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
10k
vs
= 100
VA = 100
100k
10k
10k
-10V
Lecture 15-3
Emitter Follower Example
2) Establish small signal model
10k
vs
100k
ib
10k
ro
10k
Lecture 15-4
Amplifier Input Resistance, Rib
3) Calculate input resistance
10k
vs
100k
i b
10k
ro
10k
Lecture 15-5
Transistors Input Resistance, Rib
Reflect impedances into base from emitter for simplified input resistance
calcuation
100k
Rib
3k
ib
10k
119k
10k
Lecture 15-6
Amplifier Input Resistance, Ri
RB is part of the amplifier circuit, and adds to the input resistance to the
transistor
Ri
100k
3k
ib
10k
119k
10k
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 10k, 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
100k
vi
ib
10k
ro
RL
Lecture 15-8
Emitter Follower --- Buffer
Further voltage division for the amplifier stage
Most easily seen using other small signal model
ie
vi
re
10k
ro
RL
Lecture 15-9
Analysis by Inspection
Experienced analog designers just analyze the circuit directly by inspection
+10V
10k
vs
100k
10k
10k
-10V
Lecture 15-10
Analysis by Inspection
What if RL is much less than RE?
+10V
10k
vs
10k
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
10k
vs
100k
i b
10k
119k
10k
Lecture 15-12
Current Gain
10k
vs
100k
i b
10k
119k
10k
Lecture 15-13
Output Resistance, Ro
4) Calculate output resistance
10k
100k
i b
10k
ro
Ro
Lecture 15-14
Output Resistance, Ro
Lecture 15-15
SPICE Results
dc solution
VCC
10V
+-
IC
839.882 A
RS
10E3
C2
1E-6F
+
Q1
Custom
IIN
0.000 pA
VINAC
+
0.000 pV
-
VSSIN
SIN
VIN
+
-758.300 mV
-
C1
1E-6F
+
RB
100E3
IO
0.000 pA
RE
10E3
VOUT
+
0.000 pV
-
RL
10E3
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
-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
-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 --- well 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
i 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
i e
ib
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
