04.BJT-Biasing.pdf

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Bipolar Junction Transistors - II

(BJT-II )

Transistor Biasing



Output Characteristic of CE Transistor

IB = 0µASaturation region

Cutoff Region

Active Region

IC = βIB

ICEO

IC < βIB

IC has a slope

due to “ Early

Effect ”



Transistor in Cutoff Region

Both the BC and the BE junctions are reverse biased

IE=0 IC=ICO≈0 IB=-IC=-ICO≈0

Transistor in Saturation Region

Both the BC and BE junctions are forward biased

and IC<βIB

VBE≈0.7 V VBC≈0.6 V VCE≈0.1

When transistor is used as a switch, it will be forced

to operate either in the CUTOFF region or in the

SATURATION region.



Transistor in Active Region

The BE junction is forward biased but the BC

junction is reverse biased and IC≈βIB

VBE=0.7 V

Transistor is operated in this region when we want to

use it as an amplifier!

RB

RC

RE

50kΩ

8kΩ

1.8kΩ

VBB = 5V

VCC = 20Vβ = 50

IB

IC

IEExample



Example: Calculate all the relevant voltages and currents

50 51

5 0.70.0303

( 1) 50 51*1.8

50 * 0.0303 1.52 1.55

1.55 *1.8 2.79

2.79 0.7 3.49

20 1.52*8 7.84

7.84 2.79 5.05

c B B E c B B

BB BE

B

B E

C E

E E E

B E BE

C CC C C

CE C E

I I I I I I I

V V I mA

R R

I mA I mA

V I R V

V V V V

V V I R

V V V V

β

β

= = = + =

− −= = =

+ + +

= = =

= = =

= + = + =

= − = − =

= − = − =

Note that VBC = 3.49 -7.84 = -4.35 V, i.e. the B-C junction is reverse

biased, implying that the transistor is working in the Active Mode

The Bias Point is specified by (VCE, IC, IB)



To operate as an amplifier a transistor must be biased inthe active region. In that case, the transistor can amplify

a small ac signal faithfully.

Bias Point (or Quiescent Operating Point, i.e. Q-Point) is

decided by the DC voltages applied and the values of

the resistances used. (See earlier example.)

The Bias Point is specified by the three quantities VCE, IC

and IB. This point must be in the active region of thetransistor if we want to use the transistor (with that bias

point) as an amplifier.

Biasing the Transistor



RC

VBB

VCC+

-

vBE

+

-

VCE

IB

IE

IC

RB

CC CE

C

C

V V I

R

−

=

1CC

C CE

C C

V I V

R R

= − +

Plot of IC

vs. VCE

is called the ‘Load Line”

Biasing the Transistor

IC

VCE

BB BE

B

B

V V I

R

−

=

IB=0

IB1

IB2

IB4

IB5

IB6CC

C

V

R

CC V

Bias Point

Intersection of the load linewith the IC-VCE characteristic

for the given value of IBdecides the bias point.



Notation

VMN or VD DC voltage between M-N or at D

vmn or vd AC voltage between M-N or at D

vMN

or vD

(DC+AC) voltage between M-N or at D

Note the different ways in which small and capital letters are

used to denote DC, AC or DC+AC values

Similar notation used to represent current values



Transistor Circuitunder DC Condition

Transistor Circuit with AC Signal applied over

the DC bias

RC

VBB

VCC+

-

vBE=

VBE+vbe

+

-

vCE=

VCE+vce

iB = IB+ib

iE = IE+ie

iC = IC+ic

vi

RB

vo

+

-

RC

VBB

VCC+

-

vBE

+

-

VCE

IB

IE

IC

RB

VBE



iB= IB+ IbiE = IE+ ieiC = IC+ ic

IB, IC,IE – DC currents

ib, ic, ie – AC currentsiB, iC, iE – DC + AC currents

Similarly,

VBE, VCE – DC Voltages

vbe, vce – AC VoltagesvBE, vCE – DC+ AC Voltages

Transistor Circuit with AC Signal

RC

VBB

VCC+

-

vBE=VBE+vbe

+

-

vCE=

VCE+vce

iB = IB+ib

iE = IE+ie

iC = IC+ic

vi

RB

vo

+

-



Interpreting “ Transistor Gain” (between ib and ic) from the

characteristics of the transistor

iC = βiB

(IC+ic)= β(IB+ib)so

But IC= βIB

so ic= βib

AC

CurrentGain



Operating Point or Quiescent Point

VCC/RC

A good selection of the operating point Q is essential in order

that an amplifier circuit amplifies an A.C signal without anydistortion.

The selected point should be able to

accommodate the “output signal swing”without distortion.

Ideally, the biasing should be done in

such a way that the Q-point does not

change even if there are some changes

in β or in the temperature at which thedevice is operating



VCC

A.C output signal

A.C input signal

RB RC

C1

C2

VCC

RB

RC

EB

C+

-

VCE

VBE

+

-

+

IBIC

IE

DC Equivalent

C1 and C2 will

be open circuit

under DC

Transistor Biasing - Fixed Bias

( )

( )

0.7

0.7

0.7

CC

B

B

CC C B

B

C

CE CC C C CC CC

B

V I

R

V I I

R

RV V I R V V

R

β β

β

−=

−= =

= − = − −



VCC

RB

RC

RE

IC+IB

IC

IE

IB

E

B

C

VCC

A.C output signal

A.C input signal

RB

RC

C1

C2

RE

Emitter and Collector Feedback Bias

DC Equivalent

VCC = (IC+IB)RC + IBRB +VBE +(β+1)IBRE

Using IC= βIB, we get

and

( ) )R (R 1βR

VVI

ECB

BECCB

+++

−

=

VCE = VCC- (IC+IB)(RC+RE)

Q-Point



Emitter and Collector Feedback Bias

VCC

A.C output signal

A.C input signal

RB

RC

C1

C2

RE

Typically, β is large so RB<<(β+1)(RC+RE)

Therefore

CC BE

C B

C E

V V I I R R

β −= ≅ +

and

( 1)( )

CC BE

B

C E

V V I

R R β

−=

+ +

Note the desirable feature that IC

is independent of β as long as

we choose a transistor which has a large β. This is an example of

Bias Stabilization – making the bias point insensitive to β.

Can you give a qualitative argument as to why IC tends to get stabilized?



VCC

R1RC

RER2

IR1

IR2

IB

IC

IE

VCC

A.C output signal

A.C input signal

R1 RC

C1

C2

RER2

DC Equivalent

Voltage Divider Bias

It is convenient to replace the biasingcircuit at the base by its Thevenin’s

Equivalent using -

2 1 2Th Th

1 2 1 2

CC B

R R RV V R R

R R R R= = =

+ +



VCC

RTh

RC

RE

VTh

IB

IC

IE

B

E

C

Voltage Divider Bias 2 1 2Th Th

1 2 1 2

Th Th ( 1)

CC

B BE B E

R R RV V R

R R R R

V I R V I R β

= =

+ +

= + + +

For large β, IC independent of β

Th BE

Th BE

( 1) B

E

C

E

V V I

R

V V I

R

β

−=

+

−=

CE CC C C E E V V I R I R= − −

with IC=βIB and IE=(β+1)IB

Solve the above to get the Q-point wherethe transistor has been biased, i.e. IC, IBand VCE

This biasing also provides biasstabilization against changes in

β. It also provides stabilization

against changes in temperature

(not shown here).

04.BJT-Biasing.pdf

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