The NOT gate

Chapter 3.2 The NOT gate

Lessons In Electric Circuits Volume IV – Digital Book
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Lessons In Electric Circuits Volume IV – Digital Book

  • 3.2. THE NOT GATE33OutputInput0110NOT gate truth tableInputOutputTruth tables for more complex gates are, of course, larger than the one shown for the NOT gate.A gate’s truth table must have as many rows as there are possibilities for unique input combinations.For a single-input gate like the NOT gate, there are only two possibilities, 0 and 1. For a two inputgate, there are four possibilities (00, 01, 10, and 11), and thus four rows to the corresponding truthtable. For a three-input gate, there are eight possibilities (000, 001, 010, 011, 100, 101, 110, and111), and thus a truth table with eight rows are needed. The mathematically inclined will realizethat the number of truth table rows needed for a gate is equal to 2 raised to the power of the numberof input terminals.• REVIEW:• In digital circuits, binary bit values of 0 and 1 are represented by voltage signals measured inreference to a common circuit point called ground. An absence of voltage represents a binary”0” and the presence of full DC supply voltage represents a binary ”1.”• A logic gate, or simply gate, is a special form of amplifier circuit designed to input and outputlogic level voltages (voltages intended to represent binary bits). Gate circuits are most com-monly represented in a schematic by their own unique symbols rather than by their constituenttransistors and resistors.• Just as with operational amplifiers, the power supply connections to gates are often omittedin schematic diagrams for the sake of simplicity.• A truth table is a standard way of representing the input/output relationships of a gate circuit,listing all the possible input logic level combinations with their respective output logic levels.3.2The NOT gateThe single-transistor inverter circuit illustrated earlier is actually too crude to be of practical useas a gate. Real inverter circuits contain more than one transistor to maximize voltage gain (soas to ensure that the final output transistor is either in full cutoff or full saturation), and othercomponents designed to reduce the chance of accidental damage.Shown here is a schematic diagram for a real inverter circuit, complete with all necessary com-ponents for efficient and reliable operation: