Common Base and Common Collector Amplifiers
Reminder: BJT Small Signal Model
ib
r
E
B Cic
ib
ie
Definition of input and output voltage and current directions
Amplifier
iin iout
vin vout
Common Base Amplifiers
• Uses the same stabilised bias scheme as CE configuration.
Common Base Amplifiers
• Uses the same stabilised bias scheme as CE configuration.
• The base is grounded for a.c. signals using a large capacitor.
• Signal input at the emitter (between the emitter and the common base)
• Signal output at the collector (between the collector and the common base)
Common Base Amplifiers
+VCC
GND
RE
R1
R2 CB vin
RC
voutRL
Common Base Amplifiers
• We will ignore the a.c. coupled load RL
R1
R2 CB
RC
vin
+VCC
GND
RE
vout
CB Amplifier, a.c. equivalent.
E
B
C
ib
ic= iout
RC
r
vin RE
ie
iin
vout
ib
Common Base Amplifier
1i-
i
i
i gain Current
e
c
in
out
Common Base Amplifier
π
c
in
out
r
R β
v
v gain Voltage
Ccout R i- v
β
cπbπin
ir- i r - v
Common Base Amplifier
• Input impedance to transistor
)(1
r
i-
ri-
i-
v
e
b
e
in
Common Base Amplifier
E
B
C
ib
ic= iout
RC
r
vin RE
ie
iin
vout
ib
vin
Common Base Amplifier
• Input impedance to stage (whole amplifier)
)(1
r //R
E
Common Base Amplifier
E
B
C
ib
ic= iout
RC
r
vin RE
ie
iin
vout
ib
Common Collector Amplifier
• Signal input between base and common collector.
• Signal output between emitter and common collector.
Common Collector Amplifier
VCC
R1
R2RE
vin vout
CC Amplifier, a.c. equivalent circuit
vs
RS
ib
r
B
C
ib io
REvoutRB = R1//R2vin
ieE
Common Collector Amplifier
bcbe iiii )1(
]β)R(1[riRiriv Eπ bEeπ bin
Common Collector Amplifier
• Transistor input impedance =
]β)R(1[r iv R Eπ
b
in ib
Common Collector Amplifier
E bE eout β)R(1iRiv
]R )(1 [r i
R i β)(1
v
v gain Voltage
E πb
Eb
in
out
]β)R(1[riv Eπ bin
Common Collector Amplifier
1]R β)(1 [r
R β)(1gain Voltage
E π
E
)1(i
i
i
igain Current
b
e
in
out
Common Collector Amplifier
• To find the output impedance we note that:
• Hence
πb B Sbout r i-R// R i- v
lowβ)(1
)r R// (R
i
v R
πB S
e
outout
Common Collector Amplifier
• The voltage gain is slightly less than unity.
• The CC circuit is sometimes known as an emitter follower since the output voltage at the emitter follows the input voltage at the base.
Common Collector Amplifier
• The prime function of this circuit is to connect a high resistance source to a low resistance load (“a buffer amplifier”).
• Since voltage gain is approximately one this is achievable as the current gain is high.