Discussion
Which logic family is characterised by very low power consumption?
Answer: A
Correct answer: CMOS
CMOS - complementary metal-oxide-semiconductor - consumes very little power, which is why it displaced every earlier family and is used in essentially all modern integrated circuits. The reason lies in its structure. Each gate pairs a p-channel and an n-channel transistor so that in either stable state one of the two is off, and almost no current flows from supply to ground. Power is drawn chiefly when the output changes, charging and discharging the capacitance of the next stage - which is why CMOS power consumption rises with clock frequency. TTL, transistor-transistor logic, is faster than early CMOS but draws current continuously. ECL, emitter-coupled logic, is the fastest of the three because its transistors never saturate, but it consumes the most power of all. RTL was among the earliest families and is obsolete. CMOS also tolerates a wide supply range and has high noise immunity, though it is sensitive to static discharge.
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