Special-output gates

Chapter 3.8 Special-output gates

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

  • 80CHAPTER 3. LOGIC GATES• B-series CMOS gates have ”buffered” outputs to increase voltage gain from input to output,resulting in faster output response to input signal changes. This helps overcome the inherentslowness of CMOS gates due to MOSFET input capacitance and the RC time constant therebyengendered.3.8Special-output gatesIt is sometimes desirable to have a logic gate that provides both inverted and non-inverted outputs.For example, a single-input gate that is both a buffer and an inverter, with a separate output terminalfor each function. Or, a two-input gate that provides both the AND and the NAND functions in asingle circuit. Such gates do exist and they are referred to as complementary output gates.The general symbology for such a gate is the basic gate figure with a bar and two output linesprotruding from it. An array of complementary gate symbols is shown in the following illustration:Complementary bufferComplementary AND gateComplementary OR gateComplementary XOR gateComplementary gates are especially useful in ”crowded” circuits where there may not be enoughphysical room to mount the additional integrated circuit chips necessary to provide both invertedand noninverted outputs using standard gates and additional inverters. They are also useful inapplications where a complementary output is necessary from a gate, but the addition of an inverterwould introduce an unwanted time lag in the inverted output relative to the noninverted output.The internal circuitry of complemented gates is such that both inverted and noninverted outputschange state at almost exactly the same time: