A widely used family of logic circuits that is produced in integrated-circuit form and whose principal switching components are bipolar transistors. It is available in low power and high switching speed versions (see Schottky TTL), in addition to the standard form.

The diagram shows the equivalent circuit of a TTL two-input NAND gate. The basic circuit uses a multiemitter bipolar transistor, Q₁, which is easily fabricated in integrated-circuit form. Each base-emitter junction of Q₁ effectively acts as a diode, in a similar manner to a DTL input stage. Thus if all inputs are at a high voltage (logic 1), all input ‘diodes’ are reverse biased; the collector voltage of Q₁ rises to V_cc, turning on Q₂ (which acts as a phase splitter). The emitter voltage of Q₂ rises while its collector voltage falls, turning Q₃ on and Q₄ off. The output thus falls to logic 0, i.e. zero volts.

If any one of the Q₁ inputs is returned to logic 0, 0 volts, then Q₁ is turned hard on, turning off Q₂ whose collector voltage rises; this turns on Q₄. No current is available via Q₂ for Q₃’s base, and so Q₃ turns off. The output thus increases to +5 volts, i.e. logic 1. Diode D₁ is included to establish the correct bias conditions for Q₄. The output stage, consisting of Q₃, D₁, Q₄, and R, acts as a power amplifier and is often termed a totem-pole output. The output stage has the property of providing a low-impedance drive for both positive and negative signals. It provides a good fan-out but cannot be connected to other TLL outputs as can open-collector outputs.

TTL is the most commonly used technology for SSI and MSI devices due to its low cost, high speed, and ready availability.

TTL. NAND gate