50.2. Common Gate Configuration

So, here we do have the common base configuration. 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 a common gate configuration, one of the first steps to analyze current gain is to ensure that the output terminal is unloaded. An unloaded terminal means it’s shorted to the AC ground, which allows us to focus on how much signal current flows through the circuit without any load impacting it. This is a crucial setup for accurate current gain measurement.

Imagine trying to measure the water flow rate in a pipe. If you attach a valve (load) at the end, the pressure may change, affecting your measurement. To get an accurate measurement, you would first remove the valve to measure the flow unobstructed—this is similar to what we do by unloading the output terminal.

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.

When analyzing small signals in a common gate configuration, we consider an unloaded condition where the terminating impedance is zero. This means that the collector node is grounded, which serves as a reference point. We observe the input signal current, which allows us to analyze how the input and output currents relate in this configuration.

Imagine you are listening to music through speakers; if the speakers are not connected (like having zero impedance), you can hear the music’s clarity without any distortion. In this case, grounding the collector node helps us see the clear flow of input signal, free from any external influence.

Now if you see this circuit again the base node it is grounded, voltage at the emitter we do have v. So, the voltage it is v it is − v right and part of the current is also flowing here.

In this setup, the base node is grounded, and we measure voltage at the emitter. The input voltage is defined as the difference between the emitter voltage and base voltage (Vbe). There are different parts of the current flowing through the circuit based on this voltage and the characteristics of the components involved.

Think of a river where the base of the river (ground) is fixed. The water level at different points (voltage) varies based on how much rain (input current) falls. Different currents flowing through the circuit can be likened to the varying flow of water in the river caused by variations in rainfall.

So, we can say that i is having different components; one is this part another is this part right and then we also have this current and this current.

Each component in the circuit contributes to the total current (i). By analyzing these current components individually, we can derive a formula to calculate the overall current gain in the common gate configuration. The output current depends significantly on the gain determined by the components involved.

Consider the ingredients of a recipe - the salt, sugar, and spice represent different components contributing to the flavor of a dish. Each ingredient adds its own unique taste, just like each current component contributes to the total current, affecting the overall output in the circuit.

In fact, you may recall this is nothing but, α of the transistor; that is very obvious. If I ignore this resistance if I feed a signal current here at the emitter whatever the current will be getting at the collector side, it depends on how much the current gain we do have from emitter to collector.

The current gain (α) in a common gate configuration is crucial as it indicates how much the input current at the emitter is amplified to the output current at the collector. If we simplify the model by ignoring certain resistances, we can see a direct relationship between input and output currents dictated by α.

Think of a magnifying glass that amplifies light. Just like the intensity of light (input current) increases as it passes through the lens (the transistor’s emitter to collector), the output current is also amplified based on the gain α of the transistor.

So, we can say that this circuit namely the common base, since its input resistance is low output resistance is high and the current gain is close to 1. So, it is a good circuit for current mode buffer.

The common base configuration features a low input resistance and high output resistance, making it ideal for applications where current buffering is essential. A current gain close to 1 indicates that this configuration can effectively handle current while maintaining signal integrity.

Imagine a signal booster for a radio. It takes in a weak signal (input) and boosts it to a strong signal (output) without altering the quality. This is equivalent to the common base configuration effectively buffering current with minimal loss.

The similar kind of analysis it can be done for common gate also. If I ignore this R then it is exactly = 1 that is very obvious.

The analysis for the common gate configuration is similar to that of the common base. Ignoring certain resistances allows us to see that the current gain can be approximated to 1, which means all the input current directly translates to the output without significant loss.

Think of a well that directly gives you water from the ground—whatever you draw from it (input current) comes out without delays or blockages (output current). This illustrates how common gate configurations effectively deliver current.

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