What is Emitter-Coupled Logic?

When multiple digital signals have to be combined, engineers use several types of logic gates. One of the most popular and widely used types of logic gates made of transistors is called the Emitter-Coupled Logic or ECL. It makes use of a transistor-based differential amplifier to amplify and combine digital signals. Usually, these circuits or gates, as they are commonly known, have multiple inputs and most are single output. Circuit design ensures that none of the transistors in the gate ever saturates nor are they completely turned off. That means the transistors are always working in their linear active operational region and therefore, do not have to contend with a charge storage time. That makes these gates work at extremely high speeds and this is the main advantage of Emitter-Coupled Logic gates.

For example, consider a three-input OR/NOR gate from the Motorola series of MECL devices. This circuit works on standard voltages of -5.2V (VEE) and ground (VCC). Any unused inputs have to be tied to VEE to prevent erratic behavior. The bias circuit is made up of transistors and associated diodes and resistors (not shown). The circuits are generally packaged as integrated circuits as multiple gates in a single IC. Typically, such ICs include quad 2-input, triple 3-input and dual 4-input gates. Here, the gates differ only in the number of input transistors they are built of, while a single bias circuit suffices for all the gates.

While in operation, a logic output changes its state from a high of -0.75V to a low of -1.60V, a change of only 0.85V. The differential amplifier transistors receive a fixed bias of -1.175V from the internal bias circuit. Now, if all inputs are at -1.6V or tied to the VEE rail, the input transistors are turned off, with the internal differential transistors alone conducting current. This causes the base voltage of the OR output transistor to reduce and its output voltage remains at -1.6V. Simultaneously, since none of the input transistors is affecting the base of the NOR transistor, its output rises to -0.75V, which is the emitter-base voltage VBE of the transistor itself. All transistors in the IC are designed to show a VBE of 0.75V.

As soon as an input rises to -0.75V, that transistor draws emitter current away from the internal differential transistors. This causes the outputs to switch states. Although these circuits work with very small voltage changes, which are typically dictated by the VBE of the internal transistors, the current flowing through various parts of the circuit is of greater importance. This is why Emitter-Coupled Logic has another name – Current Mode Logic or CML. Many other logic types also implement the CML and all of them suffer a major drawback – they draw a great deal of current from the power supply and tend to dissipate heat to a significant amount.

To counter this drawback, other logic systems have evolved, such as TTL and CMOS. For example, high-speed devices such as frequency counters employ ECL only at the input ends of their circuitry, following it up with high-speed CMOS or TTL counters at later stages.