81.1.3. Analysis of current mirror

So, we are going to talk about the operating principles and analysis of current mirror. In the next lectures, we will be talking about application of current mirror; specifically for amplifier and signal mirroring.

In this section, the discussion revolves around the basic principles of the current mirror circuit. A current mirror is essential in amplifiers and for signal mirroring, acting as a bias circuit that influences the overall circuit performance. It provides a stable current source that is pivotal in both amplifying signals and ensuring consistent operation despite variations in voltage and temperature.

Think of a current mirror like a water fountain. Once set up (biased), it consistently delivers a stream of water (current) regardless of changes in the water level (voltage). This ensures that plants (amplifiers) nearby receive a consistent supply of water, helping them grow without interruption.

The basic characteristics we are looking for this bias current biasing element include: 1. Output impedance should be high; 2. The current should be well defined; 3. This circuit should be operating with available supply voltage.

Current biasing elements must possess three key characteristics: high output impedance ensures that the bias current remains stable regardless of voltage changes; well-defined current means that the current flowing through the bias element should be predictable and consistent; and operating with the available supply voltage means that the bias circuit must operate efficiently without wasting excessive voltage, facilitating smooth circuit operation.

Imagine a well-built road system for fire trucks (current) to travel. High impedance is like a broad and sturdy road ensuring trucks don't get delayed by bumps (voltage changes). A defined truck load symbolizes the precise amount of water (current) it carries, while efficient routes through the town showcase how the trucks manage to reach their destinations without unnecessary detours (voltage wastage).

Before we talk about the implementation, we need to recall the basic characteristic required for this current biasing element.

This segment introduces the reader to various implementations of current biasing elements, emphasizing that before exploring details, it is vital to remember the fundamental characteristics that make these biasing elements effective. Current mirrors are a sophisticated evolution of earlier biasing methods and are designed to meet stringent requirements for stability and reliability.

Think of current mirrors like the development of highways from simple dirt roads. While dirt roads served their purpose, modern highways incorporate multiple lanes (implementations) to handle varying traffic (current) while ensuring that vehicles (circuit components) can travel at stable speeds without delays due to bumps or obstacles (voltage fluctuations).

We do have different versions of current mirror or other current biasing elements...so we have better options, we may go for that.

This section outlines a comparison of several iterations or versions of current biasing elements, starting with simple resistors that provide basic functionality but lack reliability. As the versions advance to BJTs and MOSFETs, improvements in defining current and minimizing variations are discussed. Each iteration offers enhancements in performance, reflecting the ongoing pursuit of more efficient and stable methods for current generation.

Consider a growing company. Initially, it might rely on a single person (a resistor) to manage finances, which can lead to errors and instability. As the business matures, it might adopt an accounting software (BJT/MOSFET) that organizes expenses and improves accuracy, leading to better financial decisions. Ultimately, with a dedicated finance team (current mirror), the company ensures stable monetary flow and efficiency, adaptable to fluctuating market conditions.

This is the basic structure of the fourth version of the current biasing circuit... this application circuit it can also be reused to make another current bias.

The final part discusses the structure and operational principles of the fourth generation of current biasing circuits, combining the benefits of a current mirror with a stable current reference. This design aims not only to produce consistent output current regardless of variations in supply voltage or process parameters but also allows for scalability, meaning the same reference can be leveraged for multiple applications.

Think of a modern power grid. Just as it efficiently channels electricity (current) from a few power plants (resistant sources) to multiple neighborhoods (application circuits) while maintaining stability, a current mirror uses a stable reference current to ensure all connected devices operate smoothly and efficiently, adapting to changes in demand as needed.

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