BJT AmplifiersBJT 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
α αβ
α β
BJT amplifier operates in the “Active” mode • Emitter-base : Forward (npn) • Collector-base: Reverse (npn)
BJT biasing review
Linear amplifiers
Small signal amplification
Common-emitter ac equivalent circuit
DC vs AC elements Element DC Model AC Model
Resistor R R
Capacitor Open C
Inductor Short L
Independent Constant Voltage Source
Small-Signal Equivalent Circuit Using Common-Emitter Current Gain
Problem-Solving Technique: BJT AC Analysis 1. 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
Common-Collector or Emitter-Follower Amplifier
)( ))(1(
))(1(
))(1(
21
Si
i
Eo
Darlington Pair
21ββ≅iA
Cascode Amplifier
)(1 LCmv RRgA −≅
Problem-Solving Technique: BJT AC Analysis
Common Emitter equivalent circuit
Common-Collector or Emitter-Follower Amplifier
Output Resistance:Emitter Follower
Darlington Pair
Cascode Amplifier