81.1.5. Small signal model of current mirror

Now, let us see the implementation what are the possible implementation we do have. First of all before we talk about the implementation we need to recall the basic characteristic required for this current biasing element.

In this part, we begin discussing the implementation of current mirror circuits. Before delving into specific configurations, it's crucial to remember the fundamental characteristics that a current biasing element must maintain. These include high output impedance, defined current flow, and the ability to operate efficiently within given supply voltages.

Think of a current mirror as a water distribution system. Just as water needs to flow steadily and without interruption to reach various areas, a current biasing element must maintain a stable current and ensure it effectively distributes to the connected circuits. The characteristics are like setting up the proper pipes and valves so that water flows as needed without leaks or bursts.

So, here we are enlisting different versions of current mirror or other current biasing element I should not say current mirror it is current biasing element. So 1st versions, 2nd versions and then we also have 3rd versions.

The text outlines different versions or generations of current biasing elements. The first version involves simple resistors, which provide some current but lack the high output impedance necessary for effective performance. The second version introduces BJTs or MOSFETs, which operate in active or saturation regions, improving output impedance yet might still be influenced by temperature variations. Each version progressively addresses the limitations of the previous one, with the goal of stabilizing current flow regardless of external changes.

Imagine upgrading a power supply for an electronic device. The first version is like using a simple battery, which can drain quickly and lead to inconsistent power. The second version is akin to introducing a high-capacity rechargeable battery that stabilizes power better but can still be affected by temperature. Each upgrade addresses a flaw in the previous model, just like engineers improving power sources to maximize performance.

So, here is the basic structure of the fourth version of the current biasing circuit. Now, suppose we do have say two transistors; transistor-1 and transistor-2 and the transistor-1, it is say diode connected, namely its gate it is connected to drain.

In this section, we explore the implementation of the fourth version of a current mirror circuit, which uses two transistors. The first transistor is configured as a diode (its gate connected to its drain) to provide a stable reference current. This setup helps to maintain the current through the second transistor (the output transistor), ensuring that it mirrors the current defined by the first transistor without being significantly affected by supply voltage variations, as long as both transistors are in the saturation region.

Think of this current mirror as a leader (transistor-1) guiding followers (transistor-2) along a path. The leader shows the way without wavering, even if external conditions (like a storm or dust) try to divert them. Just like a leader's steady course ensures the group stays together, a well-designed current mirror keeps the current consistent across different circuits, regardless of fluctuation in supply voltages.

So, this is a good current biasing element. But the natural question is that we are looking for of course, we have to have this current reference to make this circuit working.

Here, the discussion focuses on the advantages of current mirrors. They provide stable and reliable biasing for circuits, making them ideal for maintaining performance in amplifiers. The use of a current reference allows these mirrors to function correctly in varying conditions. Even rudimentary current sources can provide adequate performance without needing overly complex circuits.

Imagine a manager in an office ensuring that all departments operate smoothly despite changes in workload or external pressures. This manager (current reference) doesn’t need to be overly complex; even a simple and reliable one can provide the stability needed for the entire office (current mirror) to function effectively, helping employees (circuits) perform their tasks without interruption.

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