81.3.2. Fourth Version: Current Reference with Current Mirror

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. And let we flow one current call say I reference maybe that is coming from the supply voltage.

The Fourth Version introduces a refined structure using two transistors to improve current biasing. Here, one of the transistors (transistor-1) is configured in a diode connection, which means its gate is connected to its drain terminal. This configuration allows for the creation of a stable reference current (I_reference) drawn from a supply voltage, making it the starting point for deriving other currents in the circuit.

Think of this configuration like a water fountain with a dedicated reservoir (the supply voltage) that ensures a consistent flow of water (current) into a connected pipe (the circuit). The fountain's nozzle is similar to the diode-connected transistor, which maintains a steady, controlled stream of water steady regardless of changes in the connected pipes.

As long as this transistor-2 it is in a saturation region, then we can say that I it is which is I of transistor-2 in this case that is I which is in fact, the bias current for the application circuit, which is quote and quote independent of V.

Transistor-2 is designed to operate within a saturation region. This state is vital because it allows the current I to be stable, acting as the bias current needed for the application circuit. The current is ideally independent of the voltage across the transistor (V_DS), which means any variations in voltage won’t affect the amount of current flowing through, thus enhancing reliability and performance in circuit applications.

Imagine a well-tuned car engine that runs smoothly regardless of how steep the hill is (voltage across the transistor). Whether you’re driving uphill or downhill, the car's speed (current) remains steady due to the engine's excellent design, just like transistor-2's ability to maintain consistent bias current.

Based on this current and based on its dimension, it produces a voltage called V_GS1 and the same V we are deploying to transistor-2. So, instead of having independent potential divider, we are generating a voltage at this point for transistor-2...

The voltage V_GS1 (gate-source voltage of transistor-1) generated due to the current I_reference is applied to transistor-2, ensuring it's in the right operating region. This voltage not only influences how the transistor works but also helps in creating a system where the reference current is translated into the bias current for other circuits without needing separate power sources or configurations.

Think of this like a series of dominoes. The push you give the first domino (I_reference) influences the position and stability of the subsequent dominoes (V_GS1 applied to transistor-2), ensuring they all stay in line and function properly without needing individual adjustments.

This current flow through transistor-2 is independent of the threshold voltage of the inverter. Of course, here we are assuming that this V_th of both the transistor they are equal.

By assuming the threshold voltages (V_th) of both transistors are equal, the circuit minimizes the influence of variations that often occur in manufacturing. This design ensures that changes in one transistor do not adversely affect the current output, securing consistent operation across multiple devices or conditions.

This scenario can be compared to having a standardized shipping container. Regardless of the size of the truck (transistor), as long as the container’s base fits all trucks (equal V_th), loading and unloading remains smooth. This helps in maintaining efficiency no matter which truck is used for the delivery.

Even if you have a primitive current source, say for example, if you have a simple resistor connected that will also serve the purpose. Because in this case drop across this transistor it is not so big living behind a big amount of a voltage drop available across this resistors.

The design allows even simplistic structures like resistors to function as the current reference without incurring significant voltage drops in the system. This feature improves overall efficiency by ensuring minimal voltage is wasted across the current reference, allowing maximum voltage to be available for the application circuit itself.

Think of it like using a small pipe (the resistor) to feed water into a larger system. As long as the small pipe doesn't restrict the flow too much, the overall system receives enough water to function effectively, using less energy and resources.

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