The Use of Transistors in Logic Circuits You Should Know

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Transistors are commonly featured in digital electronics. One of the key application areas of transistors is in digital electronics is in the making of logical circuits. As the name suggests, these are electric circuits that are designed for performing and executing logical operations.

All logical operations are based on the concept of Boolean algebra and their real-life applications entail the implementation of the logic gates. These gates include AND, OR, NOT, NOR, XOR, and NAND. These gates are designed to accept the input signals and then perform the logical functions so that you can get the desired output on the circuit.

So, where in the logic circuits do transistors come in? They are actually used for the construction of the logic gates. The inputs of the logic circuits get into the logic gates and then come out from the other end. This implies that the actual manipulation of the circuit takes place in the transistors. Keep reading as we explore the roles of transistors in logic circuits.

Table of Contents

Overview of the logic circuits

Before we focus on the functions of transistors, it will be prudent to have a brief overview of the performance of the logical gates.

As we have explained, logic circuits perform logical operations. They have input signals, logic gates, and output signals. They receive binary inputs and still give out binary outputs.

These circuits make it possible for computers and other devices to perform both simple and complex operations.

Usually, there are several logic gates that are used for making logical circuits. The number of gates used will depend on the complexity of the operations, among many other factors.

The image below represents a logical circuit that has two gates, AND and OR connected in series;

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The logic gates in the circuits have different outputs and here is the summary:

AND Gate: This logic gate produces a high output (1) only when both the inputs are high. Otherwise, the output will be low.
OR Gate: This logic gate is designed to produce high output as long as any of the inputs is high. It will produce low output (0) only when both inputs are low.
NOT Gate: This is usually referred to as the inverter circuit. Its output will always the opposite of the input. This implies that if the input is low, the output will be high, and the reverse is true.
NOR Gate: This logic gate is made of a combination of both the NOT and OR gates. It will give the high inputs (1) only on condition that all the inputs are low. Otherwise, all the other options will give low output.
NAND gate: Here, we also have two logic gates connected next to each other. The AND and NOT gates. Here, the output will be low only when both inputs are low. All the other input options will give you high output.
XOR Gate : Also known as the Exclusive OR, it is usually used as a one-bit adder. The output will be high only when the inputs are odd.

Using transistors to implement logic gates

Now that you know what the logic gates are, let’s look at how transistors are used to implement these gates in different logic circuits.

Usually, transistors are used in logic circuits because of their fast-switching capabilities. This implies that even though transistors are used for other functions such as amplification, they normally operate as switches when used in logic circuits. However, there are special cases where transistors in logic gates are used for amplification purpose.

When used as a switch, transistors control the flow of the electric signals. They determine when the signals are ON and OFF.

MOSFET and BJT are the two common types of transistors used as logic gates. They are usually arranged and configured to form transistor-transistor logic (TTL). This configuration allows the transistors to accept the input and give the output signals depending on the input. Since they are used in digital circuits, the inputs can either be high or low (1 or 0)

Transistor working as AND Gate

When used for making the AND gates, the transistors must be connected in series. If you want to get a high input, the inputs getting the gates must be in a conducting state.

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Transistors in the OR gate

When two transistors are used in the OR gate, they have to be connected parallel to each other. With this connection, the output will be high when either of the inputs is high or in a conducting state.

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Transistors as NAND Gate

When used for making NAND gate, two transistors are connected in series. However, the output terminal is positioned above the transistors and not next to the gates as it was the case of AND gate. The output of this logic gate will always be high unless if both inputs are high.

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Transistors as NOR gate

When used as NOR gate, you will only need one transistor to complete the logic circuit. It also features a resistor and two inputs. The output will be low when either of the inputs is high.

Applications areas of transistors in logic circuits

At this point, you know how transistors work and how they are used in the implementation of logic circuits. Now let’s look at their key application areas.

Logical operations: This is the primary function of the logical circuits. When the logic gates are included in the circuit, they are used for performing boolean expressions which are mainly in binary forms comprising of 1s and 0s.
In multiplexers: Also known as MUX, they are used in devices where the output depends on the input signals. The transistors in multiplexers control the flow of data in the system.
Data operations: Data processing in computers depends on the logic circuits. The transistors play a key role in manipulating data that is used for executing various digital operations.

By understanding the role of transistors in logic circuits, you can go ahead to implement your digital electronic projects.