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TTL


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Abbrev. for transistor-transistor logic. 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 Vcc, 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.

Subjects: Computing.


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