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What Is Diode-Transistor Logic?

By G.W. Poulos
Updated May 16, 2024
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Diode-transistor logic refers to a specific class of circuit used in modern digital electronics to process electrical signals. The construction of these circuits employs bipolar junction transistors, semiconductor diodes, and resistors. A diode-transistor logic circuit uses its diodes to perform logic functions and a transistor to perform amplification functions. This is in contrast to resistor-transistor logic circuits, the predecessor of diode-transistor logic, which use bipolar junction transistors and resistors for both the logic and amplification functions.

Digital logic circuits, called gates, perform functions on electrical signals, such as addition, subtraction, multiplication, and division. An AND gate, for example, can have two inputs, numbered one and two, and one output. When a signal is high on both input one and input two, the gate will send a high signal from its output. Engineers call these logic circuits because they act logically and predictably in response to various input combinations.

In the example of an AND gate, it can only respond a certain number of ways to any combination of inputs. Potential responses for logic gates are often listed as a simple set of mathematical formulas. The possible responses for a two-input AND gate, with the first term being input one, the second term being input two, and the sum being the gate's output, are as follows: 0+0=0, 1+0=0, 0+1=0 and 1+1=1. Logic gates come in many other types, including NAND, OR, and NOR gates. Each of these logic gates provides a different set of logical functions that, when combined, can perform any combination of mathematical executions on any combination of electrical signal inputs.

The first logical functions in electronics were performed via manual switches, where a given switch would be flipped to provide an output when the operator saw the requisite signals had been provided — usually indicated by a series of lights. Later, these functions were automated with electronic relays. These devices were large and slow, and suffered from human error and mechanical failure.

With the advent of the solid-state transistor, a device that naturally requires two inputs to provide an output, gating functions became faster and more reliable, and the first true digital logic circuits were built, with the use of resistors creating resistor-transistor logic (RTL) technology. As the technology progressed, it was realized that using semiconductor diodes in place of the resistors would not only increase the operational speed of the logic gates but would also allow for greater fan-in, which in simplest terms means the gates could have more than two inputs. Thus was born diode-transistor logic technology (DTL), which became the standard for logic gates.

As transistor technology grew, new devices, such as field effect transistors, became available to engineers. These devices are faster and smaller, and consume less power than the transistors used in diode-transistor logic circuits. Using field effect transistors in place of the DTL diodes, the resulting logic gates operate much faster and can have multiple outputs. As a result, this newer transistor-transistor logic technology, dubbed TTL, has widely replaced DTL and is the new standard in logic gate construction.

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