81.2. Basic Characteristics Required for Current Biasing Element

First of all the output impedance should be high what does it mean is that, suppose we have say this element whatever the current biasing element and it is having a current flow and then, we do have some application circuit. So, we call this is current biasing elements and this is the application circuit.

Now, we want this current should be well defined by this biasing element; see let we call this is I_Bias. And we want this I_Bias should be as independent as possible on the voltage across this, we may call this is V_Bias. So, that is what we want to say that if this I_Bias it is independent of this V_Bias, which means that if we plot the I-V characteristic of this biasing element I_Bias versus V_Bias. And we want theoretically it should be independent which means it is horizontal line.

In other words, we may say that the slope of this line slope of this line it is zero slope is quote and unquote 0. Or we want resistance you can see output resistance is quote and unquote ∞; practically we want it should be as high as possible. So, if we can achieve what we will get it is that, in case these applications circuit it is defining the voltage at this point.

This chunk focuses on the first basic characteristic required for a current biasing element: high output impedance. High output impedance means that the current flowing through the biasing element should remain stable regardless of changes in voltage across it. This stability suggests that the I-V characteristic (current versus voltage plot) of the biasing element should ideally be a horizontal line, indicating minimal slope. This high impedance allows the bias current to remain consistent even if the voltage changes due to variations in the circuit, which is essential for reliable circuit performance.

Think of high output impedance like a well-built dam that can hold back a large reservoir of water. If the water level rises (similar to voltage changes), the dam (the current biasing element) must be able to contain the water without overflowing (the current flowing through it remaining unchanged). A good dam effectively prevents fluctuations in the water level from affecting the overall landscape in the area behind it, just like an effective biasing element maintains stable current regardless of nearby voltage changes.

Then second characteristic we are looking for it is the current should be well defined. So, this I_Bias should be flowing through this current biasing element, should be well defined; namely I_Bias should be easy to implement. And important thing is that variation its variation with respect to whatever the design value. We do have should be as small as possible not only its static value, but even say instant inverse value should be as less dependent on any other variation namely temperature variation or supply variation.

In this chunk, the focus is on the second characteristic: the requirement for a well-defined current. This means that the bias current (I_Bias) should be as stable and constant as possible, minimizing any changes that may occur due to external factors such as temperature fluctuations or variations in the supply voltage. A well-defined current is crucial because it ensures that the circuit behaves predictably and consistently under different operating conditions. Thus, it should be straightforward to implement and should maintain its value close to the design specifications.

Consider the example of a thermostat regulating your home heating. The thermostat aims to maintain a constant temperature (the well-defined current). If the outside temperature fluctuates (similar to supply variations), the thermostat adjusts the heating to ensure that the indoor temperature remains stable. Just as the thermostat must react appropriately to changing conditions while maintaining the desired room temperature, the current biasing element must maintain a consistent output current, unaffected by temperature shifts or supply voltage changes.

Then the third characteristic we are looking for it is that this circuit should be operating with available supply voltage, which means that suppose we do have a supply voltage here it is say V_DD and then we do have ground here. So, we want this circuit the bias circuit should not be taking too much of this supply real keeping behind large amount of voltage drop for the application circuit. So, we want this current bias circuit should be operating with a less amount of voltage V_Bias across it.

The third characteristic emphasizes that the current biasing circuit should effectively operate within the constraints of the available supply voltage (V_DD). This means that the voltage drop across the bias circuit (V_Bias) should be minimized to ensure that enough voltage remains for the actual application circuit to function correctly. A design that consumes too much voltage in the biasing stage would leave insufficient voltage for the main circuit components, potentially leading to poor performance. Therefore, achieving this requirement is critical for efficient circuit design.

Imagine your monthly budget for spending money. If you spend most of your budget on utilities (like electricity and water bills), you might have very little left for entertainment or dining out. Similarly, in a circuit, if a lot of the supply voltage is 'spent' on the current biasing circuit, there’s not much left for the main application circuit to work effectively. Just as it's important to manage expenses wisely to have enough money for other activities, it’s crucial to design the bias circuit to operate efficiently within the voltage limits to ensure the performance of the entire system.

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