Common-Collector (Emitter
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Transcript Common-Collector (Emitter
Recall Lecture 14
Introduction to BJT Amplifier
Small signal or AC equivalent circuit parameters
Have to calculate the DC collector current by
performing DC analysis first
Common Emitter-Emitter Grounded
Voltage gain
Current gain
Common-Collector (EmitterFollower) Amplifier
Remember that for Common Collector
Amplifier,
the output is measured at the emitter terminal.
the gain is a positive value
β = 100
VBE = 0.7V
VA = 80
Perform DC analysis to obtain the value of IC
BE loop:
25IB + 0.7 + 2IE – 2.5 = 0
25IB + 0.7 + 2(1+ β)IB = 2.5
IC = βIB = 0.793 mA
Calculate the small-signal parameters
r = 3.28 k and ro = 100.88 k
β = 100
VBE = 0.7V
VA = 80
Small-signal equivalent circuit
of the emitter-follower amplifier
Output at emitter
terminal
Redraw the small signal equivalent circuit so that all signal
grounds connected together.
x
Vb
r = 3.28
k
RS = 0.5 k
Vb
RTH = 25
k
x
ro =
100.88
k
RE = 2 k
STEPS
OUTPUT SIDE
1.
Get the equivalent resistance at the output side, ROUT
2.
At node x, use KCL and get io in terms of ib where io = ib + ib
3.
Get the vo equation where vo = io ROUT
INPUT SIDE
4.
Find vb in terms of ib using supermesh
5.
Calculate Rib – input resistance seen from base: Rib = vb / ib
6.
Calculate Ri
7.
Get vb in terms of vs.
8.
Go back to vo equation and replace where necessary
vb
1.
Get the equivalent resistance at the output terminal,
ROUT ROUT = ro ||RE = 1.96 k
2.
At node x, use KCL and get io in terms of ib
io = ib+ib = ( 1+ )ib = 101 ib
3.
Get the vo equation where vo = io ROUT
vo = Rout ( 1+ )ib = 197.96 ib
x
vb
4.
Find vb in terms of ib using supermesh:
vb = ibr + io(Rout)
vb = ib (r +101 (1.96)) = 201.24 ib
5.
Calculate Rib Rib = vb / ib 201.24 k
6.
Calculate Ri Ri = RTH||Rib = 22.24 k
Small-Signal Voltage Gain
7. Get vb in terms of vs using voltage divider
Vb
vb =
22.24
vs
22.24 + 0.5
vb = 0.978vs
8. Go back to vo equation and replace where necessary
vo = 197.96 ib but ib = vb / Rib
vo = 197.96 (vb / Rib) = 197.96 ( 0.978 vs) = 0.9621 vs
201.24
AV = vo / vs = 0.9621