Logic circuits play a vital role in designing digital electronics circuits. A logic circuit generates desired output on the basis of inputs applied and on the basis of rules which the corresponding gate follows. Logic Circuits include devices such as multiplexers, registers, ALU’s and Microprocessors. All these devices are designed using gates and gates are designed using transistors.
There are four categories in logic families:
RTL (Resistor Transistor Logic): RTL which is the first member of this family is not available in monolithic form. Such type of digital circuits are built using resistor as an input network and transistors are used for performing switching function. Advantage of using RTL is the use of minimum number of transistors but disadvantage is power dissipation is high when transistors are switched on.
DTL (Diode Transistor Logic): DTL is commercially available IC in 53/73 series. This class of digital circuit is direct forebear of TTL and in this gating function is performed by diode and transistor is used to perform amplifying function.
TTL (Transistor Transistor Logic): TTL is designed using resistors and BJTs. In this, gating function and amplifying function are performed by using transistors.
ECL (Emitter Coupled Logic): ECL belongs to non saturated class of digital logic family. In this current is forced between two terminals of an emitter coupled pair so sometimes it is called as Current Steering logic (CSL), Current Mode Logic (CML), Current Switch Emitter-Follower (CSEF) logic.
· There is a disadvantage of high power dissipation in RTL circuits when transistor is switched on because of which high current is to be supplied and heat is generated inside the circuit. So to overcome these problems TTL circuits with Totem-Pole output are used.
· TTL circuits are developed to provide higher speed and lower power dissipation for optimization of design. A TTL circuit has simplified design in comparison to other members of logic families and has high speed in comparison to RTL and easy design layout in comparison to ECL.
· DTL has advantage of more fanout over RTL but propagation delay is relatively large.
· In ECL each gate continuously draws current so it requires more power in comparison to other members of logic families.
Author - Somya Bisaria
(Intern at Silicon Mentor)