23.5.4. Summary of Current Amplifier Model

Whenever we are talking about current amplifier similar to voltage amplifier, what does it mean is that, it is an equivalent linear circuit, which provides dependency of the output signal output current signal on the input current signal.

A current amplifier is a type of amplifier that, like a voltage amplifier, represents an equivalent linear circuit. The key difference is that a current amplifier works by relating the output current to an input current rather than voltage. Essentially, it amplifies the input current signal to produce a larger output current signal.

Think of a current amplifier like a water pipeline where the pressure of water (representing input current) at the start of the pipeline controls the flow of water (output current) at the end. The stronger the water pressure at the beginning, the greater the flow at the outlet, similar to how the input current influences the output current.

Here we do have one example having this is also amplifier having 1 BJT. And, as you can see here, what are the things we do have is the BJT is at the center place, and then it is having a DC bias through the R C.

In the given example, a Bipolar Junction Transistor (BJT) is used in the current amplifier configuration. The BJT is connected with a DC bias using a resistor, which helps it operate in the active region. This ensures that the BJT can amplify the current properly as it allows the transistor to switch and amplify signals efficiently.

Imagine the BJT as a valve controlling water flow in a pipe. The DC bias is like the valve's initial setting that ensures a consistent supply of water through the main pipeline—without this bias, the flow might be too low or non-existent, affecting the overall system performance.

At the base of the transistor what you can say that the base current to the transistor it is having two components. Namely the I_B, the DC part, and also the time varying part.

The base current in the BJT amplifier consists of two components: a DC component and an AC component (time-varying part). The DC part maintains a constant biasing level, and the AC part represents the actual signal being amplified. This combination is essential for achieving the desired amplification while keeping the transistor in the correct operating region.

Consider the base current components like a musical note. The DC part is like the steady tone of a piano note, and the AC part is the melody that varies over time. Just like a good musician needs to maintain the steady tone while playing a beautiful melody, the current amplifier needs these two parts working together to function effectively.

We like to short this output node to ground and then we like to extract the entire signal, but while you are doing this, we have to make sure that the output node it is not really or other the collector node it should not get shorted to DC ground.

To extract the output current from the current amplifier, the output node is short-circuited to ground. This allows us to capture the entire AC signal while ensuring that we do not interfere with the DC component. Using a DC blocking capacitor can prevent the DC current from affecting the measurement and keeps the circuit’s operational integrity.

This can be compared to a water tap. When you want to measure how much water is flowing from the tap, you can place a bucket beneath it (short-circuiting). However, you need to make sure that you don't let any water stay in the pipe (DC component) that could alter your measurement of the flow, so you keep it clean and free of obstructions.

Now, if we are having load resistance R_L, which is non-zero, then whatever the reduced current will be seen that reduce current will be again it can be calculated by considering this R_L.

In practical scenarios, when a load resistance is connected to the output of the current amplifier, it divides the total output current. This means that not all of the internal current reaches the output. The load resistance effectively reduces the current we can see at the output, which must be taken into account when designing and analyzing the amplifier.

This is like sharing a pizza among friends. If you have a pizza (output current) to share and your friend takes a slice (load resistance), there will be less pizza left for others. Similarly, when the load resistance takes some of the output current, the current available for the rest of the circuit decreases.

So, the current amplifier model it is shown here by this dotted line. So, this is what the current amplifier model. And, then as I said that it is having 3 important parameters namely the unloaded current gain A, which gives the internal output current.

The final model representation of a current amplifier includes three critical parameters: unloaded current gain (A), which indicates how much the input current is amplified to produce the output current. Additionally, input and output resistances describe how the amplifier interacts with the rest of the circuit. These parameters shape how the amplifier behaves in various scenarios and are essential for understanding its performance.

Think of the model representation like a recipe for making a cake. The unloaded current gain is your main ingredient that determines how tall your cake will rise. The input and output resistances are like the baking time and temperature—important factors that ensure the final cake is delicious and comes out perfectly every time.

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