TTL Logic Families and its Types.pptx

This material has enough information about TTL logic family.

1. TTL Logic Families and its Types

2. Introduction: Transistor-Transistor logic (TTL) is a class of digital circuits built from Bipolar Junction Transistors (BJT’s) and resistors. As the name suggests, both logic and amplifying functions are performed by the transistors. The BJT’s in TTL act on direct current. TTL ICs usually have Four-digit identification starting from 74 or 54. The first TTL IC was developed in 1965 and are known ”Standard TTL” that are not in use nowadays.

3. TTL Sub-families *There are several series of TTL technology such as:- Standard – 74 Low power – 74L High speed – 74H Schottky – 74S Low-power Schottky – 74LS Advanced Schottky – 74AS Advanced low-power Schottky – 74ALS

4. Identification of an IC: The commercial TTL ICs have a specific number starting with 74 followed by suffixes (S, LS, etc..) For example, if an IC is specified 74ALS, then we have to know that that particular IC belongs to the series of Advanced Low-power Schottky. The TTL family also includes a number series starting from 54 that is counterpart of 74 series.

5. Calculations: The Speed-power product is an important parameter for comparing various TTL series. This is the product of propagation delay and the power dissipated is measured in picojoules ( pJ ). The propagation delay of a transistor mainly depends on two factors. Storage time RC time constants.

6. Low Power TTL, 74L series: There are 4 high resistances in the circuit. (R1=40k, R2=20k, R3=500, R4=12k) The larger the resistance the reduction in current therefore reducing the power consumption (1mW). Also the speed is reduced with 33ns switching speed. The power consumption of the Low power TTL is 1/10 of that of standard TTL.

7. High speed TTL, 74H Series Having the same circuit of as the low power TTL, but in the changes in resistances. (R1=2.8k, R2=760, R3=50, R4=470) And the diode is replaced by another pair named Darlington pair (Q3-Q5) and is connected through a resistor of 4k. It has a high speed switching of 6ns and higher power dissipation of 22mW. The switching speed is of two times more than that of Standard TTL.

8. Schottky TTL, 74S Series In Schottky TTL’s, Schottky diodes are used before the transistors. The Schottky diode has a lower forward bias of 0.25V. When BC junction becomes forward biased, the Schottky diode bypasses the current preventing the transistor from going into saturation.

9. Low Power Schottky TTL, 74LS Series: Lower power and slower speed version of 74S series. Uses Schottky clamped transistor, but with larger resistances than 74S series.

10. 74AS and 74ALS series’: The ‘ALS’ and ‘AS’ series provides a considerable higher speeds than the ‘LS’ and ‘S’ series. The ‘ALS’ series offers a substantial reduction of power consumption over the ‘LS’ series and the ‘AS’ series offers power reduction over the ‘S’ series

11. Comparison of various TTL’s:

12. Family Evolution: