TTL and CMOS logic families.

TTL logic families:

  • The standard TTL gate was the first version in the TTL family, this gate was designed with different resistors values to produce gates with lower power dissipation or with higher speed.

  • In the low power TTL gate, the resistor values are higher than in the standard gate to reduce the power dissipation, but the propagation delay is increased.

  • The schoyley TTL gate was the next improvement in the technology. the effect of the schottley transistor is to remove the storage time delay by preventing the transistor from going into saturation.This series increases the speed of operation without an excessive increase in power dissipation.

  • The differences in TTL series are not in digital logic that they perform, but rather in the internal construction of basic NAND gate. in any case, TTL gates in all available series come in 3 different types of output configuration: Open collector output, totem-pole output and three state (Tri-state) output.

CMOS logic families:

  • Complementary MOS (CMOS) circuits take advantage of the fact that both n-channel and p-channel devices can be fabricated on same substrate.

  • CMOS circuits consist of both types of MOS devices interconnected to form logic functions.

  • The original design of CMOS ICS is recognized from 4000 number designation. the performance characteristics of 74c series are same as 4000 series.

  • The high speed CMOS 744C series is an improvement of the 74c series with a tenfold increase in switching speed.

  • The 74HCT series is electrically compatible with TTL ICs. this means that the circuits in this series can be connected to inputs and outputs of TTL ICs without need of additional interfacing circuits.

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