Analog & Digital Electronics
Course No: PH-218
Lec-10: CC and CB BJT Amplifiers
Course Instructors:
Dr. A. P. VAJPEYI
Department of Physics,
Indian Institute of Technology Guwahati, India
1
Common Collector Amplifier (Emitter Follower)
vout = vin - vBE
If vin is 2V, vout = 1.3V
If vin is 3V, vout = 2.3V.
vin
vout
Since vout follows exactly the vin
therefore, there is no phase inversion
between input and output
Since there is no resistance in collector circuit, therefore collector is
ac grounded.
A CC amplifier is like a heavily swamped CE amplifier with a
collector resistor shorted and output taken across emitter resistor.
The voltage gain of this amplifier is nearly one – the output “follows”
the input - hence the name: emitter “follower.
2
Common Collector Amplifier : Input Impedance
vin = ib rπ + iE RE
vin = ib ( β + 1)re' + ( β + 1)ib RE
vin = (re' + RE )( β + 1)ib
v
Zin(base) = in = β (re' + RE )
ib
'
e
Zi = [ R1 \ \ R2 \ \ β ( RE + r )]
Input Impedance is very large
3
Common Collector Amplifier : Output Impedance
If a high impedance source is connected to low impedance amplifier then most of
the signal is dropped across the internal impedance of the source. To avoid this
problem common collector amplifier is used in between source and CE amplifier. It
increases the input impedence of the CE amplifier without significant change in
input voltage.
4
Common Collector Amplifier : Voltage gain
vs = ( Rs + rπ )ib + ie RE
vs = ( Rs + rπ )ib + ( β + 1)ib RE
ib =
vs
( Rs + rπ ) + ( β + 1) RE
vo = ie RE = ( β + 1)ib RE
( β + 1) RE vs
vo =
( Rs + rπ ) + ( β + 1) RE
vo
( β + 1) RE
Av = =
vs ( Rs + rπ ) + ( β + 1) RE
Since
( Rs + rπ )
<< RE
( β + 1)
Av =
vo
RE
=
vs ( Rs + rπ ) + R
E
( β + 1)
Therefore, Av=1
5
Common Base Amplifier
RL = R 3|| R 7
AC input signal is applied across emitter and the output signal is
taken across the collector.
6
Common Base Amplifier: Performance Parameters
AC Equivalent Ckt
'
e
Input Impedance: Zi = RE \ \ r
Output Impedance:
Voltage Gain:
Since
'
e
r << RE
Therefore
Z in = re'
Zout = RC \ \ RL
vout ic ( RC \ \ RL )
Av =
=
vin
ie re'
( RC \ \ RL )
Av =
re'
Input voltage and output voltage is in same phase
7
Summary of BJT Amplifier Performance Parameters
Characteristic
Input impedance
Common
Base
Low
Common
Emitter
Medium
Common
Collector
High
Output impedance
Very High
High
Low
Phase Angle
0o
180o
0o
Voltage Gain
High
Medium
Low
Current Gain
Low
Medium
High
Power Gain
Low
Very High
Medium
The CB mode is generally only used in single stage amplifier circuits such as
microphone pre-amplifier or RF radio amplifiers due to its very good high
frequency response.
The Emitter follower configuration is very useful for impedance matching
applications because of the very high input impedance, in the region of hundreds
of thousands of Ohms, and it has relatively low output impedance.
8
Multistage Amplifier: Gain Calculation
Procedure:
AvT = Av1 Av 2 Av 3 K
1. Do dc analysis
AiT = Ai1 Ai 2 Ai 3 K
2. Find r’e for each stage
ApT = AvT AiT
3. Find rC for each stage
4. Using r’e and rC to find Av for
each stage
Input impedance of next stage is the load of current stage.
(Zin of next stage is RL of current stage)
9
Determine Av of the 1st stage, 2nd stage and overall voltage gain of the 2 stage
amplifier. Assume that r’e for the 1st stage is 19.8 Ω and r’e for the 2nd stage is
found to be 17.4 Ω. For the 2nd stage, hfe is 200.
10
Z in (base ) = ( h fe + 1) re′ = 201×17.4 = 3.497kΩ
Z in = R5 R6 Z in (base ) = 1.329kΩ
rC for the 1st stage can be found as
rC = R3 Z in = 5kΩ 1.33kΩ=1.05kΩ
rC
1.05kΩ
Av = − = −
= −53.03
′
re
19.8Ω
rC for the 2nd stage can be found as
rC = R7 RL = 3.33kΩ
Av for the 2nd stage is found as
rC
3.33kΩ
Av = − = −
= −191.38
re′
17.4Ω
AvT = Av1 Av 2 = ( −53.03)( −191.38 ) = 10.15 × 103
11
Multistage Amplifier: Characteristics
12