Op Amp Made from BJT: A Comprehensive Guide

Operational amplifiers, or op-amps, are fundamental building blocks in electronic circuits. They are versatile devices that can be used for a wide range of applications, from signal amplification to filtering and oscillation. One of the most intriguing aspects of op-amps is their ability to be constructed using bipolar junction transistors (BJTs). In this article, we will delve into the intricacies of creating an op-amp using BJTs, exploring its design, operation, and applications.

Understanding the Basics

Before we dive into the specifics of building an op-amp from BJTs, it’s essential to have a solid understanding of what an op-amp is and how it functions. An op-amp is an electronic device with two input terminals and one output terminal. It amplifies the difference between the voltages at its two input terminals and produces an output voltage that is proportional to this difference.

BJTs, on the other hand, are three-terminal semiconductor devices that can be used as switches or amplifiers. They have an emitter, a base, and a collector, and their operation is based on the control of current flow between these terminals.

Designing an Op-Amp from BJTs

Designing an op-amp from BJTs involves creating a circuit that can amplify the difference between two input voltages and produce an output voltage that is proportional to this difference. One common configuration for an op-amp made from BJTs is the long-tailed pair, which consists of two BJTs with their emitters connected together and their collectors connected to the output terminal.

Here’s a step-by-step guide to designing an op-amp from BJTs:

• Choose the appropriate BJTs: For an op-amp, you’ll need two BJTs with similar characteristics, such as gain and current gain. The BC547 and BC557 are popular choices for this purpose.

• Connect the emitters of the two BJTs together: This creates the long-tailed pair, which is the heart of the op-amp.

• Connect the collectors of the two BJTs to the output terminal: The output voltage will be proportional to the difference between the input voltages.

• Connect the bases of the two BJTs to the input terminals: The input voltages will determine the amplification factor of the op-amp.

• Adjust the biasing resistors: The biasing resistors will determine the operating point of the BJTs and, consequently, the gain of the op-amp.

Operation of the Op-Amp

Once the op-amp is designed and built, it’s essential to understand how it operates. The long-tailed pair configuration ensures that the output voltage is proportional to the difference between the input voltages. When the input voltage at one terminal is higher than the other, the BJT with the lower input voltage will conduct more current, causing the output voltage to increase. Conversely, when the input voltage at one terminal is lower than the other, the BJT with the higher input voltage will conduct more current, causing the output voltage to decrease.

Here’s a table summarizing the operation of the op-amp:

Applications of Op-Amps Made from BJTs

Op-amps made from BJTs have a wide range of applications, including signal amplification, filtering, and oscillation. Some common applications include:

• Signal Amplification: Op-amps can be used to amplify weak signals, making them suitable for applications such as audio amplifiers and sensor interfaces.

• Filtering: Op-amps can be used to filter out unwanted frequencies from a signal, making them