How it works
The simplest biasing, fixed bias, uses a single resistor RB from VCC to the base: IB = (VCC − VBE)/RB and IC = βIB. It is simple but β-dependent — if β varies from 100 to 200 between transistors, IC doubles, shifting the Q-point badly. Voltage divider bias (self-bias) solves this by using R1 and R2 to fix VB ≈ VCC × R2/(R1+R2), independent of β when the divider is stiff. Adding emitter resistor RE introduces negative feedback: if IC rises, VE = IC × RE rises, VBE = VB − VE falls, reducing IB and pulling IC back. Stability factor S = ΔIC/ΔICO; for voltage divider bias S can be as low as 1, whereas fixed bias gives S = β+1, which can be over 100 — a huge difference in temperature stability.
Key points to remember
Four biasing schemes: fixed bias (S = β+1, very unstable), collector feedback bias (S improved, single resistor from collector to base), emitter feedback bias, and voltage divider bias (S approaching 1, most stable). For voltage divider bias, the divider is considered stiff when R1 ∥ R2 ≤ 0.1 × β × RE — this condition must be verified in design problems. The Q-point should be centred at IC(max)/2 and VCE = VCC/2 for maximum undistorted swing. Temperature increase raises ICEO, VBE decreases by about 2 mV/°C, and β increases — all three shift the Q-point toward saturation if biasing is poor. Stability factor S is the single most important figure of merit for a biasing network.
Exam tip
Every Anna University paper on BJT biasing asks you to calculate the Q-point of a voltage divider bias circuit — find VB first, then VE = VB − 0.7, then IE ≈ IC = VE/RE, and finally VCE = VCC − IC(RC + RE).