Transistors (NPN/PNP)

1. Definition

1.1 Transistor

A transistor is a semiconductor device used to amplify or switch electronic signals.
It has three terminals that control the flow of current: Base (B), Collector (C), and Emitter (E).

Transistor examples

Transistor Symbol

Transistor symbols

1.2 NPN vs PNP Transistors

NPN Transistor

  • Symbol: Arrow points out of the emitter.
  • Operation: Current flows from collector to emitter when a small current enters the base.
  • Biasing: Base is more positive than emitter.
  • Common Use: Low-side switching (connect emitter to ground).

PNP Transistor

  • Symbol: Arrow points in to the emitter.
  • Operation: Current flows from emitter to collector when a small current leaves the base.
  • Biasing: Base is more negative than emitter.
  • Common Use: High-side switching (connect emitter to supply voltage).

Side-by-side comparison:

Feature NPN PNP
Arrow direction Outward Inward
Base voltage Higher than emitter Lower than emitter
Current flow Collector → Emitter Emitter → Collector
Common connection Emitter to GND Emitter to +V
Typical use case Switching to ground Switching to positive supply

2. Features

  • Type: NPN or PNP.
  • Gain (hFE): Ratio of collector current to base current.
  • Maximum ratings: Voltage (Vce), Current (Ic), Power (P).
  • Package: TO-92, SOT-23, TO-220, etc.

4. How to Use

Safety Note

Exceeding maximum ratings for voltage, current, or power can destroy the transistor.

4.1 Identify pins

  • Use the datasheet : pin order varies between packages and manufacturers.
  • Typical TO-92: E-B-C (Emitter, Base, Collector).

Transistor pinout

4.2 Switching example (NPN)

For a 5V LED circuit:

  1. Connect emitter to ground.
  2. Connect collector to LED, LED to resistor, resistor to +5V.
  3. Apply small base current via a resistor from microcontroller.

4.3 Amplification example

  • Small audio signal at base → larger output signal between collector and emitter.
  • Requires proper biasing resistors.

4.4 Testing with a multimeter

  • Use diode mode to check base-emitter and base-collector junctions.
  • For NPN: Base to emitter & base to collector should show ~0.6–0.7V drop (forward).
  • For PNP: Emitter to base & collector to base show ~0.6–0.7V drop.

5. Application

  • Signal amplification.
  • Switching (LEDs, motors, relays).
  • Oscillator circuits.

6. Video Explanation

video coming soon