15) transistors
TRANSCRIPT
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Mand Labs presents
Transistor
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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
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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
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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
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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
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Transistor Types
• A semiconductor device with three legs
• We shall use BJT- Bipolar Junction Transistors
• Two types- NPN and PNP
NPN PNP
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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
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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
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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.
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Type Identification
• Download Data-sheet
• Google “Number printed on transistor”
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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
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hFE
Rotate meter dial to hFE
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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
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Gain Measurement
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Gain Measurement
Reading is called Beta of the Transistor
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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
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NPN
A layer of p-type semiconductor sandwiched between layers of two n-type semiconductors
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PNP
A layer of n-type semiconductor sandwiched between layers of two p-type semiconductors
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Schematic Symbol of NPN
Note: Direction of current is outwards and coming out of emitter
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Schematic Symbol of PNP
Note: Direction of current is inwards and towards emitter
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How a transistor works
• Think of transistor as input-output system
• Output depends upon the biasing condition
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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
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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
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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
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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
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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
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Connect Tr on BB
Note: Face the curved surface and insert E, B and C legs in different columns
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Schematic Example 1
• Emitter is grounded
• A 220k Ω resistor is connected in series with base
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Schematic Example 2
• An LED is connected with its -ve terminal at the collector
• +ve terminal of LED is connected to a resistor
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Schematic Example 2
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Schematic Example 3
• Another resistor is connected to the collector
• The other end of the resistor is grounded
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Schematic Example 3
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Schematic Example 4
• Emitter of 1st transistor is connected to base of 2nd transistor
• Collector of both transistors are joined
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Schematic Example 4
For convenience: Connect two transistors on the upper and lower halves of the breadboard