Electronics Lecture6 7 BJT - Aziz Mohaisen · Department of Electrical, Electronics and...

Korea University of Technology & Education

Department of Electrical, Electronics and Communications Engineering

Department of EECE

Bipolar Junction Transistors

Electronic Circuits

Manar Mohaisen Office: F208

Email: [email protected]



₪ Explain the Operation of the Zener Diode

₪ Explain Applications of the Zener Diode

₪ Explain the Operation and Applications of the Varactor Diode

₪ Explain the Operation and Applications of the Optical Diodes

₪ Other Types of Diodes

Review of Precedent Class



₪ Bipolar Junction Transistor (BJT)

₪ BJT Characteristics and Parameters

₪ BJT as an Amplifier and as a Switch

₪ Phototransistor and its Applications

₪ Transistor Categories and Packaging

₪ Discussions

Class Objectives



₪ BJT ■ Consists of three regions called emitter, base, and collector.

►The base region is thin and lightly doped.

►The emitter region is heavily doped wide region.

►The controller region is moderately doped wide region.

■ Consists of two pn junctions ►Namely, base-emitter junction and base-controller junctions.

Bipolar Junction Transistor

B(base)

C (collector)

n

p

n

Base-Collectorjunction

Base-Emitterjunction

E (emitter)

B

C

p

n

E

p

npn pnp



₪ BJT Operation ■ Base Current

►Electrons flow from the emitter to the base generating hole current.

● Electrons from the emitter recombine with the holes in the base.

►Since the base region is thin

● The number of holes is small.

● The number of electrons combining with the holes is also small.

● These valence electrons become free in the metallic base. This is called the base current.

Bipolar Junction Transistor – contd.



₪ BJT Operation – contd. ■ Collector Current

►Base region is thin.

● most of the electrons entering the base don’t combine with holes.

● The uncombined free electrons are attracted by the collector supply voltage.

● The uncombined free electrons move through the collector into the external circuit.




₪ Transistor Currents


E B CI I I= +

IE IE

IC

IB

IC

IBn

p

n

p

n

p

+

– +

–

–+

IE

IC

IB

+

–

+

IE

IC

IB

+

–

–

(1) BJT currents (2) DC Beta

CDC

B

IIβ =

(3) DC Alpha

CDC

E

IIα =



₪ Transistor DC Model [Unsaturated BJT] ■ Input circuit: forward-biased diode with a current of IB.

■ Output circuit: a dependent current source of βDC IB.

BJT Characteristics and Parameters



₪ BJT Circuit Analysis ■ VBE = 0.7 V

►Forward-biased diode.

►Therefore,

■ IB is given by:

■ IC is given by:

■ VCE is given by:

■ VCB is given by:

BJT Characteristics and Parameters – contd.

BR BB BEV V V= −

BB BEB

B

V VI R−

=

BC DCI Iβ=

CE CC C CV V I R= −

BECB CEV V V= −



₪ Example 4-2 ■ βDC = 150

►Currents:

►Voltages:


BB BEB

B

5V 0.7V 430 A10kV VI R μ− −= = =

Ω

BC DC 150 430 A 64.5 mAI Iβ μ= = × =

64.9mAE B CI I I == +

CE CC C C 10V (64.5mA)(100 ) 3.55VV V I R= − = − Ω =

BECB CE 3.55V 0.7V 2.85VV V V= − = − =



₪ Collector Characteristic Curves ■ Saturation region

►Both junctions are forward- biased

● VCC = 0.

► IC is independent of IB.

►As VCC increases, IC and VCE increase.

► Ideally, when VCE exceeds 0.7, the base-collector junction becomes reverse-biased.

● The BJT goes into the linear region (active region).

● In linear region, IC is dependent on IB.


IC

BC

A0 0.7 V VCE(max)

VCE

Saturation region

Active region

Breakdown region

BC DCI Iβ=



₪ Collector Characteristic Curves – contd. ■ At higher voltages of VCE

►The base-collector junction goes in breakdown.

►The collector current increases rapidly.

►A transistor should never be operated in the breakdown region.

BJT Characteristics and Parameters – contd. IC

BC

A0 0.7 V VCE(max)

VCE

Breakdown region

Saturation region

Active region



₪ Cutoff ■ This region of operation happens when IB = 0.

►There will be a very small collector leakage current ICEO.

►This ICEO is neglected and therefore VCE = VCC.

►Both junctions are reverse-biased.




₪ Saturation ■ As VBB increases, the base-emitter junction is forward-biased.

►The base current IB increases.

■ The base current increases and the collector current increases as a result (IC = βDCIB). ►As a result, VCE decreases till the saturation value VCE (sat) and the base

-controller junction becomes forward-biased (VCE = VCC – ICRC).

► IC does not increase anymore even when IB increases.

■ Saturation occurs below the knee of the collector curves ►A few tenths of a volt.




₪ DC Load Line ■ In between cutoff and saturation along the load line is the active

region. ►Details will be covered in Chapter 5.


0

IC

VCE

IB = 0 Cutoff

VCE(sat) VCC

IC(sat)

Saturation



₪ Example 4-4 ■ Determine whether or not the transistor is in saturation or not.

►VCE(sat) = 0.2 V.

■ Solution ► If the transistor is in saturation, then

► If the transistor is not in saturation, then

● and

►Because IC can’t be greater than IC(sat), the transistor is saturated.


CC C(sat)C(sat)

B

10V 0.2V 9.8mA1.0 kV V

I R− −= = =

Ω

BB BEB

B

2.3 V 0.23 mA10kV VI R

−= = =Ω

BC DC (50)(0.23mA) 11.5 mAI Iβ= = =



₪ More About βDC

■ βDC is not really constant but rather varies with IC and the temperature. ►Usually we specify the minimum βDC in the transistor datasheet.




₪ Maximum Transistor Rating ■ Limitations (maximum ratings) are specified by the manufacturer

for: ►Collector-to-base voltage

►Collector-to-emitter voltage

►Collector current

►Power dissipation

■ Note that ►Collector-to-emitter voltage and collector current can’t be maximum

at the same time.

► If VCE is maximum, IC is given by:

► If IC is maximum, VCE is given by:


D(max)C

CE

PI V=

D(max)CE

C

PV I=



₪ Example 4-5 ■ Collector-to-emitter voltage = 6 V.

■ Maximum power rating = 250 mW.

■ Find the maximum collector current.

■ Solution:


D(max)C

CE

250 mW 41.7 mA6 VP

I V= = =



₪ Example 4-6 ■ PD(max) = 800 mW,

■ VCE(max) = 15 V,

■ and IC(max) = 100 mA.

■ Determine the maximum value of

VCC before exceeding the rating.

■ Solution:

● IC is much less than the maximum collector maximum current.

● IC is dependent on IB, and will not change as long as IB is fixed.

● Therefore,

● Has the maximum power rating been exceeded? No.


BB BEB

B195 AV VI R μ−= = BC DC (100)(195 A) 19.5mAI Iβ μ= = =

C C C (19.5mA)(1.0k ) 19.5 VRV I R= = Ω =

CCC(max) CE(max) 15V 19.4 V 34.5 VRV V V= + = + =

D CCE(max) (15V)(19.5mA) 293 mWP V I= = =



₪ Derating PD(max)

■ PD(max) is usually specified at 25oC.

■ For higher temperatures, PD(max) is less.

■ Derating is calculated using a derating factor.

■ Example

►A derating factor of 2 mW/oC

● This means that the maximum power dissipation is reduced 2 mW for each degree Celsius increase in temperature.




₪ Voltage Gain ■ Defined as the ratio between output voltage and input voltage.

■ Let us define re’ as the resistance between base and emitter.

■ Then,

►Voltage gain

■ Example: ► re

’ = 50 Ω

►RC = 1.0 k Ω

The BJT as an Amplifier

'e ebV I r= c e CV I R≅

C C' '

ecve e eb

I R RVA V I r r== ≅

C'

1000 2050ve

RAr

= =≅



₪ Switch ■ Condition in Cutoff

■ Conditions in Saturation

The BJT as a Switch

RB

0 V

RC IC = 0

+VCC

RC

C

E

+VCC

IB = 0 –

+RB

RC IC(sat)

+VCC

RC

C

E

+VCC

IB

+VBB

IC(sat)

Cutoff – open switch Saturation – closed switch

CCCE(cutoff)V V=

CC CE(sat)C(sat)

C

V VI R

−= C(sat)

B(min)DC

II β=



₪ Application of a Transistor Switch ■ In cutoff diode is off.

■ In saturation diode is on.

₪ Example ■ The diode requires 30 mA to be on.

■ Use double the minimum base current to ensure saturation. ►VCC = 9V, VCE(sat) = 0.3 V, RC = 220 Ω, RB = 3.3 kΩ.

►Determine the value of the square wave.

The BJT as a Switch – contd.

LEDCC CE(sat)C(sat)

C

9V 1.6V 0.3V 32.3mA220V V V

I R− − − −= = =Ω

C(sat)B(min)

DC

32.3mA 646 A50I

I μβ= = =

B BEB(min)2 (1.29mA)(3.3k ) 0.7 V 4.96VinV I R V= + = Ω + =



₪ Phototransistor ■ The base current is produced when light strikes the sensing

semiconductor base region. ► In absence of light, there will be a small leaking current ICEO.

■ When the light strikes the collector-base pn junction ►A base current, Iλ, is produced that is directly prop. with the light

intensity.

■ Except for the base current generation, ►The phototransistor behaves

as a conventional BJT.

►The base-collector region

is wider than that in the case

of the BJT.

● To produce greater current.

The Phototransistor

Collector

Emitter

Light

n p

n

Base

RC

+V

VO U T

CC

C DCI Iλβ=



₪ Three-lead phototransistor ■ The base lead is electrically available where the phototransistor

can be used as a BJT with or without light-sensitivity.

₪ Two-lead phototransistor ■ The base lead is not electrically available.

►This is the usual configuration.

The Phototransistor – contd.



₪ Collector Characteristics Curves ■ Each curve corresponds to a certain value of light intensity.

■ Phototransistors are not sensitive to all lights ►They are sensitive to wavelengths in the red and infrared bandwidths.


RC

+V

VO U T

CC

Dark current

50 10 15 20 25 30VCE (V)

10

8

6

4

2

IC (mA)

50 mW/cm2

40 mW/cm2

30 mW/cm2

20 mW/cm2

10 mW/cm2



₪ Applications ■ Light activating.

■ Light deactivating.




₪ Optocouplers ■ A device that has light-based connection.

₪ Types of Optocouplers ■ LED-to-photodiode

■ LED-to-phototransistor

₪ Current transfer ration ■ Ration between change in input current (LED’s) and that of the

out current.




₪ Applications of the Optocouplers ■ Isolate sections of the circuit that are incompatible in terms of

current or voltage. ►Ex.: Isolate low-current control or circuits from noisy power supplies

or high current motors, etc.

►Ex.: Isolating patients from monitoring instruments.

►Ex.: Traffic lights where control circuit is isolated from the power circuit.




₪ General-purpose / Small-signal Transistors ■ Used for low- or medium-power amplifiers or switches.

■ The packages are either metal or plastic.

■ Certain types of packages contain multiple transistors.

Transistor Categories and Packaging

12 3

1 Emitter

2Base

3 Collector

TO-92

2 Emitter

1Base

3 Collector

12

3

SOT-23

1 Emitter

2Base

3 Collector

3 21

TO-18



₪ Power Transistors ■ Used to handle large currents ( > 1 A) and/or large voltages.

►A power amplifier is used to drive the speakers.

■ Usually it has a metal case for the collector ►Usually connected to a heat sink for heat dissipation.

Transistor Categories and Packaging – contd.



₪ RF Transistors ■ Operate at extremely high-frequencies.

■ Usually used in communication systems.

Transistor Categories and Packaging – contd.



Discussion & Notes

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K

K

AK

A

A

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A

K A

K

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AK

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A

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Electronics Lecture6 7 BJT - Aziz Mohaisen · Department of Electrical, Electronics and...

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