NPN Transistor Basics

Overview

This lecture introduces the basics of NPN bipolar junction transistors (BJTs), explaining their function as electronic switches and how to use them in simple circuits.

Introduction to Transistors

• Transistors are fundamental components in computers and integrated circuits.
• Transistors can function as electronic switches or signal amplifiers.
• There are different types of transistors; BJTs are commonly used in many circuits.

NPN Bipolar Junction Transistor (BJT)

• NPN transistors have three pins: base, collector, and emitter.
• No current flows from collector to emitter unless voltage/current is applied to the base.
• Applying a small current and about 0.6V to the base allows a larger current to flow from collector to emitter.
• This property lets transistors amplify or switch signals.

Simple Transistor Switch Circuit

• Circuit includes an NPN transistor, LED with resistor, and power source.
• Emitter connects to battery ground (negative).
• Collector connects to LED and resistor, then to battery positive.
• Base is controlled via a push button and a high-value resistor (e.g., 100 kΩ) connected to the battery positive.
• Pressing the button sends a small current to the base, allowing LED to light up (acts as a closed switch).
• Without base current, LED remains off (open switch).

Alternate Methods and Applications

• A wire can replace the push button; touching the wire to skin allows LED to light up (skin acts as resistor).
• Other components like photoresistors can be used at the base for light-sensitive switching circuits.

Key Terms & Definitions

• Transistor — Semiconductor device used to switch or amplify electrical signals.
• BJT (Bipolar Junction Transistor) — A type of transistor with base, collector, and emitter terminals.
• NPN Transistor — A BJT where current flows from collector to emitter when base is supplied with a small current.
• Base — The control terminal of a transistor.
• Collector — The terminal where current enters in an NPN transistor.
• Emitter — The terminal where current exits in an NPN transistor.
• Resistor — Component that limits current flow in a circuit.
• LED (Light Emitting Diode) — A diode that emits light when current flows through it.

Action Items / Next Steps

• Review the circuit schematic and practice building the simple switch circuit with an NPN transistor.
• Optional: Read more about transistors via suggested resources for deeper understanding.