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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)]
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
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
- Resistor-transistor logic gates use Transistors to combine multiple
input signals, which also amplify and invert the resulting combined
signal. Often an additional 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 often
available. However, they do draw a significant amount of current from the
power supply for each 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.
Although they are not designed for linear operation, RTL integrated
circuits are sometimes used as inexpensive small-signal amplifiers, or as
interface devices between linear and digital circuits.
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.
- 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
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
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.
- 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
Most logic families share a common characteristic: their inputs require
a certain amount of current in order to operate correctly. CMOS gates work
a bit differently, but still represent a capacitance that must be charged
or discharged when the input changes state. The current required to drive
any input must come from the output supplying the logic signal. Therefore,
we need to know how much current an input requires, and how much current
an output can reliably supply, in order to determine how many inputs may
be connected to a single output.
However, making such calculations can be tedious, and can bog down
logic circuit design. Therefore, we use a different technique. Rather than
working constantly with actual currents, we determine the amount of
current required to drive one standard input, and designate that as a
standard load on any output. Now we can define the number of standard
loads a given output can drive, and identify it that way. Unfortunately,
some inputs for specialized circuits require more than the usual input
current, and some gates, known as buffers, are deliberately
designed to be able to drive more inputs than usual. For an easy way to
define input current requirements and output drive capabilities, we define
two new terms:
- The number of standard loads drawn by an input to ensure reliable
operation. Most inputs have a fan-in of 1.
- The number of standard loads that can be reliably driven by an
output, without causing the output voltage to shift out of its legal range
Remember, fan-in and fan-out apply directly only within a given logic
family. If for any reason you need to interface between two different
logic families, be careful to note and meet the drive requirements and
limitations of both families, within the interface circuitry.