BJT as an Amplifier and Its Biasing - …storage.googleapis.com/wzukusers/user-19186504/documents...BJT as an Amplifier and Its Biasing . ... is measured by stability factor S

Slide 1 BITS Pilani, Dubai Campus

Lecture on Microelectronics Circuits Dr. Vilas

Microelectronic Circuits

BJT as an Amplifier and Its Biasing



Transfer Characteristics & Biasing



BJT Current-Voltage relationship

The collector current Emitter current

TVBEvsC eIi

TVBEvsB e

Ii

C

E

ii

TVBEvsE e

Ii

BC ii



BJT Current-Voltage relationship

We know that

BC ii

EC ii

EB ii

BB ii 1

1

1



BJT as amplifier Large signal operation

Consider a BJT , CE amplifier circuit

Case I : vI < 0.5 V

TRANSFER CHARACTERISTICS

CCCEo Vvv

IBEvv

Output Voltage

for vI < 0.5 V Transistor will be in cut-off , iC will be

negligible and vO will be equal to VCC




Case I I: Base input voltage vI ≥ 0.5 V


CCCCCEoiRVvv

Output Voltage

for vI > 0.5 V Transistor will be

in active mode , iC increases

and vO decreases.




Case III: Base input voltage vI increased more


CCCCCEoiRVvv

Output Voltage

Active mode ends when vo or vCE falls by

0.4V, at this point CBJ turns on and

transistor enters into saturation region

Further increase in vBE causes vCE to

decrease slightly.




Saturated BJT will exhibit very small resistance between Collector and Emitter (There is a low resistance path between collector and ground (Closed switch)

C

CEsatCC

CsatR

VVI



Amplifier Gain

•For a BJT amplifier to operate in as a linear amplifier , it has to be biased in active region. •Q (quiescent point) •Q point is characterized by and

BEvCEv

Ci

T

BE

Vv

sCeIi

CCCCCEIRVV



Amplifier Gain

The signal vi is superimposed on VBE.

The slope of the tangent to the curve at Q is the slope of the linear segment, which is the voltage gain of the amplifier for the small input signal around Q.

For vI=vBE

C

vv

S

T

v

vvI

o

v

ReIV

AThus

dv

dvA

T

BE

BEI

1

CECCCRC

T

RC

T

CC

v

VVRV

V

V

V

RIA

acrossdropvoltagedc



Amplifier Gain

•Observe that output is inverting, i.e., output is 180o out of phase with the input. •We can observe that for the larger gain, large voltage drop across RC •Operate on smaller vCE



Effect of vCE on Amplifier Gain

Lower VCE

Reduced gain and clipping on

the Negative half Cycle



Effect of vCE on Amplifier Gain

Higher VCE

Reduced gain and clipping on the positive half

Cycle



Graphical Analysis of An Amplifier

B

BBBE

B

B

BEBBBB

R

Vv

Ri

viRV

1

asitwritecanwe

Let us consider vi=0, We get

let us determine the dc biasing point for this circuit.




The dc base current IB can found out graphically as below.

B

BB

R

V




Operating point will be on the iC-vCE characteristics curve shown below for the dc base current IB

C

CC

CE

C

C

CCCCCE

R

Vv

Ri

RiVv

1

asitwritecanwe

Q point gives dc collector current

IC and dc collector to emitter voltage

VCE

C

CC

R

V




•Q point should lie on the active region

•Q-point should be located in a region where it

allows a reasonable signal swing as the input vi is

applied.



Graphical Analysis

•When the Vi is applied • Curve BEB vi



Graphical Analysis

•When the Vi is applied • Curve CEC vi



Effect of Q-point location on signal Swing

•Now consider the effect of Q-point location on signal Swing •Positive peak of the can not go beyond the , otherwise transistor enters the cutoff region. •Negative peak of the can not extend below the 0.3 v, otherwise transistor enters the saturation region.

Curve CEC vi

cev

CCV cev



Effect of RC on signal Swing

The load line position is based on the value of Rc

Lower value of Rc results into very large value of closed to , thus positive swing of will severely limited.

For large value of Rc, is too low and near to 0 value of thus negative swing of will severely limited.

CEV

CCV

CEV CEv

cev

cev



Biasing of BJT

Transistor used in amplifier circuit must have constant (DC) levels of collector, base and emitter current and constant terminal voltages.

The level of IC and VCE defines the transistor dc operating points or Quiescent point.

The circuit that provides this state is called as bias circuit.

Ideally, currents and voltages should remains constant in bias circuit. However these quantities are affected due to temperature change and transistor current gain.

The best bias circuits hold the current and voltage constant regardless of the variation in current gain and temperature.



Biasing of BJT

DC Load Line : It is a straight line drawn on the output characteristics . For Common Emitter configuration

Consider two cases and •For • For

These two point gives the DC Load Line

CCCCCE IRVV

0CI 0CEV0CI

CCCE VV

0CEV

CCCC RVI /



DC Load Line

CCC RV /

CCCE VV

Dc Load line has Two points

Point A at

Point B at

Q point lies on this DC load line at the center

CCC RV /

CCCE VV



Stability of the circuit

It is desirable and necessary to keep IC constant with respect to the

variations of ICBO . The collector leakage current ICBO is greatly influenced

by temperature changes. A rise of 10°C doubles the collector leakage current which may be as high as 0.2 mA for low powered germanium transistors.

The extent to which a biasing circuit is successful in achieving this goal is measured by stability factor S. The value of stability S should be 1.

CBO

C

dI

dIS



Basic Biasing Methods

Essentials of biasing:

(i) It should ensure proper zero signal collector current. (ii) It should ensure that VCE does not fall below 0.5 V for Ge transistors and 1 V for silicon transistors at any instant. (iii) It should ensure the stabilization of operating point.

Biasing Techniques

Collector -to-Base feedback resistor Bias

Two power Supply Bias (Emitter bias)

Voltage divider Bias

Constant Current source bias





1

1

BC

BECCE

BEBE

CE

BEBBCECC

RR

VVI

CurrentBiasEmitter

VRI

RI

VRIRIV





Advantages :

1. It provides the better stability less than 1- β

2. It is a simple method as it requires only one resistance RB.

Disadvantages : 1. stability factor is fairly high

2. This circuit provides a negative feedback which reduces the

gain of the amplifier




Two power Supply Bias (Emitter bias)

Advantages:

If RE >> R B then current will be independent of β

Thus it may provide better stability

disadvantages:

Emitter bias requited extra power which increases

the cost as well as power consumption

E

B

BEEE

E

RR

VVI

1




Voltage divider bias

This is the most widely used bias method for providing the biasing and stabilization of the transistor .

R1 and R2 connected to Vcc to provide the biasing and RE connected for stabilization.

The “voltage divider” name comes from the

voltage divider formed by R1 and R2.




Apply KVL at Base-Emitter circuit

Thevenin’s Equivalent Circuit

E

B

BEBB

E

E

B

EEBBBEBB

RR

VVI

II

RIRIVV

1

1




•To make IE insensitive to temperature variation and β value circuit required to satisfy two constraints

The variation in VBE is taken care by the large VBB

•When VBB is large , then voltage drop across RC and voltage VCB will be decreased. •As a rule of thumb, one designs for

1

B

E

BEBB

RR

VV Constraint 1

Constraint 2

CCCCCCCECBCCBBVaboutRIandVaboutVorVVaboutV

3

1

3

1)(,

3

1




Advantages:

RE provides negative feedback action that stabilizes the bias current.

Stability is closed to unity.

Disadvantages:

Negative feedback action reduces the gain of the amplifier.

Selection of RB is a challenging task.



Constant current source bias

Q1 and Q2 are matched transistors Q1 is connected as a diode by shorting collector and base. If Q1 and Q2 have high β values we van neglect their base currents.

Emitter current is independent of β and RB, thus

RB can be made large enabling high input

resistance at the base without effecting the bias.

R

VVVII

VsamehaveQandQ

R

VVVI

BEEECC

REF

BE

BEEECC

REF

)(

)(

21

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BJT as an Amplifier and Its Biasing - …storage.googleapis.com/wzukusers/user-19186504/documents...BJT as an Amplifier and Its Biasing . ... is measured by stability factor S