Transcript Common-Collector (Emitter

Recall Lecture 14
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Introduction to BJT Amplifier
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Small signal or AC equivalent circuit parameters
Have to calculate the DC collector current by
performing DC analysis first
Common Emitter-Emitter Grounded
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Voltage gain
Current gain
Common-Collector (EmitterFollower) Amplifier
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Remember that for Common Collector
Amplifier,
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the output is measured at the emitter terminal.
the gain is a positive value
β = 100
VBE = 0.7V
VA = 80
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Perform DC analysis to obtain the value of IC
BE loop:
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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