81.1. Current Mirror Circuits (Part- A)

So, dear students welcome back to our online certification course on an Analog Electronic Circuits. Myself Pradip Mandal from E and EC Department of IIT Kharagpur today’s topic of discussion is Current Mirror Circuits. We are in module-8 and presently we are in week-9. We are going to talk about current mirror.

In this introduction, the speaker welcomes the students and introduces the topic of current mirror circuits, which are essential elements in analog electronic circuits. The significance of current mirrors is highlighted by placing them in the context of the course structure, showing that they are foundational components necessary for understanding more complex circuits and applications.

Imagine the current mirror circuit as a blueprint for a building. Just as a blueprint helps us visualize how a structure will look and function, current mirrors help us understand how to regulate current in more complex electronic systems.

We shall start with motivation and a need of current mirror. Current biasing elements help to improve circuit performance. Current mirror circuits are useful not only as a biasing element but can also be considered as a buffer circuit in current mode amplifiers.

This chunk discusses why current mirrors are important. They serve the dual purpose of stabilizing and biasing circuits, which enhances their overall performance. Furthermore, current mirrors help in managing signal amplification, making them versatile components in analog electronics.

Think of current mirrors like a traffic management system. Just as traffic lights and signs regulate the flow of vehicles to prevent congestion and ensure smooth transit, current mirrors ensure that electrical signals flow properly without interference in electronic circuits.

There are essential characteristics we look for in current biasing elements. 1) High output impedance. 2) Well-defined current. 3) Operates with available supply voltage.

These three characteristics are crucial for a current biasing element to perform effectively. High output impedance means that changes in voltage do not affect the current substantially, which provides stability. A well-defined current allows for predictable circuit behavior, and operating within the supply voltage ensures that the biasing circuit can be effectively integrated with other components.

Consider the behavior of water in pipes. A high output impedance is like using a pipe that maintains a consistent flow regardless of pressure changes. A well-defined current is akin to knowing exactly how much water will come out of a faucet, and operating within supply voltage is like making sure the water system can work without exceeding the city's water pressure.

Before we enter the basic structure, we shall talk about the evolution of the current biasing element leading to current mirror circuits. The current biasing element consists of a mirror pair and a current reference.

Current mirrors consist of pairs of transistors that work together to maintain a consistent output current based on a reference current. This evolution signifies the transition from simple current sources to more sophisticated designs that ensure better control and performance in circuits.

This can be likened to a pair of identical twins. If one twin (acting as the reference) decides to perform a task, the other mimics this action precisely, ensuring that both twins function in harmony, just as the transistors in a current mirror do.

We will talk about analysis of current mirror, including deriving the expression for output current in terms of input or reference current and discussing mirroring ratio.

The analysis of current mirrors involves calculating how much current is mirrored from the reference point to the output. This process demonstrates how effective a current mirror is at maintaining consistent current levels, which is critical for the reliability of the circuit's operation.

Think of it as a relay team in a race, where one runner (the reference current) reliably passes the baton (current) to the next runner (the output current). The ability to pass the baton smoothly while maintaining speed is critical, much like how current mirrors transfer currents.

We shall talk about improvements of the basic current mirror to enhance performance, specifically for improving output resistance. We will also discuss both BJT and MOS implementations.

Improvements to current mirror designs focus on increasing output resistance to make them more effective in various applications. These enhancements ensure better performance under different operational conditions, making current mirrors even more versatile in integrated circuits.

Consider improving a car's suspension system to handle rough terrain. Just as better suspension allows a car to ride smoothly over bumps, enhancements in current mirrors allow them to perform reliably even under changing conditions.

The current mirror can also be used for current mode amplifiers. It works as a buffer, helping impedance matching between stages to prevent loading effects.

Current mirrors can act as buffers that prevent signal degradation when connecting multiple circuit stages. They allow for better communication between different parts of a circuit by ensuring that each stage operates efficiently without affecting the others.

Think of a relay station in communication lines that amplifies signals without distorting them. Just as the relay station ensures clear communication across long distances, a current mirror buffer maintains strong signal integrity across circuit stages.

Let us see the implementation of current mirrors. Different versions and their characteristics lead to the fourth version of current biasing element which combines various principles for optimal performance.

The implementation phase discusses various designs and methods of creating effective current mirrors, culminating in a design that balances the initial conditions with practical performance requirements. Each version improves upon the last by addressing previous limitations.

It’s like designing a new smartphone: each model improves upon the last by adding features and fixing issues. A current mirror evolves in a similar way to provide enhanced functionality in electronic circuits.

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