Bipolar junction transistor 1 Objectives Describe the basic - - PowerPoint PPT Presentation

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Bipolar junction transistor 1 Objectives Describe the basic - - PowerPoint PPT Presentation

Apr 19, 2023 110 likes •401 views

Chapter 4 Bipolar junction transistor 1 Objectives Describe the basic structure of the bipolar junction transistor (BJT) Explain and analyze basic transistor bias and operation operation Discuss the parameters and

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SLIDE 1

Chapter 4

Bipolar junction transistor

1

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SLIDE 2

Objectives

  • Describe the basic structure of the bipolar junction

transistor (BJT)

  • Explain

and analyze basic transistor bias and

  • peration
  • peration
  • Discuss the parameters and characteristics of a

transistor and how they apply to transistor circuits

2

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SLIDE 3

What is transistor?

three-terminal device whose output current, voltage and/or power are controlled by its input. 2 basic transistor types: BJT and FET

Transistor

3

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SLIDE 4

Transistor Construction

There are two types of transistors:

  • pnp
  • npn

The terminals are labeled:

  • E – Emitter
  • B - Base

pnp

  • C - Collector

npn

4

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SLIDE 5

Transistor Operation

Biasing:

  • two pn junction must be correctly biased with external dc voltages to operate the

transistor properly.

  • The figure shown the proper bias arrangement for both npn and pnp transistor for

active operation as an amplifier.

5

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SLIDE 6

Transistor Operation

Transistor Voltages:

  • VCC – collector supply voltage. This is a power supply voltage applied directly to

collector of transistor.

  • VBB – base supply voltage. this is dc voltage used to bias base of transistor.
  • VEE – emitter supply voltage. dc biasing voltage and in many cases, VEE is simply a

ground connection.

6

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SLIDE 7

Transistor Operation

Transistor Voltages:

  • VC – dc voltage measured from collector terminal of component to ground
  • VB – dc voltage measured from base terminal to ground.
  • VE – dc voltage measured from emitter terminal to ground.

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SLIDE 8

Transistor Operation

Transistor Voltages:

  • VCE – dc voltage measured from collector to emitter terminal of transistor.
  • VBE – dc voltage measured from base to emitter terminal of transistor.
  • VCB – dc voltage measured from collector to base terminal of transistor.

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SLIDE 9

Currents in a Transistor

Emitter current is the sum of the collector and base currents:

Current gain (β) factor by which current increases from base of transistor to its collector.

C B E

I I I + =

I I β =

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B DC C

I I β =

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SLIDE 10

Operating Regions

  • Active – Operating range of the amplifier.
  • Cutoff – The amplifier is basically off. There is voltage, but little current.
  • Saturation – The amplifier is fullon. There is current, but little voltage.

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SLIDE 11

Operating Regions

Cutoff region

  • Both

transistor junctions are reverse biased.

  • With

large depletion region between C-B and E-B, reverse current, ICEO passes from emitter to collector and can be neglected.

  • So, VCE = VCC

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SLIDE 12

Operating Regions

Saturation region

  • Both

transistor junctions are forward-biased.

  • IC reaches its maximum value as

determined by VCC and total resistance in C-E circuit. resistance in C-E circuit.

  • IC

is independently from relationship of β and IB.

  • VBE is approximately 0.7V and

VCE < VBE.

E C CC C

R R V I + =

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SLIDE 13

Operating Regions

Active region

  • BE

junction is forward biased and the BC junction is reverse biased.

  • All terminal currents have some

measurable value. measurable value.

  • The magnitude of IC depends on

the values of β and IB.

  • VCE is approximately near to 0.7V

and VCE falls in ranges VBE<VCE<VCC.

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SLIDE 14

Operating Regions

Transistor Operating Regions: 1.Cutoff region:

  • Both transistor junctions are reverse biased
  • All terminal current are approximately equal

to zero. Since ICEO neglected, VCE = VCC 2.Active region:

  • The BE junction is forward biased and the BC junction is reverse biased
  • The BE junction is forward biased and the BC junction is reverse biased
  • All terminal currents have some measurable value
  • The magnitude of IC depends on the values of and IB
  • VCE is approximately near to 0.7V and VCE falls in ranges VBE<VCE<VCC

3.Saturation:

  • Both transistor junctions are forward biased
  • IC reaches its maximum values- determine by

the component in the CE circuit, and independent

  • f the values of and IB
  • VBE is approximately 0.7V and VCE < VBE

β β

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SLIDE 15

Approximations

Emitter and collector currents:

I ≅ I C E

Base-emittervoltage:

VBE = 0.7 V (for Silicon)

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SLIDE 16

Alpha (α α α α)

Alpha (α) is the ratio of IC to IE :

IE α I = C dc

Ideally: α = 1 Ideally: α = 1 In reality: α is between 0.9 and 0.998

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SLIDE 17

Beta (β β β β)

In DC mode: β represents the amplification factor of a transistor. (β is sometimes referred to as hfe, a term used in transistor modeling calculations) β = IC I B

dc

Relationship between amplification factors β and α

17

β β + 1 α = α α −1 β = Relationship Between Currents

IC = βIB

IE = (β + 1)IB

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SLIDE 18

Current and Voltage Analysis:

When the BE junction is forward-biased, like a forward biased

diode and the voltage drop is

Since the emitter is at ground (0V), by Kirchhoff’s voltage law, the

voltage across is: …….(1)

V VBE 7 . ≅

B

R

BE BB R

V V V

B

− =

BJT CHARACTERISTICS & PARAMETERS

voltage across is: …….(1)

Also, by Ohm’s law:

……..(2)

From (1) ->(2) : Therefore, the dc base current is:

B

R

BE BB R

V V V

B

− =

B B R

R I V

B =

B B BE BB

R I V V = −

B BE BB B

R V V I − =

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SLIDE 19

Current and Voltage Analysis:

  • The voltage at the collector with respect to the grounded emitter is:
  • Since the drop across

is:

C

R CC CE

V V V − =

R

C C RC

R I V = BJT CHARACTERISTICS & PARAMETERS

  • Since the drop across

is:

  • The dc voltage at the collector with respect to the emitter is:
  • where
  • The dc voltage at the collector with respect to the base is:

C C CC CE

R I V V − =

BE CE CB

V V V − =

C

R

C C RC

R I V =

B DC C

I I β =

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SLIDE 20

BJT CHARACTERISTICS & PARAMETERS

20

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SLIDE 21

DC Load Line:

Cutoff and saturation can be illustrated in relation to the collector characteristic curves by the use of a load line. DC load line drawn on the connecting cutoff and saturation point. The bottom of load line is ideal cutoff where IC=0 & VCE=VCC. The top of load line is saturation

BJT CHARACTERISTICS & PARAMETERS

The top of load line is saturation where IC=IC(sat) & VCE =VCE(sat) In between cutoff and saturation is the active region of transistor’s

  • peration.

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SLIDE 22

BJT CHARACTERISTICS & PARAMETERS

Collector Characteristic Curve:

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SLIDE 23

A transistor when used as a switch is simply being biased so that it is in:

  • 1. cutoff (switched off)
  • 2. saturation (switched on)

BJT AS A SWITCH

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SLIDE 24

Conditions in Cutoff

CC cutoff CE

V V =

) (

Neglect leakage current and all currents are zero. BE junction is reverse biased.

BJT AS A SWITCH

C sat CE CC sat C

R V V I

) ( ) (

− =

DC sat C B

I I β

) ( (min) =

Conditions in Saturation Since VCE(sat) is very small compared to VCC, it can be neglected.

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SLIDE 25

Transistor Specification Sheet

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SLIDE 26

Transistor Specification Sheet

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SLIDE 27

Transistor Testing

  • Curve Tracer

Provides a graph of the characteristic curves.

  • DMM

Some DMMs measure βDC or hFE.

  • Ohmmeter

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SLIDE 28

Transistor Terminal Identification

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