Theory of TTL Logic Family

Introduction:

Transistor-transistor logic (TTL) is a class of digital circuits made from bipolar junction transistors (BJT) and resistors. This is termed as transistor–transistor logic since both the logic gating function (example: AND) and the amplifying function are executed by the transistors (contrary with DTL and RTL).

TTL is noteworthy for being a widespread integrated circuit (IC) family employed in most of the applications like computers, test equipment industrial controls, instrumentation, and the consumer electronics, synthesizers, and so on. The designation TTL is sometimes employed to mean TTL-compatible logic levels, even when not related directly with TTL integrated circuits, for illustration as a label on the inputs and outputs of the electronic instruments.

TTL became the base of computers and other digital electronics. Even subsequent to much larger scale integrated circuits build multiple-circuit-board processors obsolete, TTL devices still found widespread use as the "glue" logic interfacing more compactly integrated components. TTL devices were originally made up in ceramic and plastic dual-in-line (DIP) packages, and flat-pack form. TTL chips are now as well made up in surface-mount packages. Successors to initial bipolar TTL logic frequently are interchangeable in function with original circuits, however with enhanced speed or lower power dissipation.

Totem pole in TTL:

What is totem pole?

The addition of an active pull up circuit in the output of a gate is termed as totem pole.

Why totem pole?

To raise the switching speed of the gate that is limited due to parasitic capacitance at the output.

TTL Transistor switching trouble:

  • Transistors are driven into deep saturation to entirely conduct or cutoff to switch off.
  • The answer of deep saturation is that the two junctions are at present forward biased.
  • The forward biasing of BC junction forces a big number of minority carriers to collector region.
  • Whenever the transistors switch off, this minority carrier requires to be eliminated. This takes a finite amount of time termed as the storage time (main component of the propagation delay) and therefore raises the switch off time.

Solution:

Prevent the transistor from going deep in saturation. This is accomplished by preventing the BC junction from becoming the forward biased.

The Schottky diode is employed to do the above by placing it across BC junction. Since of its lower barrier potential, this will conduct current from the base directly to collector prior to BC is forward biased. Therefore fewer carriers are then stored in the collector region and the switching becomes much quicker.

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