what is a linear regulator?

Answer 1

The main element of a linear regulator is the "pass transistor". It is operated in its linear region (not in saturation or not cut-off) and all the output current to the load passes through this transistor. That current multiplied by the voltage across the collector and emitter of the pass transistor is power that is wasted, and sometimes it is 50% or more of the power coming into the linear regulator (hence the regulator is very inefficient compared with switching type regulators).

The regulation occurs by an analog comparator driving the base of the pass transistor (through gain stages and level shifters), comparing the output of the regulator with a known reference (like a zener diode voltage) or a user set-point (potentiometer control knob), and controlling the base current to control the collector-to-emitter voltage, thus keeping the output voltage steady (even though the input voltage might be varying).

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