Transistor As A Switch

Question

A transistor used to turn a lamp on and off in a digital circuit is deliberately driven between only two operating states rather than the active region.

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NCERT explains that when a transistor is used as a switch it is operated in only two states rather than in the linear active region used for amplification. In cut-off, the base input is too small to turn the transistor on, so negligible collector current flows and the output voltage is high, near the supply value; this represents the OFF or logic high state. In saturation, a large enough base current fully turns the transistor on, so it conducts heavily, the collector-emitter voltage is very small, and the output is low; this represents the ON or logic low state. The option pairing active and breakdown is wrong because breakdown damages the device and the active region gives analog, not switching, behaviour. The option using saturation and active is wrong because the active region produces a variable amplified output, not clean switching. The option using cut-off and active is wrong for the same reason, since the active region is continuously variable rather than a distinct switch state. As stated in NCERT Class 12, Chapter 14, a switching transistor toggles between cut-off and saturation. A consistency check confirms the output swings sharply between high and low, exactly as a digital switch requires.

Transistor As A Switch

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