Transistor as a Logic Switch

Transistor logic switches, also known as Transistor-Transistor Logic (TTL) became widespread after the invention of the bipolar junction transistor (BJT). It is difficult for students to believe that prior to the invention of the transistor, logic gates used valve technology. A switching transistor could perform the logic function and the amplifying function, which changed the digital electronics industry overnight. A simple transistor circuit may perform all the functions of Boolean logic, such as "AND", "OR", and "NOT". With the invention of the integrated circuits (IC), their incorporation into logical building blocks occurred readily to form the famous 74 series of chips that we use today.

Transistor NOT function

Here is a simple inverter switch, which behaves as a Boolean NOT gate. When there is no signal at the base junction, the transistor is not conducting, and the LED remains switched ON due to the potential across it. When there is a signal at the base, the transistor conducts and the LED switches OFF.

When the transistor conducts, the voltage drop across its collector-emitter junctions becomes almost zero. The flow of current is then almost entirely through the transistor and not the LED and therefore the LED turns OFF.

Transistor Gates

If you want to see more transistor gates then I have a separate article dedicated just for this. Please use the following link.

Basic Logic Gates, Truth Tables, and Functions Explained

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