A

CHAPTER 21 (2): BIPOLAR JUNCTION TRANSISTORS

Chapter 21: Bipolar Junction Transistors (Part 2)

Objectives

  • Differentiate between three regions of operation: active, cut-off, and saturation.

  • Understand the use of transistors as electronic switches.

21-4: The Transistor as a Linear Amplifier

  • A Bipolar Junction Transistor (BJT) amplifies small signals while preserving wave shape.

  • To function as a linear amplifier, the BJT must be in its active or linear region.

  • Key relationships observed during operation:

    • Current relationships (Input and output functions)

    • Voltage parameters across the transistor.

21-5: Transistor as a Switch

  • Functionality:

    • A BJT serves as an electrically controlled switch.

  • Control Mechanism:

    • Controlled by base current (IB).

    • When IB = 0, the transistor is in cut-off mode (switch open).

    • When IB is large, the transistor saturates (switch closed).

Conditions for Cut-off (Switch Open)

  • Condition: VCE(CUTOFF) = VCC – ICEO

  • During cut-off:

    • Assumes a scenario where the switch between collector (C) and emitter (E) is open.

    • Collector current (IC) = ICEO at VCE = 0V and IB = 0.

Conditions for Saturation

  • Condition: Base current is sufficiently large to drive the transistor into saturation.

  • VCE(SAT): Approximately 0.2 to 0.3V.

  • With VCE(SAT) effectively treated as 0V when considering circuit function.

  • Transistor behaves like a short circuit between collector and emitter (switch closed).

    • Requires a sufficient base current to achieve the desired collector current (IC).

Example 21-2

  • Task:

    • Determine VCE when VIN = 0V.

    • Calculate minimal base current (IB) required for saturation with a given current gain (bDC = 100).

    • Identify maximum value of RB when VIN = 4V.

Example Solution

  • a) When VIN = 0V, the transistor operates in cut-off; thus, VCE = VCC = 15V.

  • b) For saturation without VCE(SAT), IB must reach a minimal threshold.

  • c) To determine maximum RB, analyze the base-emitter configuration.

Switching an LED

  • Operation:

    • When VIN = 0V, the transistor is cut-off; no collector current and LED remains off.

    • When VIN is high, saturating the transistor allows current through to the LED, turning it on (1s ON, 1s OFF) with a periodic square wave input.

Example 21-3

  • Task:

    • Calculate the amplitude of input voltage required to ensure transistor saturation with given LED and circuit parameters.

  • Minimum base current necessary for saturation is calculated, considering the known values (e.g., VCC, VCE(SAT)).

Summary

  • A transistor operates as an amplifier in its active region and as a switch in cut-off (open) and saturation (closed) regions.

    • Cut-off: behaves like an open switch between collector and emitter.

    • Saturation: behaves like a closed switch between collector and emitter.

Chapter Closure

  • End of Chapter 21. All rights reserved by Singapore Polytechnic.