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 −≅