector-to-emitter voltage Vce varying from 1 V to 2 V for a peak-to-peak value of 2 – 1 = 1 V.
Transistor AC Model
very small
very large
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd7
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd8
Transistor AC Model
The five resistance parameters (r-parameters) can be used for detailed analysis of a BJT circuit.
For most analysis work, the simplified r-parameters give good results.
The simplified r-parameters are shown in relation to the transistor
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd9
βacIb
C
B
E
βacIb
C
B
E
Ib ′re
′re
shown in relation to the transistor model.An important r-parameter is re'. It appears as a small ac resistance between the base and emitter.
'
E
25 mVer
I=
assuming an abrupt junction between the nand p regions, and an ambient temperature of 20°C
QuizQuizQuiz
Q. The equation for finding the ac emitter resistance of a BJT is
a. '
B
25 mVer
I=
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd10
b.
c.
d.
B
'
E
'
B
'
E
25 mV
0.7 V
0.7 V
e
e
e
rI
rI
rI
=
=
=
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd11
IE = 25 mV / 8 Ω = 3.125 mA
Comparison of βac to βDC
Curve of IC versus IB is nonlinear!
© 2012 Pearson Education. Upper Saddle River, NJ, 07458. All rights reserved.
Electronic Devices, 9th edition
Thomas L. Floyd12
slope slope
h (hybrid) parameters
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Electronic Devices, 9th edition
Thomas L. Floyd13
Relationships of h Parameters and r Parameters
forward current gain, common-base
forward current gain, common-emitter
voltage feedback ratio, common-emitter
output admittance (conductance), common-emitter
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Electronic Devices, 9th edition
Thomas L. Floyd14
input impedance (resistance), common-emitter