Inside Logic Gates

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]

I have received a number of requests, asking just what goes on inside logic gates to actually perform logic functions. So, by popular demand, here are the internal schematics of various gates, as implemented by several different logic families.

I won't cover the internal operation of individual semiconductor devices in these pages, except to state the basic behavior of a given device under specific conditions. More detailed coverage of semiconductor physics and internal behavior is a job for another set of pages, which will come later.

There are several different families of logic gates. Each family has its capabilities and limitations, its advantages and disadvantages. The following list describes the main logic families and their characteristics. You can follow the links to see the circuit construction of gates of each family.

Diode Logic (DL)

Diode logic gates use diodes to perform AND and OR logic functions. Diodes have the property of easily passing an electrical current in one direction, but not the other. Thus, diodes can act as a logical switch.

Diode logic gates are very simple and inexpensive, and can be used effectively in specific situations. However, they cannot be used extensively, as they tend to degrade digital signals rapidly. In addition, they cannot perform a NOT function, so their usefulness is quite limited.

Resistor-Transistor Logic (RTL)

Resistor-transistor logic gates use resistors to combine multiple input signals, and a transistor to amplify and invert the resulting combined signal. Often a second transistor is included to re-invert the output signal. This combination provides clean output signals and either inversion or non-inversion as needed.

RTL gates are almost as simple as DL gates, and remain inexpensive. They also are handy because both normal and inverted signals are usually available. However, the use of resistors to perform a logical OR function forces a limit on the number of inputs that can be handled by the gate. Another limitation is that RTL gates cannot switch at the high speeds used by today's computers, although they are still useful in slower applications.

Diode-Transistor Logic (DTL)

By letting diodes perform the logical AND or OR function and then amplifying the result with a transistor, we can avoid some of the limitations of RTL. DTL takes diode logic gates and adds a transistor to the output, in order to provide logic inversion and to restore the signal to full logic levels.

Transistor-Transistor Logic (TTL)

The physical construction of integrated circuits made it more effective to replace all the input diodes in a DTL gate with a transistor, built with multiple emitters. The result is transistor-transistor logic, which became the standard logic circuit in most applications for a number of years.

As the state of the art improved, TTL integrated circuits were adapted slightly to handle a wider range of requirements, but their basic functions remained the same. These devices comprise the 7400 family of digital ICs.

Emitter-Coupled Logic (ECL)

Also known as Current Mode Logic (CML), ECL gates are specifically designed to operate at extremely high speeds, by avoiding the "lag" inherent when transistors are allowed to become saturated. Because of this, however, these gates demand substantial amounts of electrical current to operate correctly.

CMOS Logic

One factor is common to all of the logic families we have listed above: they use significant amounts of electrical power. Many applications, especially portable, battery-powered ones, require that the use of power be absolutely minimized. To accomplish this, the CMOS (Complementary Metal-Oxide-Semiconductor) logic family was developed. This family uses enhancement-mode MOSFETs as its transistors, and is so designed that it requires almost no current to operate.

CMOS gates are, however, severely limited in their speed of operation. Nevertheless, they are highly useful and effective in a wide range of battery-powered applications.

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]

Go to the Logic Family base page.

Go to the Logical Story pages.

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