Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

C H A P T E R 6

Basic BJT Amplifier

Configurations

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

The three basic configurations of BJT amplifier. The biasing

arrangements are not shown.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Characterizing Amplifiers

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

The Common-Emitter (CE) Amplifier

)||)(( 00 rRvgv Cm π−=

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

)arg(

)||(

00

00

00

elisrRR

rRgA

v

vAvv

C

Cmv

i

vi

≈

−=

≡= π

)||||(

)||||(

0

0

0

rRRgRr

r

v

vG

rRRgA

LCm

sigsig

v

LCmv

+−=≡

−=

π

π

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Figure 6.51 Performing the analysis directly on the

circuit with the BJT model used implicitly.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Example 6.17

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Figure 6.52 The CE amplifier with an emitter resistance Re; (a) Circuit without

bias details; (b) Equivalent circuit with the BJT replaced with its T model.

))(1( eein RrR ++= β

Co RR =

em

Cm

e

e

Cm

voRg

Rg

r

R

RgA

+−≅

+

−=1

1

CE amplifier with an

emitter resistance Re

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

))(1( eein RrR ++= β

Co RR =

em

Cm

e

e

Cm

voRg

Rg

r

R

RgA

+−≅

+

−=1

1

-Rin increased by (1+gmRe).

-Gain from base to collector reduced by (1+gmRe).

-For the same nonlinear distortion, the input signal vi can be

increased by the factor (1+gmRe).

-The overall voltage gain is less dependent on the value of β.

-High frequency response is significantly improved.

CE amplifier with an

emitter resistance Re

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Example 6.18

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Figure 6.53 (a) CB amplifier with bias details

omitted; (b) Amplifier equivalent circuit with

the BJT represented by its T Model.

Common-Base Amplifier

)||( LCmv RRgA =-Low input impedance

-High gain

-High output impedance

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Common-Collector Amplifier or Emitter Follower

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Illustrating the need for a unity-gain buffer amplifier.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

eL

L

i

o

vrR

R

v

vA

+==

C

T

eI

Vr =

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Thevenin representation of the Emitter-Follower Output

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Example 6.19.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.

Microelectronic Circuits, Sixth Edition Sedra/Smith Copyright © 2010 by Oxford University Press, Inc.