CS302 Digital Logic Design GDB Solution Feb 14

GDB Topic:
‚ÄúDiode, Transistor, capacitor and Resistor are the electronic elements which can be used to form basic digital electronic components (i.e. gates). Every element has its own composition and working. According to you, which element has the vital role in the construction of gates? Give arguments in the support of your answer.”
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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 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.


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.