Electronic Circuits ELCT604
(Spring 2020)
Chapter 2 (Cont.)
BJT AmplifiersDr. Eman Azab
Assistant Professor
Office: C3.315
E-mail: [email protected]
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
1
Analog Voltage AmplifiersCircuit Design and Configurations
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
2
Objective
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
3
Implementing a voltage amplifier Circuit on the
transistor level
What are the Specifications for an Ideal Voltage Amp.?
Infinite Input Resistance: Rin
Infinite Voltage Gain: Avo
Finite Output Resistance (Short Circuit): Rout
'out
vo
in
vA
v
outv
sig
vA
v
in Lv vo
sig in out L
R RA A
R R R R
Signals in Amplifier Circuits
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
4
Information signals that we want to amplify must be AC
signal
However, we will have three types of signals in the circuit.
Thus; we need to distinguish between them by symbol:
Type of Signal Signal Symbol Example
DC SignalsCapital Letter and
SubscriptVBE, VCE, IC
AC Signals
(Small Signals)
Small Letter and
Subscriptvbe ,vce, ic
Instantaneous
(Large Signals)
Small Letter and
Capital SubscriptvBE, vCE, iC
BJT Modes of OperationElectrical Equations of BJT I-V characteristics
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
5
Mode BEJ BCJ Equations Condition
Cutoff Reverse Reverse 𝐼𝐶 = 𝐼𝐸 = 𝐼𝐵=0𝑉𝐵𝐸 < 0.7𝑉𝐵𝐶 < 0.5
Active
(Forward)
ForwardReverse
𝑉𝐵𝐸 = 0.7𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵𝐼𝐶 = 𝛽𝐹𝐼𝐵= 𝛼𝐹𝐼𝐸
𝛼𝐹 =𝛽𝐹
1 + 𝛽𝐹
𝑉𝐵𝐶 < 0.5Or
𝑉𝐶𝐸 > 0.2
Saturation
Forward
Forward
𝑉𝐵𝐸 = 0.7𝑉𝐵𝐶 = 0.5𝑉𝐶𝐸 = 0.2𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵
𝐼𝐶 < 𝛽𝐹𝐼𝐵
Reverse
Active
Reverse
Forward
𝑉𝐵𝐶 = 0.5𝐼𝐶 = 𝐼𝐸 + 𝐼𝐵𝐼𝐸 = 𝛽𝑅𝐼𝐵= 𝛼𝑅𝐼𝐶
𝛼𝑅 =𝛽𝑅
1 + 𝛽𝑅
𝑉𝐵𝐸 < 0.7
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
6
Q
Q
BJT NPN Modes of Operation
IC versus VCE
The Early effect
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
7
CEBEC S
T A
vVi I exp( ) 1
V V
C Ao
CE C
i Vr
v I
Q
BJT Active mode I-V CKHs
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
BJT Large Signal Model in
Active Mode
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
8
Q
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
BJT Large Signal Analysis
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
9
Voltage Amplifier using BJT
Assume that we have instantaneous input voltage signal ‘vI’
The instantaneous output voltage ‘vO’ is measured from collector
referring to the ground
Using KVLs:
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
𝑣O = VCC − 𝑖CRC
𝑣I = 𝑣BE
𝑣O = 𝑣CE
BJT Large Signal Analysis
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
10
Voltage Amplifier using BJT
The transistor mode of operation depends on the value of ‘vI’
Cutoff Mode
Active Mode
Saturation Mode
0 ≤ 𝑣I ≤ 0.5
𝑣O = VCC
0.5 ≤ 𝑣I ≤ 𝑣BE,sat 𝑣O > 𝑣CE,sat
𝑣O = VCC − ISRCexp𝑣IVT
𝑣I ≥ 𝑣BE,sat
𝑣O = 0.2
𝑖C = 0
𝑖C = ISexp𝑣BEVT
BJT as an Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
11
We can easily separate DC and AC Signals (Superposition)
Under the assumption that the AC signal amplitude is very small, such
that the transistor’s mode will remain the same for the complete cycle
BJT must work in Active Mode to avoid signal distortion
DC Sources are used to make sure BJT operates in Active
mode
Input terminals of the amplifier are Base/Emitter
Output terminals of the amplifier are Collector/Emitter
Equivalent Circuit for Small Signal Analysis can be derived
𝑣BE = VBE + 𝑣𝑏𝑒 𝑖C = ISexpVBE + 𝑣𝑏𝑒
VT
BJT as an Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
12
Assume BJT is in active mode & vbe<<VT
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
𝑖C = ISexpVBE + 𝑣𝑏𝑒
VT
𝑖C = ISexpVBEVT
exp𝑣𝑏𝑒VT
𝑖C ≅ IC 1 +𝑣𝑏𝑒VT
𝑔m =𝜕𝑖C𝜕𝑣𝑏𝑒
=ICVT
𝑖C = 𝑔m𝑣𝑏𝑒 = 𝛽𝑖b
BJT Small Signal Model
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
13
We can place a resistance between Base and Emitter to
have a path for the base current
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
𝑖C = 𝑔m𝑣𝑏𝑒 = 𝛽𝑖b 𝑟π =𝑣𝑏𝑒𝑖b
=β
gm=VTIB
BJT Small Signal Model
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
14
Note that: Early effect can be taken into consideration
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
𝑟o =VAIC
Analysis of BJT Amplifiers
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
15
Objective: Calculate the voltage gain, Input and Output
Resistances
1. Determine the DC operating Point (Deactivate AC signals)
2. Calculate the small signal model parameters: gm, rπ
3. Replace the BJT with its small signal model (DC sources
are deactivated)
4. Analyze the circuit to calculate the voltage gain, Input
and Output Resistances
BJT Amplifiers
Configurations
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
16
Common Emitter Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
17
Objective: Calculate the voltage gain, Input and Output
Resistances
Input terminal Base
Output Terminal Collector
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Emitter Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
18
1. Calculate the DC Current
2. Calculate gm and rπ
IC =β
1 + βI ≅ I
Common Emitter Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
19
3. Draw the equivalent small signal model (Include ro if
given)
Common Emitter Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
20
3. Draw the equivalent small signal model (Include ro if
given)
4. Calculate the gain, input and output Resistance
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Av =𝑣O𝑣sig
= −gm(ro ∕∕ RC ∕∕ RL)RB ∕∕ rπ
RB ∕∕ rπ + Rsig
Rin = RB ∕∕ rπ Rout = ro ∕∕ RC ∕∕ RL
Common Emitter Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
21
Notes on Common Emitter Configuration:
Inverting Amplifier
Gain is greater than unity
High Input Resistance
High Output Resistance
Common Base Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
22
Objective: Calculate the voltage gain, Input and Output
Resistances
Input terminal Emitter
Output Terminal Collector
Common Base Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
23
Voltage gain, input and Output Resistance (ro is
neglected)
Av =𝑣O𝑣sig
=gm(RC ∕∕ RL)
1 +RS
(RE ∕∕rπ
1 + β)
Rin = RS + (RE ∕∕rπ
1 + β)
Rout = RC ∕∕ RL
Common Base Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
24
Notes on Common Base Configuration:
Non-Inverting Amplifier
Gain is greater than unity
Low Input Resistance
High Output Resistance
Common Collector Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
25
Objective: Calculate the voltage gain, Input and Output
Resistances
Input terminal Base
Output Terminal Emitter
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.
Common Collector Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
26
Voltage gain, input and Output Resistance
Av =𝑣O𝑣sig
=1 + β (ro ∕∕ RL)
rπ + 1 + β (ro ∕∕ RL) 1 +RsigRB
+ Rsig
Common Collector Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
27
Voltage gain, input and Output Resistance
Rin = RB ∕∕ rπ + 1 + β ro ∕∕ RL
Rout = ro ∕∕rπ + RB ∕∕ Rsig
1 + β
Common Collector Amplifier
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
28
Notes on Common Collector Configuration:
Non-Inverting Amplifier
Gain is less than unity
Emitter Follower (Buffer)
High Input Resistance
Low Output Resistance
Common Emitter with RE
Dr. Eman Azab
Electronics Dept., Faculty of IET
The German University in Cairo
29
Exercise:
Find the Voltage gain, input and Output Resistance
Figure from Sedra/Smith, Copyright © 2010 by Oxford University Press, Inc.