# Common Collector (Emitter Follower) Amplifier

```Common Collector (Emitter Follower) Amplifier
• Gain is never better than unity, however, has some desirable input and output
impedance characteristics --- acts as a buffer
VCC
RC
RS
C3
C1
C2
vs
RB
RE
RL
-VEE
Lecture 15-1
Common Collector (Emitter Follower) Amplifier
• Without RC there is no need for C3
VCC
RS
C1
C2
vs
RB
RE
RL
-VEE
Lecture 15-2
Emitter FollowerExample
• Calculate the voltage gain, current gain, input resistance and output resistance
• Assume capacitors are infinite
• 1) Calculate dc operating point
+10V
10kΩ
vs
β = 100
VA = 100
100kΩ
10kΩ
10kΩ
-10V
Lecture 15-3
Emitter Follower Example
• 2) Establish small signal model
10kΩ
vs
100kΩ
rπ
βib
10kΩ
ro
10kΩ
Lecture 15-4
Amplifier Input Resistance, Rib
• 3) Calculate input resistance
10kΩ
vs
100kΩ
rπ
βi b
10kΩ
ro
10kΩ
Lecture 15-5
Transistor’s Input Resistance, Rib
• “Reflect” impedances into base from emitter for simplified input resistance
calcuation
100kΩ
Rib
3kΩ
βib
10kΩ
119kΩ
10kΩ
Lecture 15-6
Amplifier Input Resistance, Ri
• RB is part of the amplifier circuit, and adds to the input resistance to the
transistor
Ri
100kΩ
3kΩ
βib
10kΩ
119kΩ
10kΩ
• Amplifier input resistance is limited by RB in this circuit. Why not make it
bigger?
Lecture 15-7
Emitter Follower --- Buffer
• Even though the load resistance is 10kΩ, the input resistance is much higher
• This is a desirable feature of a buffer amplifier, especially if RS is large, or RL
is small
• Is Rin big enough for our example? What is vi?
RS
vs
100kΩ
vi
rπ
βib
10kΩ
ro
RL
Lecture 15-8
Emitter Follower --- Buffer
• Further voltage division for the amplifier stage
• Most easily seen using other small signal model
αie
vi
re
10kΩ
ro
RL
Lecture 15-9
Analysis by Inspection
• Experienced analog designers just analyze the circuit directly by inspection
+10V
10kΩ
vs
100kΩ
10kΩ
10kΩ
-10V
Lecture 15-10
Analysis by Inspection
• What if RL is much less than RE?
+10V
10kΩ
vs
10kΩ
RL
-10V
Lecture 15-11
Emitter Follower Current Gain
• No voltage gain, but acts as a buffer to drive small impedance loads
• Provides good current amplification
10kΩ
vs
100kΩ
rπ
βi b
10kΩ
119kΩ
10kΩ
Lecture 15-12
Current Gain
10kΩ
vs
100kΩ
rπ
βi b
10kΩ
119kΩ
10kΩ
Lecture 15-13
Output Resistance, Ro
• 4) Calculate output resistance
10kΩ
0
100kΩ
rπ
βi b
10kΩ
ro
Ro
Lecture 15-14
Output Resistance, Ro
Lecture 15-15
SPICE Results
dc solution
VCC
10V
+-
IC
839.882 &micro;A
RS
10E3Ω
C2
1E-6F
+
Q1
Custom
IIN
0.000 pA
VINAC
+
0.000 pV
-
+
VSSIN
SIN
VIN
+
-758.300 mV
-
C1
1E-6F
+
RB
100E3Ω
IO
0.000 pA
RE
10E3Ω
VOUT
+
0.000 pV
-
RL
10E3Ω
+
VEE
-10V
Lecture 15-16
SPICE Results
voltage gain
0.0
0.1
0.2
0.3
0.4
0.5
time
0.6 ms
10
mV
0
-10
VINAC
VOUT
Lecture 15-17
SPICE Results
current gain
0.0
0.1
0.2
0.3
0.4
0.5
time
0.6 ms
1
uA
0
-1
IO
IIN
Lecture 15-18
Common Base Amplifier
• Gain is close to unity
• Very low input impedance --- great for impedance matching (e.g. 50
ohms)
• Large output impedance -- approximately RC
• Great high frequency behavior -- more on this later...
VCC
RC
vo
RS
C1
I
vs
-VEE
Lecture 15-19
Current Source Biasing
• Instead of resistors, a current source is used to bias the transistor in this
example
• Current sources can be used to bias other amplifier types too
• Building a current source is less expensive than building a resistor on an
IC --- we’ll be addressing IC issues such as this much more extensively
beginning with the next lecture
VCC
RC
vo
RS
C1
I
vs
-VEE
Lecture 15-20
Current Buffer
• Can be used as a current buffer
• Macromodel:
ii
vi
io
Ri
αi i
Ro
vo
Lecture 15-21
Small Signal Equivalent Circuit
• Since the base is grounded, we would probably select the T-model from
Lecture 13
ic
C
αi e
ib
B
ie
re
E
• What parameters are being ignored in this simplified T-model? And what is
the potential impact?
Lecture 15-22
Common Base Small Signal Analysis
Lecture 15-23
Common Base Small Signal Analysis
Lecture 15-24
```