Electronics >> Bipolar Transistors
Bipolar transistors are semiconductor components that have three regions that are doped with impurity. They are placed under two main categories; NPN and PNP. These categories correspond to the type of material with respect to their three connecting leads.
The three connecting leads of a bipolar transistor are called:
- Emitter (E)
- Base (B)
- Collector (C)
A bipolar transistor has two junctions. One junction is between the emitter and the base, the other is between the collector and the base. Each junction of a transistor functions like a diode, and has forward and reverse bias directions.
Voltage drop of a transistor in forward biased junction is 0.7 volt. This indicates the low amount of voltage that is required to operate a transistor in a circuit. Reverse biased junction of a transistor can withstand higher voltage; the amount of this voltage depends on how the device is manufactured (from 5Vdc up to 100s of volts).
Transistors can act as on/off switches, amplifiers, oscillators, modulators, and several other devices.
Practical NPN and PNP transistor circuits.
Commonly the letter (Q) and a number is used to identify a transistor in a circuit.
Description of the circuits:
RB » Base Resistor (limits the amount of base current)
RC » Collector Resistor (sets the amount of collector current)
VCC » Voltage Source
VB » Voltage Base
VBE » Voltage Base-Emitter
VCE » Voltage Collector-Emitter
Transistor gain allows a small base current to be amplified into a much larger collector current. The base current is provided by the transistor input circuit, VB and RB. The amplified collector current is provided by VCC and RC. The relationship between base current (IB) and collector current (IC) is transistor's gain and is determined by: Gain=IC/IB
The current gain of a transistor is also called beta, and is represented by the Greek symbol β (beta).
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Darlington Pair Transistor Circuit:
Darlington pair is a connection of two transistors whose total current gain equals to the product of each transistor's current gain.
Current gain in Darlington pair transistor circuit:
β = β(Q1) . β(Q2)
Example: In the above circuit, if Q1's gain would be 100 and Q2's gain would also be 100, the total gain for the circuit will be 10000.
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Doping of semiconductors:
By adding impurity atoms to a pure base crystal, we alter its electrical conductivity in a controlled fashion and produce semiconductors with specific characteristics.
In semiconductors, n-type material has a larger number of free electrons, and p-type material has a larger amount of holes in their atomic compositions.
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