Introduction

A transistor (BJT) whose terminals are not connected to any kind of voltage source is called an unbiased transistor. In this state the device does nothing useful, so an unbiased transistor is never used in actual practice. To make a transistor operate in a chosen region — for amplification, oscillation or switching — an external DC voltage of the correct polarity and suitable magnitude must be applied across its terminals. Applying such a suitable DC voltage across the transistor terminals is called transistor biasing.

Need for biasing

The biasing circuit is what sets the transistor's DC operating conditions: the base current $I_B$, the collector current $I_C$, the collector-to-emitter voltage $V_{CE}$ and the base-to-emitter voltage $V_{BE}$. Unless the transistor is properly biased, it will not stay in the region needed for the intended application.

Requirements for good biasing

A well-designed bias arrangement should meet three conditions:

1. The operating point should sit near the centre of the active region, so that when the input signal is applied the operating point does not drift towards the saturation region or towards the cutoff region.
2. The collector current should stay stable against changes in temperature.
3. The operating point should not shift when the transistor is replaced by another of the same type.

This chapter of the Karnataka 2nd PUC Electronics course develops these ideas step by step — through the DC load line, the Q point, and the practical biasing methods that a II PUC board exam expects you to analyse.