Transistor-Transistor Logic

Select the Logic Family you wish to examine:

[Diode Logic (DL)] [Resistor-Transistor Logic (RTL)] [Diode-Transistor Logic (DTL)]
[Transistor-Transistor Logic (TTL)] [Emitter-Coupled Logic (ECL)] [CMOS]

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 left, 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.

In fact, this works quite nicely. What's even better is that we can easily construct a transistor with multiple emitters, using almost the same space on the IC as a standard transistor. The figure below and to the right shows the result.

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.

Select another Logic Family you wish to examine:

[Diode Logic (DL)] [Resistor-Transistor Logic (RTL)] [Diode-Transistor Logic (DTL)]
[Transistor-Transistor Logic (TTL)] [Emitter-Coupled Logic (ECL)] [CMOS]

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