Transistor biasing
Biasing means setting the steady DC voltages and currents that put a transistor at a comfortable resting point before any signal arrives — like idling a car engine so it's ready to accelerate smoothly. Get it wrong and the transistor sits too close to 'off' or 'full on', so the signal you want to amplify gets clipped, distorted, or never appears at all.
The aim is to park the operating point (the quiescent or Q-point) in the middle of the device's active region, leaving headroom to swing up and down symmetrically. The classic trick for a BJT is a voltage-divider on the base plus an emitter resistor, which uses negative feedback to hold the bias current steady even as temperature changes or as transistors vary part-to-part. Good biasing is what separates a textbook amplifier from one that drifts and distorts in the real world.
A stiff divider plus R_E makes the Q-point nearly independent of β.
An emitter (or source) degeneration resistor is the single most important stabiliser: if temperature pushes the current up, the voltage it drops rises and pulls the base-emitter voltage back down, automatically reining the current in.