How a small current controls a large one, and how switches become logic. The junction transistor and its characteristics, the transistor as an amplifier and as a switch, and the logic gates OR, AND, NOT, NAND, NOR and XOR with their truth tables. It serves the Indian syllabus (NCERT, NEET, JEE Main) and the matching topic of A-Level and IB.
3 lessons, about 1 h 40 min4 simulations10 problems6 practice questionsPython in every lesson
A junction transistor is a thin base between an emitter and a collector. With the emitter junction forward biased and the collector junction reverse biased, a small base current controls a collector current that is times larger, with . The input and output characteristics show three regions: cut-off, active and saturation.
In a common-emitter amplifier a small change of the base current makes a change times larger in the collector current. The output voltage across the collector resistor changes by times the input, which gives a voltage gain with a phase change of . Driven between cut-off and saturation, the same transistor is a switch, and it inverts the signal.
A logic gate has inputs and an output that are each or . The truth table lists the output for every input. OR, AND and NOT are the basic gates, NAND and NOR are their opposites, and XOR gives when its inputs differ. NAND alone, or NOR alone, can make every other gate, so each is a universal gate.
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