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BJT Transistor

A BJT is a current-controlled transistor: a small base current controls a much larger collector current. In the active region the collector current is proportional to the base current through the gain β. In saturation (used as a switch) $V_{CE}$ is very low, and the collector current is then set by the rest of the circuit, not by β anymore.

I_C=\beta I_B

holds in the active region

V_{CE}\ \text{low}\ (\approx0.1\text{-}0.3\ \text{V})

characteristic of saturation (transistor as a closed switch)

Symbols

$I_B$	base current	A
$I_C$	collector current	A
$\beta$	current gain
$V_{CE}$	collector-emitter voltage	V

Example

$\beta=100$ , $I_B=20$ µA (active region):

$I_C=\beta I_B=100\cdot20=2000$ µA $=2$ mA.

I_C=\beta I_B

only holds in the active region – in saturation

I_C

is set by the load circuit.

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