Transistor-transistor logic (TTL) is a digital logic design in which bipolar transistor s act on direct-current pulses. Many TTL logic gate s are typically fabricated onto a single integrated circuit (IC). TTL ICs usually have four-digit numbers beginning with 74 or 54.
A TTL device employs transistor s with multiple emitters in gates having more than one input. TTL is characterized by high switching speed (in some cases upwards of 125 MHz ), and relative immunity to noise . Its principle drawback is the fact that circuits using TTL draw more current than equivalent circuits using metal oxide semiconductor ( MOS ) logic. Low-current TTL devices are available, but the reduced current demand comes at the expense of some operating speed.