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Jan 28, 2005EE171
LECTU
RE 7
•B
JT Small Signal A
nalysis •
BJT am
plifiers–
Com
mon Em
itter–
Com
mon C
ollector (Emitter Follow
er)–
Com
mon G
ate•
BJT as a digital logic sw
itch
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Jan 28, 2005EE171
Illustration of the Q-point base current IBQ , signal current i b (t), and total current iB (t).
Small Signal A
nalysis:•
After proper biasing, sm
all signal is injected.
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Jan 28, 2005EE171
Small-signal equivalent circuits for the B
JT.
Small Signal Equivalent C
ircuit•
It is convenient to represent a BJT w
ith its small
signal equivalent.
T Cm
C
T
V Ir
gI V
r=
==
ππ
ββ
,
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Jan 28, 2005EE171
.
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Jan 28, 2005EE171
Com
mon-em
itter amplifier.
Com
mon Em
itter Am
plifier•
C1 couples the signal source to the base of the transistor.
•C
2 couples the amplified signal at the collector to the load.
•C
E (Bypass) provides low
impedance for the A
C current.
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Jan 28, 2005EE171
Com
mon-em
itter amplifier.
•W
e can analyze our amplifier looking at its sm
all signal equivalent circuit.
•W
e can find the voltage gain (Av), input im
pedance (Zin) and output im
pedance (Zout) writing dow
n the node equations from
small signal equivalent circuit.
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Jan 28, 2005EE171
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Jan 28, 2005EE171
Com
mon-em
itter amplifier.
•To find the output im
pedance, the input source is replaced by a short circuit (w
hy?) and a test source is applied at the output port.
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Jan 28, 2005EE171
Com
mon-em
itter amplifier of Exam
ple 4.9.
•Exam
ple: To find the gain, input and output im
pedance of the circuit given below for β=100.
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Jan 28, 2005EE171
Emitter follow
er.
Emitter Follow
er•
In this case output port is at the emitter. (C
omm
on C
ollector).
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Jan 28, 2005EE171
Emitter follow
er.
•A
gain the most im
portant is a correct small signal
equivalent circuit. Then it is easily to figure out gain, input and output im
pedance of the amplifier.
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Jan 28, 2005EE171
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Jan 28, 2005EE171
•W
e follow the sam
e strategy to figure out the output im
pedance. Short the input source and apply a test source at the output port.
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Jan 28, 2005EE171
Figure 4.37a Emitter follow
er of Example 4.10.
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Jan 28, 2005EE171
Figure 4.37b & c Em
itter follower of Exam
ple 4.10.
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Jan 28, 2005EE171
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Jan 28, 2005EE171
Com
mon-base am
plifier.
Com
mon B
ase Am
plifier•
This time the input is fed from
the emitter and the
output port is the collector. Base is com
mon node.
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Jan 28, 2005EE171
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Jan 28, 2005EE171
BJT as a digital logic sw
itch
•In early tim
es of digital world B
JT was used
as logic switch in a logic fam
ily known as
resistor-transistor logic (RTL).
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Jan 28, 2005EE171
RTL inverter.
RTL inverter
•If input is zero, the B
JT operates in cutoff. No
current passes through Rc
and Vo equal to supply
voltage (input low-output high).
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Jan 28, 2005EE171
Load-line analysis of RTL inverter under no-load conditions.
Load-Line analysis of the inverter
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Jan 28, 2005EE171
Transfer characteristic for RTL inverter under no-load conditions.
Transfer characteristic of our inverter:
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Jan 28, 2005EE171
Three-input RTL N
OR
gate.
Another exam
ple: Three input Nor gate
•If anyone of the inputs is high, then the corresponding B
JT will conduct pulling the output
node down.
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