Multiple-input gates

Chapter 3.4 Multiple-input gates

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

  • 48CHAPTER 3. LOGIC GATESoutput.The schematic diagram for a buffer circuit with totem pole output transistors is a bit morecomplex, but the basic principles, and certainly the truth table, are the same as for the open-collector circuit:OutputInputVccD1R1R2R3Q1Q2Q3Buffer circuit with totem pole outputQ4Q5Q6D2D3R4R5InverterInverter• REVIEW:• Two inverter, or NOT, gates connected in ”series” so as to invert, then re-invert, a binary bitperform the function of a buffer. Buffer gates merely serve the purpose of signal amplification:taking a ”weak” signal source that isn’t capable of sourcing or sinking much current, andboosting the current capacity of the signal so as to be able to drive a load.• Buffer circuits are symbolized by a triangle symbol with no inverter ”bubble.”• Buffers, like inverters, may be made in open-collector output or totem pole output forms.3.4Multiple-input gatesInverters and buffers exhaust the possibilities for single-input gate circuits. What more can be donewith a single logic signal but to buffer it or invert it? To explore more logic gate possibilities, wemust add more input terminals to the circuit(s).Adding more input terminals to a logic gate increases the number of input state possibilities.With a single-input gate such as the inverter or buffer, there can only be two possible input states:either the input is ”high” (1) or it is ”low” (0). As was mentioned previously in this chapter, a twoinput gate has four possibilities (00, 01, 10, and 11). A three-input gate has eight possibilities (000,