Electronic Circuits For Mechatronics ELCT609
Lecture 6: Analysis of BJT Amplifiers (Cont.)
Dr. Eman Azab
Assistant Professor
Office: C3.315
E-mail: eman.azab@guc.edu.eg
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Objective
 Study of voltage Amplifiers using BJT
 Required:
Calculate theVoltage Gain: Avo
Calculate the Input Resistance: Rin (Thevinen at the Input Port)
Calculate the Output Resistance: Rout (Thevinen at the Output
Port)
A v  A vo
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R in
RL
R sig  R in R out  R L
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BJT Small Signal Model
 Amplifiers Analysis using Small signal Model
𝑖C = 𝑔m 𝑣𝑏𝑒 = 𝛽𝑖b
IC
𝑔m =
VT
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𝑣𝑏𝑒
𝛽
VT
𝑟π =
=
=
𝑖b
𝑔𝑚 IB
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
VA
𝑟o =
IC
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Analysis of BJT Amplifiers
 Objective: Calculate the Voltage gain, Input Resistance and
Output Resistance
 Solution Steps:
1.
2.
3.
4.
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Determine the DC operating Point (Deactivate AC signals & All
External Capacitors impedances are considered open Circuit)
Calculate the small signal model parameters: gm & rπ
Replace the BJT with its small signal model (DC sources are
deactivated & All External Capacitors impedances are considered
Short Circuit)
Analyze the circuit to calculate the voltage gain, Input and Output
Resistances
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
BJT Amplifier Configurations
Basic Configurations and their Characteristics
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Common Emitter Amplifier
 Objective: Calculate the voltage gain, Input and Output
Resistances
Input terminal Base
Output Terminal Collector
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Emitter Amplifier
1.
Calculate the DC Current
2.
Calculate gm and rπ
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
β
IC =
I≅I
1+β
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Emitter Amplifier
3.
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Draw the equivalent small signal model (Include ro if
given)
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Emitter Amplifier
Draw the equivalent small signal model (Include ro if
given)
4. Calculate the gain, input and output Resistance
3.
𝑣O
R B ∕∕ rπ
Av =
= −g m (ro ∕∕ R C ∕∕ R L )
𝑣sig
R B ∕∕ rπ + R sig
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
R in = R B ∕∕ rπ
R out = ro ∕∕ R C ∕∕ R L
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Emitter Amplifier
 Notes on Common Emitter Configuration:
Inverting Amplifier
 Gain is greater than unity
 High Input Resistance
 High Output Resistance

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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Common Base Amplifier
 Objective: Calculate the voltage gain, Input and Output
Resistances
Input terminal Emitter
Output Terminal Collector
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Common Base Amplifier
 Voltage gain, input and Output Resistance (ro is neglected)
𝑣O
g m (R C ∕∕ R L )
Av =
=
RS
𝑣sig 1 +
r
(R E ∕∕ π )
1+β
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
R in
rπ
= R S + (R E ∕∕
)
1+β
R out = R C ∕∕ R L
Common Base Amplifier
 Notes on Common Base Configuration:
Non-Inverting Amplifier
 Gain is greater than unity
 Low Input Resistance
 High Output Resistance

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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Common Collector Amplifier
 Objective: Calculate the voltage gain, Input and Output
Resistances
Input terminal Base
Output Terminal Emitter
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Collector Amplifier
 Voltage gain, input and Output Resistance
𝑣O
Av =
=
𝑣sig
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1 + β (ro ∕∕ R L )
rπ + 1 + β (ro ∕∕ R L )
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
R sig
1+
+ R sig
RB
Common Collector Amplifier
 Voltage gain, input and Output Resistance
R in = R B ∕∕ rπ + 1 + β ro ∕∕ R L
R out
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rπ + R B ∕∕ R sig
= ro ∕∕
1+β
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Common Collector Amplifier
 Notes on Common Collector Configuration:





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Non-Inverting Amplifier
Gain is less than unity
Emitter Follower (Buffer)
High Input Resistance
Low Output Resistance
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Common Emitter with emitter Resistance
 Exercise:
 Find theVoltage gain, input and Output Resistance
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Solved Mid-term Problems
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Problem 1

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The figure below shows a circuit with three inputs and one output with the
assumption that Q1 and Q2 are identical with β=50. The inputs A, B, and C could
be at one of the two voltage levels: 1.3V or 0V. The inputs A and B are connected to
the anodes of diodes D1 and D2 respectively. Assume a voltage drop of 0.6V for
forward-biased diode and base-emitter junction to calculate Vo in the following
cases: (indicate with reason clearly the state of each transistor)
1.
A=1.3V, B=0V, and C=0V. (Ans.: 4.9V)
2.
A=1.3V, B=1.3V, and C=1.3V. (Ans.: 0.2V)
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Problem 2

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For the circuit shown below, β = 100 and Vdiode-ON = 0.7V. Find Io and Rout.
(Hint: Use the DC analysis to get Io and AC analysis to get Rout).
 (Ans.: Io=21.76mA, Rout=∞)
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
Problem 3
In the circuit shown, the Zener diode has VZ = 6.8V and rZ = 20Ω
while the normal diode D has Vth = 0.7V. The input VS is a 20-V peakto-peak sinusoidal signal with an average value of 0V. Derive an
expression for the output signal VO over the range of the input signal
VS.
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Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo