Transistor
Simple Explanation
A transistor is a semiconductor device that can amplify a small electrical signal into a much larger one, or act as an electronic switch. It has three terminals (commonly base, collector, and emitter in a bipolar junction transistor), and a small current at the base controls a much larger current flowing between the collector and emitter.
Why Do We Need It?
The transistor is often called one of the most important inventions of the 20th century β it replaced older, bulkier, less reliable vacuum tubes and made modern electronics, including every computer and smartphone, possible.
Formula
Transistor Current Gain
Ξ² = I_C / I_B
Current gain measures how much a transistor amplifies a small base current into a much larger collector current.
- Ξ²
- β Current gain (no units) β typically ranging from tens to hundreds
- I_C
- β Collector current, in amperes
- I_B
- β Base current, in amperes
When to use it: Use to find how much a transistor amplifies a small input (base) current, or to find the required base current for a target collector current.
Worked Example
Finding collector current from current gain
A transistor has a current gain (Ξ²) of 100 and a base current of 0.02 mA. Find the collector current.
Why Does This Work?
Inside a transistor, a small current injected at the base controls the flow of a much larger current between the collector and emitter, because the base current effectively 'opens the gate' for a much larger current to flow through the device's semiconductor junctions β this amplifying relationship is exactly what the current gain Ξ² quantifies.
Real-Life Example
Amplifying a microphone signal
A microphone produces a very weak electrical signal that must be amplified before it can drive a loudspeaker.
Transistors in an amplifier circuit take this weak input signal (controlling the base current) and use it to control a much larger current from a power supply, producing a much stronger output signal capable of driving a speaker.
Practice
A transistor has a current gain of 150 and a collector current of 30 mA. Find the base current.
MediumCommon mistake
Thinking the transistor generates extra energy from nothing β the amplified collector current comes from an external power supply; the small base current only CONTROLS how much of that supply current flows, it doesn't create energy.
Quick Review
- Ξ² = I_C/I_B: current gain measures amplification of base current to collector current.
- A transistor can amplify signals or act as an electronic switch.
- The amplified current comes from an external power supply, controlled by the small base current.