Sometimes TTL-compatible logic levels are not associated directly with TTL integrated circuits, for example, they may be used at the inputs and outputs of electronic instruments. TTL integrated circuits (ICs) were widely used in applications such as computers, industrial controls, test equipment and instrumentation, consumer electronics, and synthesizers.
Its name signifies that transistors perform both the logic function (the first "transistor") and the amplifying function (the second "transistor"), as opposed to resistor–transistor logic (RTL) or diode–transistor logic (DTL). The high collector current of T 1 shifts the stored charge in T 2 and T 3 and hence, T 2 and T 3 go to cut-off and T 1 saturates and then output Y returns to high.įig.Transistor–transistor logic ( TTL) is a logic family built from bipolar junction transistors. The consequent emitter base junction of T 1 starts conducting and T 1 base voltage drops to a low value. Because this current is flowing in the base of T 2, the transistors T 2 and T 3 saturate and then output Y is low.Ĭondition 3: The circuit is operating under II while one of the inputs becomes low. The collector current of T 1 flows in reverse direction. Therefore, T 1 is in reverse active mode. The collector base junction of T 1 is forward biased. The emitter base junctions of T 1 are reverse biased. Output is high (that is Y=1).Ĭondition 2: Each input is high. T 4 functions as an emitter follower and couples a high voltage to load. T 2 is cut off and forces T 3 to cut off. Thus, the base voltage of T 2 is almost zero. The operation can be described briefly by three conditions as specified below:Ĭondition 1: At least one input is low (that is, 0). The voltage drop of diode D remains the base-emitter junction of T 4 reverse biased therefore only T 3 conducts while output is low. The diode D is added to make sure that T 4 is cut off while output is low. The transistors T 3 and T 4 by the totem pole output, the capacitance CL shows the stray capacitance and so on. The transistor T 2 functions as a phase splitter since the emitter voltage is out of phase along with the collector voltage. In this figure, T 1 has 3 emitters thus there can be three inputs A, B, C. Multiple emitter transistors along with about 60 emitters have been developed. Such transistor can be thought of like a combination of various transistors along with a common collector and base. T 1 is a multiple type emitter transistor. The low output impedance means a short time constant RC therefore the output can change rapidly from one state to the other. The totem pole output implies that transistor T 4 sits atop T 3 in order to give low output impedance. Operation of TTL NAND Gate: Fig.(d) Demonstrates a TTL NAND gate with a totem pole output. Give the circuit of a TTL NAND gate and explain its operation in brief.
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