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Transistor-Transistor Logic
4 input DTL NAND gate.

With the rapid development of integrated circuits (ICs), new problems were encountered and new solutions were developed. One of the problems with DTL circuits was that it takes as much room on the IC chip to construct a diode as it does to construct a transistor. Since "real estate" is exceedingly important in ICs, it was desirable to find a way to avoid requiring large numbers of input diodes. But what could be used to replace many diodes?

Well, looking at the DTL NAND gate to the right, we might note that the opposed diodes look pretty much like the two junctions of a transistor. In fact, if we were to have an inverter, it would have a single input diode, and we just might be able to replace the two opposed diodes with an NPN transistor to do the same job.



TTL inverter.

In fact, this works quite nicely. The figure to the left shows the resulting inverter.

In addition, we can add multiple emitters to the input transistor without greatly increasing the amount of space needed on the chip. This allows us to construct a multiple-input gate in almost the same space as an inverter. The resulting savings in real estate translates to a significant savings in manufacturing costs, which in turn reduces the cost to the end user of the device.



Commercial TTL NAND gate circuit.

One problem shared by all logic gates with a single output transistor and a pull-up collector resistor is switching speed. The transistor actively pulls the output down to logic 0, but the resistor is not active in pulling the output up to logic 1. Due to inevitable factors such as circuit capacitances and a characteristic of bipolar transistors called "charge storage," it will take a certain amount of time for the transistor to turn completely off and the output to rise to a logic 1 level. This limits the frequency at which the gate can operate.

The designers of commercial TTL IC gates reduced that problem by modifying the output circuit. The result was the "totem pole" output circuit used in most of the 7400/5400 series TTL ICs. The final circuit used in most standard commercial TTL ICs is shown to the right. The number of inputs may vary — a commercial IC package might have six inverters, four 2-input gates, three 3-input gates, or two 4-input gates. An 8-input gate in one package is also available. But in each case, the circuit structure remains the same.


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