What is a Transistor?
A transistor is a semiconductor device used to amplify or switch electronic signals. It's the building block of modern electronics — from audio amplifiers to every digital logic gate in a computer.
A transistor is a three-terminal semiconductor device that amplifies current or acts as an electronic switch; in a BJT, collector current Ic = β·Ib.
- •Current flows from collector to emitter
- •Base current flows into the base
- •Most common type in modern circuits
- •Turns ON with a positive base voltage
- •Current flows from emitter to collector
- •Base current flows out of the base
- •Less common in new designs
- •Turns ON with a negative base voltage
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Step-by-step worked examples
A BJT has β = 150 and Ib = 0.02 mA. Find the collector current Ic.
Ic = β × Ib = 150 × 0.02 = 3 mA
A transistor has Ic = 100 mA and β = 100. Find the base current Ib.
Ib = Ic / β = 100 / 100 = 1 mA
A BJT switch has Vin = 5 V, Vbe = 0.7 V and base resistor Rb = 10 kΩ. Find Ib.
Ib = (Vin − Vbe) / Rb = (5 − 0.7) / 10000 = 4.3 / 10000 = 0.00043 A = 0.43 mA
Flashcards
Quick quiz
Q1.How many terminals does a standard BJT transistor have?
Q2.β (beta) in a transistor represents:
Q3.The formula for collector current in terms of β and Ib is:
Q4.In an NPN transistor, the majority charge carriers are:
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Common mistakes
Assuming transistor terminals are interchangeable. — Correct: Base, collector and emitter are fixed by construction and cannot be swapped.
Using Ic = Ib + β. — Correct: The correct relation is Ic = β·Ib.
Thinking transistors only amplify signals. — Correct: Transistors are also used as switches, forming the basis of all digital logic.
Assuming NPN and PNP behave identically. — Correct: They have opposite current directions and turn on with opposite base voltage polarity.
FAQ
What is a transistor?
A transistor is a semiconductor device used to amplify electronic signals or act as an electronic switch.
What is the transistor current gain formula?
For a BJT, collector current Ic = β·Ib, where β is the current gain.
What are examples of transistor use?
Transistors are used in audio amplifiers, digital logic gates, power switches and voltage regulators.
How do you calculate collector current in a transistor?
Multiply the current gain β by the base current: Ic = β·Ib.