81.3. Different Versions of Current Mirroring

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, the 1st version, 2nd version, and then we also have 3rd version. So, first one it is the simple resistor.

In this chunk, we introduce the concept of current biasing elements, specifically outlining that there are different versions. The first version mentioned is a simple resistor, which serves as a basic way to control current in a circuit. The focus here is on understanding the basics of current biasing.

Think of a simple resistor biasing current like adjusting the brightness of a lamp with a dimmer switch. You can make it brighter or dimmer, but it's limited and can lead to inconsistent performance.

So, we do have the application circuits connected here. And we want the current here it should be well defined. And we also want as we said that it should be well defined the output resistance here it should be high and also we want that the drop across this resistance should be as low as possible.

The use of a simple resistor as a biasing element is explained here. While it can define current, it has limitations. The output resistance of this configuration is low, which means that variations in voltage can affect the current significantly. The ideal characteristic for a current biasing circuit is to have high output resistance and minimal voltage drop across it, factors that are not met by a simple resistor.

Imagine trying to keep the water flow steady through a simple tube (the resistor). If the tube has a wide diameter (low resistance), changes in pressure (voltage) can easily alter the flow rate (current), making this arrangement unreliable.

Now, if you consider the second version, we do have one BJT or maybe a MOS transistor having a meaningful bias here as V_BE or V_GS.

The second version of current biasing involves using a BJT (Bipolar Junction Transistor) or MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). These components can provide better control of current because they operate based on their voltage input, making them less sensitive to fluctuations in voltage than a simple resistor. They can maintain a more stable current output when used appropriately.

Think of this as using a more sophisticated water valve that can maintain a steady flow despite changes in water pressure, rather than a basic faucet that allows flow to vary significantly with pressure changes.

So, we do have now 3rd version ... It is a strong function of these parameters particularly the β of the transistor.

In the third version, the biasing circuit uses resistors along with transistors (BJTs or MOSFETs) to improve the definition of current. This setup helps achieve a more precise current flow, though it still suffers from dependency on variations in transistor parameters, particularly the current gain (β). These parameters can change with temperature and manufacturing differences, hence posing a challenge for stability.

This is akin to having a more advanced irrigation system that adjusts water distribution based on specific crops' needs, yet still, it could be affected by changes in soil type or weather conditions, which makes its performance unpredictable.

In fact, that leads to the requirement of current mirror. So, the fourth version or fourth generation of current biasing element is basically a current reference along with a current mirror.

The fourth and final version is the current mirror concept, which combines a current reference and transistors to create a stable current source. The current mirror can provide a controlled output current that is less sensitive to variations in supply voltage or transistor parameters. It offers a high output impedance, making it a preferred choice for precision analog circuits.

Consider this like having a highly sophisticated water recycling system in a garden. This system maintains consistent water levels for the plants irrespective of changes in seasonal rainfall or temperature, ensuring that every plant gets the right amount of water consistently.

--