Introduction to Transistors

A transistor is a three-terminal semiconductor device that can amplify signals or act as an electronically controlled switch. Invented in 1947 at Bell Labs, the transistor replaced bulky vacuum tubes and launched the modern electronics revolution. Today, a single microprocessor contains billions of transistors, each smaller than a virus. Understanding transistors unlocks the ability to build amplifiers, digital logic, motor controllers, and countless other circuits.

Objectives

• Understand the fundamental difference between BJTs and MOSFETs
• Calculate base resistor values for BJT switching circuits
• Identify transistor pinouts and read package markings
• Design basic transistor switch circuits for LEDs, relays, and motors
• Recognize the three operating regions (cutoff, active, saturation)
• Build simple logic gates from discrete transistors
• Understand common configurations (common emitter, source follower)
• Apply transistors in real-world projects

Key Takeaways

• BJTs are controlled by base current: Ic = β × Ib
• MOSFETs are controlled by gate voltage: Vgs > Vth to turn ON
• Always check pinouts - they vary between manufacturers and part numbers
• Use flyback diodes with ALL inductive loads (motors, relays, solenoids)
• For switching, drive BJTs into saturation (Vce ≈ 0.2V)
• Logic-level MOSFETs can be driven directly from 3.3V/5V microcontrollers
• Standard MOSFETs need gate drivers for proper switching
• Transistor NOT gates are the foundation of all digital logic
• Power transistors need heat sinks - calculate P = I² × RDS(on) for MOSFETs

Applications

• LED Driver (Low-Side Switch): NPN transistor switches LED on/off from microcontroller signal
• Relay/Motor Driver: Transistor controls inductive load with flyback diode protection
• NOT Gate (Inverter): Single transistor creates logic inverter - foundation of digital electronics
• Common Emitter Amplifier: Most common BJT amplifier configuration with voltage gain
• H-Bridge Motor Control: Four transistors enable forward/reverse motor control
• Current Mirror: Copies reference current to other parts of circuit - fundamental analog building block
• Darlington Pair: Two transistors cascaded for very high gain (β₁ × β₂)
• Touch Sensor: High-gain Darlington detects tiny body capacitance currents

Practice Problems

Problem 1: A 2N2222 transistor (β = 100) needs to switch a 150mA load. What is the minimum base current required?

Problem 2: Calculate the base resistor to drive a transistor into saturation with Ic = 100mA, β = 100, from a 3.3V microcontroller.

Problem 3: A MOSFET has RDS(on) = 50mΩ and is switching 5A. Calculate the power dissipation.

Problem 4: What is the collector current if a transistor has Ib = 50μA and β = 200?

Problem 5: A transistor NOT gate has a 10kΩ collector resistor and 5V supply. What is the output voltage when the transistor is in cutoff? When saturated (Vce(sat) = 0.2V)?