A transistor's working principle fundamentally involves controlling current flow between two terminals (Collector and Emitter) by varying the current or voltage applied to a third terminal (Base). It acts like an electronic switch or amplifier.
Transistor Basics
A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It's the fundamental building block of modern electronic devices. There are primarily two types: Bipolar Junction Transistors (BJTs) and Field-Effect Transistors (FETs).
Working Principle Explained
The core idea is that a small change in the current or voltage at one terminal (the control terminal) can produce a large change in the current flowing through the other two terminals. This allows transistors to be used as amplifiers or switches.
Transistor as a Switch
- OFF State: When no voltage or current is applied to the Base (for BJTs), the transistor is in a non-conducting state, effectively acting as an open switch. Little to no current flows between the Collector and Emitter.
- ON State: According to the provided reference, when a voltage of approximately 0.7V or more is applied to the Base terminal of a transistor (with the Emitter grounded), a small current flows into the Base. This small Base current causes the transistor to turn ON, allowing a larger current to flow between the Collector and the Emitter. This creates a closed circuit.
- For example, imagine a simple circuit with an LED and a resistor connected in series with the Collector and Emitter of an NPN transistor. When the Base voltage is below 0.7V, the LED is off. When the Base voltage exceeds 0.7V, the transistor turns on, allowing current to flow, and the LED lights up.
Transistor as an Amplifier
As an amplifier, a small input signal applied to the Base terminal modulates the larger current flowing from the Collector to the Emitter, thus amplifying the signal.
Key Concepts
- Base (B): The control terminal. A small current or voltage applied here controls the current flow between the Collector and Emitter.
- Collector (C): One of the terminals through which current flows, influenced by the Base.
- Emitter (E): The other terminal through which current flows; often connected to ground.
Different Types of Transistors
| Feature | Bipolar Junction Transistor (BJT) | Field-Effect Transistor (FET) |
|---|---|---|
| Control Mechanism | Base Current | Gate Voltage |
| Input Impedance | Low | High |
| Types | NPN, PNP | JFET, MOSFET |
Practical Insights
- Biasing: Transistors need to be properly biased (i.e., have specific DC voltages applied) to operate correctly as amplifiers.
- Saturation: When the Base current is high enough, the transistor is said to be in saturation, where further increases in Base current do not significantly increase the Collector current. This is often used in switching applications.
- Cut-off: When the Base current (or voltage) is too low, the transistor is in cut-off, meaning no current flows between the Collector and Emitter.
