Chapter 9 explores details of the two bipolar circuit families that have been used extensively in logic circuit design since the mid-1960s. Emitter-coupled logic (ECL) historically has been the fastest form of logic available. The ECL circuit uses bipolar transistors operating in a differential circuit that is often called a current switch. For binary logic operation, two states are needed, and in ECL, the transistors operate in the forward-active region with either a relatively large collector current or a very small collector current, actually near cutoff. The transistors avoid saturation and an attendant delay time that substantially slows down BJT switching speed.

Transistor-transistor logic (TTL or T²L) was the dominant logic family for systems designed through the mid-1980s, when it began being replaced by CMOS. TTL was the family that established 5 V as the standard power supply level. The main transistors in TTL switch between the forward-active -- but essentially nonconducting -- and saturation regions of operation. In the TTL circuit, we find one of the few actual applications for the reverse-active region of operation of the BJT. Because various transistors in the TTL circuit enter saturation, TTL delays tend to be poorer than those that can be achieved with ECL. However, an improved circuit, Schottky clamped TTL, is substantially faster than standard TTL or can achieve delays similar to standard TTL at much less power dissipation.