50.1. Common Base Configuration

So, here we do have the common base configuration. We do have the corresponding circuit here and to get the current gain what we have to do? At the output node we have to make their corresponding terminal unloaded. What do you mean by unloaded? We have to basically short this node to ac ground and then we have to find how much the current it is coming from the circuit signal current.

In the common base configuration, the goal is to determine the current gain of the circuit. To do this, we need to ensure that the output node is 'unloaded,' which means connecting it directly to an AC ground. This setup allows us to analyze the current coming from the input signal without external influence.

Imagine trying to measure the water flow from a hose. To get an accurate measurement, you need to ensure that the hose's outlet is not blocked and is open to the air. Similarly, by making the output node unloaded, we can accurately measure the current flowing in the circuit.

We are putting this capacitor, so that the operating point of the transistor it is not getting affected and at the same time signal wise we are observing the short circuit output current.

A capacitor is placed in the circuit to ensure that the measurement of the signal does not affect the transistor's operating point. It allows AC signals to pass while blocking DC, thus ensuring that we can analyze only the signal current without altering the base voltage of the transistor.

Think of a water filter that allows only certain sizes of particles to pass through while blocking larger ones. Similarly, the capacitor filters out DC influences, enabling only the AC signal components to be analyzed.

And we know that if the signal it is in current form unloaded condition should be the corresponding impedance or the terminating impedance should be 0. So, small signal model if you see the corresponding situation here it is this node the corresponding collector node it is ground and we are observing the corresponding signal current i for their input signal it is i.

In the unloaded condition, the impedance seen at the output should ideally be zero. This setup simplifies the analysis of the small signal model, allowing us to assess the relationship between the input signal current and the output current. At this point, the collector node is effectively grounded, and we can measure how the input current flows through the circuit.

Consider a seesaw balanced at the center. If one side has no weight (impedance zero), it shows how well the forces (currents) on either side interact. Similarly, having a zero load allows us to see the relationship between input and output currents clearly.

So, in summary we can say that i it is it can be directly written in terms of v. On the other hand if you see the current at the output terminal here. So, if this is the current. In fact, this current of course, this node it is grounded. So, the current here it is actually 0 because this is also ground this is also ground.

To calculate the current gain (often denoted as α), we relate the output current to the input current. Since the output terminal is grounded, no current flows directly at that terminal, implying that all the output current is due to the input current in the circuit. Therefore, we analyze how much of the input signal (i) leads to the output (i).

Think of a teacher (input current) impacting students (output current). If the classroom (output terminal) has no students present, the impact can't be observed directly. However, any teaching efforts immediately show results by how many students actually engage (current relationship in the circuit).

So, we can say that this current gain it is less than 1, but it is very close to 1. So, that gives us good you know conclusion that this circuit namely the common base, since its input resistance is low output resistance is high and the current gain it is it is close to 1. So, it is a good circuit for current mode buffer.

The current gain in a common base configuration is typically slightly less than one but close to it. This indicates that while the circuit does not increase the current, it effectively transmits the input current with minimal loss. Additionally, the configuration offers low input resistance and high output resistance, making it suitable for current buffer applications.

Think of a quality speaker system at a concert. It doesn’t generate its own sound but effectively transmits the volume level of the instruments and vocals to a larger audience. Similarly, the common base configuration helps transmit current efficiently without significant gain.

--