Bipolar Junction Transistors (BJT)

In-depth guide to BJT physics, biasing techniques, amplifier configurations, and practical applications.

The Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that uses current to control current. Unlike MOSFETs which are voltage-controlled, BJTs require a continuous base current to maintain conduction. This fundamental characteristic makes BJTs excellent for linear amplification where precise gain control is needed. While MOSFETs dominate digital and power switching, BJTs remain essential for analog amplifiers, current sources, voltage references, and high-frequency circuits.

Objectives

  • Understand BJT physics using the Ebers-Moll model
  • Design and analyze biasing configurations (fixed, voltage divider, collector feedback)
  • Calculate DC operating points and verify stability
  • Design Common Emitter, Common Base, and Common Collector amplifiers
  • Understand and prevent thermal runaway in power applications
  • Build Darlington pairs for high-gain switching
  • Design current mirrors for analog circuits
  • Calculate frequency response and bandwidth limitations
  • Select appropriate BJTs for specific applications
  • Salvage and test BJTs from electronic waste

Key Takeaways

  • BJTs are current-controlled: Ic = β × Ib
  • Voltage divider bias provides stable, β-independent operation
  • CE amplifiers provide voltage gain; CC provides current gain and buffering
  • Thermal runaway is prevented by emitter degeneration resistors
  • Darlington pairs achieve very high gain for switching applications
  • Current mirrors enable precise current replication
  • Always check datasheets for pinouts and maximum ratings
  • gm = Ic/26mV - transconductance is proportional to collector current

Applications

  • Audio Amplifiers: Preamps, power output stages, tone controls
  • Current Mirrors: Precise current replication in analog ICs
  • Voltage References: Band-gap references use BJT Vbe characteristics
  • Temperature Sensors: Vbe changes -2mV/°C - precision thermometry
  • LED Drivers: Simple current-controlled switching
  • Darlington Touch Sensors: High gain detects skin resistance
  • Push-Pull Output Stages: Complementary NPN/PNP for efficient amplification

Practice Problems

Problem 1: A voltage divider biased amplifier has Vcc=15V, R1=47kΩ, R2=10kΩ, Rc=2.2kΩ, Re=1kΩ. Find the Q-point.

Problem 2: Calculate voltage gain of CE amplifier with Ic=2mA, Rc=4.7kΩ, Re=1kΩ (no bypass).