Q. DTL AND TTL LOGIC CIRCUITs?

Bipolar transistorswere the first solid-state switching devices commonly used to implement digital logic circuits in the 1950s and 1960s. These circuits used diodes at the input of the gate for logic operation followed by a transistor (BJT) output device for signal inversion.One of the bipolar logic families that emerged was called DTL (diode-transistor logic). The TTL (transistor-transistor logic) soon replaced DTL and then became the principal bipolar technology for the next two decades. It is still often used today. In TTL circuits, the diodes used in DTL at the gate input are replaced with a multiemitter transistor for increased performance. Primarily by reducing the  size of the transistors and other components, speed and performance improvements have been made in TTL circuits.

Both DTL and TTL are called saturating logic families, because the BJTs in the circuit are biased into the saturated region to achieve the effect of a closed switch. The inherent slow switching speed is a major difficulty with saturating logic because saturated BJTs store significant charge and switch rather slowly. Schottky TTL is a nonsaturating logic family that was later developed to achieve higher speed performance by preventing the transistors from saturating.

Another bipolar nonsaturating logic family is ECL (emitter-coupled logic), which has BJTs that remain biased in the active region. These circuits consume more power, are less dense, but are extremely fast.

Using the transistor switch, logic gates can be constructed to performthe basic logic functions such as AND, OR, NAND, NOR, and NOT. Since the individual gates are available in the form of small packages (such as the dual-in-line package, or DIP), it is generally not necessary to design individual gates in order to design an overall digital system. However, the designer needs to observe fan-out restrictions (i.e., themaximumnumber of gates thatmay be driven by the device), fan-in restrictions (i.e., the maximum number of gates that may drive the device), propagation delays, proper supply voltage to the unit, and proper connections to perform the intended logicfunction. Gates of the same logic family can be interconnected since they have the same logic voltage levels, impedance characteristics, and switching times. Some of the logic families are discussed in this section.