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BJT AmplifiersLE 340
BJT Amplifiers outline
• Review BJT basic concepts• Discuss linear amplifier for small time-varying signal• Basic transistor amplifier circuits
• Common-emitter amplifier• Emitter-follower amplifier• Common-base amplifier
• Multistage amplifiers
BJT review
ααβ
αβ
β
−=
=+=
=+=
1
)1(
EC
BE
BC
BCE
iiii
iiiii
BJT amplifier operates in the “Active” mode• Emitter-base : Forward (npn)• Collector-base: Reverse (npn)
BJT biasing review
• Bias stable = Q-point not affected by β
Linear amplifiers
Bipolar linear amplifier
Common-emitter with time-varying input vs
Small signal amplification
bBT
beBQB iI
VvIi +=+≅ )1(
Small signal -> AC and DC add linearly!
Common-emitter ac equivalent circuit
Setting all DC currents and voltages to ZERO
DC vs AC elementsElement DC Model AC Model
Resistor R R
Capacitor Open C
Inductor Short L
Diode +Vγ, rf – rd = VT/ID
Independent Constant Voltage Source
+ VS - Short
Independent Constant Current Source
IS Open
Small-Signal Hybrid π Model
β
β
π
π
=
=
=
rgIVr
VI
g
m
CQ
T
T
CQm
Small-Signal Equivalent Circuit Using Common-Emitter Current Gain
Problem-Solving Technique: BJT AC Analysis1. Analyze circuit with only dc sources to find Q
point.
2. Replace each element in circuit with small-signal model, including the hybrid π model for the transistor.
3. Analyze the small-signal equivalent circuit after setting dc source components to zero.
Common Emitter equivalent circuit
))((B
Cmv RrrRgA+
−=π
π
Exercise: Common-emitter with emitter resistor
Common-Collectoror Emitter-Follower Amplifier
oEo rRrRβ
π
+=
1
)())(1(
))(1(
))(1(
21
Si
i
Eo
Eov
ibi
Eoib
RRR
RrrRr
A
RRRR
RrrR
++++
=
=
++=
ββ
β
π
π
Small-Signal Equivalent Circuit:Emitter Follower
)())(1(
))(1(
))(1(
21
Si
i
Eo
Eov
ibi
Eoib
RRR
RrrRr
A
RRRR
RrrR
++++
=
=
++=
ββ
β
π
π
Output Resistance:Emitter Follower
oEo rRrRβ
π
+=
1
Common-base amplifier
Common-base equivalent circuit
]1
)[(
)(
ELC
Cmi
LCmv
RrRR
RgA
RRgA
βπ
++=
=
Common Emitter Cascade Amplifier
Small-Signal Equivalent Circuit:Cascade Amplifier
Darlington Pair
21ββ≅iA
Cascode Amplifier
Small-Signal Equivalent Circuit:Cascode Amplifier
)(1 LCmv RRgA −≅