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MOS TTL (Transistor-Transistor Logic)

With the rapid development of integrated circuits (ICs), were found new problems were and new solutions were developed. One problem with DTL circuits was taking much more space in the structure of the IC to build a transistor diode. Since the "real estate" are exceedingly important in ICs, it was desirable to find a way to avoid requiring a large number of diodes in the input. But what could be used to replace many diodes? Well, watching the DTL NAND gate at the top, you might notice that the opposing diodes are quite similar to the two junctions of a transistor. In fact, if we had an investor would have a single input diode, and it might be possible to replace the two diodes opposed by an NPN transistor to do the same job.

In fact, this works quite well. The figure above shows the resulting inverter. We can also add multiple emitters to the input transistor without worrying about the increase in the amount of space required on the chip. This allows us to build a multi-input gate in the same space as an investor. The resulting savings in real estate translates to significant savings in manufacturing costs, which in turn reduces the cost to the user of the device.

A problem shared by all logic gates with one output transistor's collector resistor is the switching speed. The transistor actively pulls down the output to logic 0, but the resistor is not active in pulling the output up to logic 1. due to unavoidable factors such as capacitance of the circuit and a characteristic of bipolar transistors called "charge storage," will take a certain amount of time the transistor to fully turn off and the output rises to a logic 1 level. This limits the frequency at which the gate can operate. Designers of commercial doors TTL IC reduced this problem by modifying the output circuit. The result was "the output circuit totem pole" used in most of the 7400/5400 series TTL ICs. The final circuit used in the majority in the standard TTL is shown in the top part. Does the number of entries may vary? a commercial package IC investors could be six, four 2-input gates, three 3-input gates, or two 4-input gates. An 8-input gate in a package is also available. But in each case, the circuit structure remains the same, differing only the number of entries.