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Transistor

Introduction

• Transfer + Resistance

• Resistor or semiconductor that amplifies signals when transferred from input to output

• Alternative to Vacuum Tubes; Computation was slow

• Example: Light bulb, CRT Monitor

• Bulky, Consumed Power, Generated heat, Limited functionality

History

• Size of Computer in 1950s - 10,000 vacuum tubes covering 93 square meters

• Vacuum Tubes would leak and metal emitting electrons in tube burned out

• William Shockley, John Bardeen and Walter Brattain at Bell Labs built world’s first transistor in 1947

First Single Point Transistor

Transistor Advantages

• Small in size

• Helps in miniaturization

• Faster switching

• Power Efficient

• Generate less heat

• Long lasting and reliable

• Amplify signals

First Single Point Transistor

Integrated Circuits

• First Commercial successful product: Transistor Radio

• Pack more transistors in less space for mass production and circuit complexity

• Nerve Cell of Information Technology

• Moore’ Law: No. of transistors on chip doubles every year

Transistor Types

• A semiconductor device with three legs

• We shall use BJT- Bipolar Junction Transistors

• Two types- NPN and PNP

NPN PNP

Type Identification• Rotate meter to beep symbol .)))

• Connect two probes at first two or last two legs

• Beep sound/reading indicates: Red probe connection is P and Black probe connection is N

Type Identification

If the reading or beep comes, then

Red Probe represents P and Black Probe represents N

In this case, Ist leg is N and 2nd leg is P

Type Identification

In this case, 2nd leg is P and 3rd leg is N

Ist leg is N, 2nd leg is P and 3rd leg is N. Hence, it is a NPN transistor.

Type Identification

• Download Data-sheet

• Google “Number printed on transistor”

Naming of Legs

• Three legs are named as Emitter, Base and Collector

• Keep the curved surface in front of your eyes and start from the left side: E---B---C

• E: Emitter, B: Base and C: Collector

hFE

Rotate meter dial to hFE

Gain Measurement

• Rotate meter dial to hFE

• Take a NPN transistor

• Inserts its Emitter, Base and Collector legs into the respective E, B and C slots on the multimeter

• Press the transistor and take the reading

Gain Measurement

Gain Measurement

Reading is called Beta of the Transistor

Beta

• Amplification Factor or hFE of the transistor

• Factor by which output current is amplified

• Ratio of output current to input current

Example: Mic-Amplifier System

1.Electric Mic converts voice (audio) to electronic signal

2. Electronic signal enters a transistor and the transistor amplifies the signal

3.Speaker converts the signal back into the amplified voice

NPN

A layer of p-type semiconductor sandwiched between layers of two n-type semiconductors

PNP

A layer of n-type semiconductor sandwiched between layers of two p-type semiconductors

Schematic Symbol of NPN

Note: Direction of current is outwards and coming out of emitter

Schematic Symbol of PNP

Note: Direction of current is inwards and towards emitter

How a transistor works

• Think of transistor as input-output system

• Output depends upon the biasing condition

To make NPN ON

• Base is given more +ve voltage than Emitter

• Collector is given more +ve voltage than Emitter

• Input Current is formed from base to emitter: Base Current

• Output Current is formed from collector to emitter: Collector Current

To make PNP ON

• Emitter is given more +ve voltage than Base

• Emitter is given more +ve voltage than Collector

• Input Current is formed from emitter to base: Base Current

• Output Current is formed from emitter to collector: Collector Current

NPN Transistor as Amplifier

BC Junction is Reverse Biased: Voltage at Base < Voltage at Collector

Transistor as Amplifier

Ib Ic = β Ib

BE Junction = Forward BiasedBC Junction = Reverse Biased

NPN Transistor as Switch

BC Junction is Reverse Biased: Voltage at Base > Voltage at Collector

Transistor as Switch

Ib Ic ≠ β Ib

BE Junction = Forward BiasedBC Junction = Forward Biased

Region Function Biasing Condition Relation between Ib and Ic

Cut-off Off (Switch)BE Junction is not

forward biased by +0.7VIb = 0 and Ic = 0

Active Amplifier

BE Junction: forward biased and

BC Junction is reverse biased (Base voltage <

Collector voltage)

Ic = hFE × IbTransistor amplifies the base current

Saturation ON (Switch)

BE Junction is forward biased and

BC Junction is forward biased (Base voltage >

Collector voltage)

Ic ≠ hFE × IbIc depends upon the external circuit

connected between emitter and collector of the transistor

Transistor Table

Connect Tr on BB

Note: Face the curved surface and insert E, B and C legs in different columns

Schematic Example 1

• Emitter is grounded

• A 220k Ω resistor is connected in series with base

Schematic Example 2

• An LED is connected with its -ve terminal at the collector

• +ve terminal of LED is connected to a resistor

Schematic Example 2

Schematic Example 3

• Another resistor is connected to the collector

• The other end of the resistor is grounded

Schematic Example 3

Schematic Example 4

• Emitter of 1st transistor is connected to base of 2nd transistor

• Collector of both transistors are joined

Schematic Example 4

For convenience: Connect two transistors on the upper and lower halves of the breadboard

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