Using the Hybrid- Model

k 10

032.0350

mS 3225

8.0

mA 8.0

m

T

Cm

C

gr

V

Ig

I

rbb and ro are omitted (insignificant) RB represents parallel combination of RB1 and RB2

At high frequencies C1, C2 and C3 approximate short circuits.

Problem : CBC influences the input and output halves of the circuit

The Miller Effect (input capacitance)

outinC

outinC vvsC

Z

vvi

inout Avv inininC vAsCAvvsCi 1

AsCi

v

C

in

11

The Miller Effect (output capacitance)

inoutC

inoutC vvsC

Z

vvi

Avv outin /

outoutoutC vAsCAvvsCi /11/

AsCi

v

C

out

/111

The Miller Effect – Summary

AsCi

v

C

in

11

AsCiv

C

out

/111

Using the Miller Effect

kHz 770

21

||||21

inSinBSc CRCrRRf

100/11 & 1001100 BCOUTBCBEIN CCCCCA

Extending the Upper Cut-Off

Use a different transistor – lower CBC. Reduce the gain; CIN is proportional to

gain. Reduce the source resistance. Eliminate the Miller effect – use a

different amplifier configuration.

Common-Base Configuration

Common-emitter

amplifier

Common-base amplifier

Common-Base Quiescent Conditions

CCC

EC

EE

EREE

EBE

BB

RIV

II

RRV

II

VV

VI

15

5.1415

V 5.0V 5.0

00

i.e. exactly the same as common emitter amplifier.

Common-Base Voltage Gain

in

c

c

out

in

out

v

i

i

v

v

v

i.e. same as C-E but non-inverted.

CCCCC

out

c

out RRIII

V

i

v 15dd

d

d

ein

c

e

in

e

bec rv

i

r

v

r

vi

1

e

C

eC

in

out

r

R

rR

v

v

1

Common-Base Output Resistance

RCeout iii

iRC

ie

C

outRCout

cebe

R

vii

iiv

00

but

Cout

out Ri

v

Common-Base Input Resistance

incineRE iiiii

eE

in

eEin

e

in

E

in

e

be

E

in

cREin

rR

v

rRv

r

v

R

v

r

v

R

v

iii

||11

eeEin rrRr ||

High Frequency Effects

Neither CBC or CBE connects vin to vout.

There is, therefore, no Miller effect.

Cin = CBE

Cout = CBC

C-B vs. C-E Comparison

Identical quiescent conditions Identical voltage gain (except C-E

inverts) Identical output resistance Common-Base input impedance is very

low Common-Emitter suffers Miller effect

Summary

Common-emitter upper cut-off frequency is disappointingly low due, mainly, to the Miller effect.

Common-base configuration does not suffer Miller effect but has impractically low input impedance.

Solution : combine the two ?