23.2.2. Department of Electronics and Electrical Communication Engineering

So, let us talk about other kinds of amplifier called current amplifier. And, 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.

In this section, we introduce the concept of current amplifiers, differentiating them from voltage amplifiers. A current amplifier is defined as a linear circuit where the output current is proportional to the input current. This illustrates how the amplifier operates; by taking an input current signal and producing an amplified output current signal.

Think of a current amplifier like a water pump. If you push a certain amount of water (in this case, the current) into the pump, it will pump out a larger volume of water at the other end (the output current). Just as different pumps can handle different volumes of water, different current amplifiers can amplify varying levels of input current.

So, similar to the previous case 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 , we are giving proper voltage at the collector of the transistor.

This chunk discusses the basic configuration of a current amplifier using a Bipolar Junction Transistor (BJT). The BJT is the core component, and it needs a direct current (DC) bias to operate. This biasing ensures that the transistor is in the appropriate operating region to amplify the input current effectively.

Imagine a BJT like a gate that needs a specific key (DC bias) to open up. Without the right key, the gate cannot function properly. Similarly, the BJT must be properly biased to allow it to amplify the current correctly.

So, 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 , the DC part, and also the time varying part which is shown here.

The base current in a current amplifier consists of two components: a direct current (DC) part and a time-varying (AC) part. The DC part keeps the transistor active, while the AC part carries the signal we want to amplify. Understanding these components helps in analyzing how the transistor responds to signals.

Consider a speaker system where the direct current is like a constant power supply keeping the system on, while the time-varying current is the music signal being played. Both are necessary for the speaker to function and produce sound.

So, when we like to extract the signal in the form of current, we must 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 obtain the output signal in a current amplifier, it is essential to connect the output node to AC ground but avoid connecting it directly to DC ground. This process ensures that we can capture the varying signal without a DC bias interfering with our measurement.

Imagine trying to measure the flow of a river (the current signal). If you place your measuring device too deep (or connect to DC ground), you may only get the water level instead of the flow rate. However, strategically placing it at the water surface (AC ground) allows you to accurately gauge the current flow.

So, the moment we connect to load at the output instead of directly shorting, so if I am having a finite resistance at the output so, it is expected that the practically the current flowing through the circuit may not be same as the internal current.

When connecting a load to the output of a current amplifier, a finite load resistance can affect the flow of current. The current measured may be different from the internal current due to this loading effect, which must be considered for accurate circuit analysis.

Think about how turning on multiple appliances in a house (the load) can affect the voltage available from a generator. If the generator isn't powerful enough, each appliance may receive less power than expected, just as the current amplifier's output can be diminished under load.

So, the as we said that we have discussed about voltage amplifier and then also we have discussed about the current amplifier and you might have seen that, whenever you are talking about the model of the current amplifier, we are basically referring to one equivalent linearized circuit, which is representing the entire circuit to find the input to output relationship.

This section summarizes the relationship between the different types of amplifiers—voltage and current amplifiers. It emphasizes the role of linear models in understanding circuits, where the key focus is on the input-output relationships and how voltage and current are treated in these models.

Consider an amplifier as a translator between languages. If you consistently translate from one language to another, you build a relationship between the words and meanings (input to output). Similarly, the models we create help us understand the relationships between input and output in circuits.

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