23.5.1. Current Source and Internal Current Gain A

There may be based on the signal at the input and signal at the output we may be having different types of amplifiers. 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 electronics, amplifiers can have different purposes based on the input and output signals. While voltage amplifiers increase voltage signals, current amplifiers primarily deal with current signals. A current amplifier functions similarly to a voltage amplifier, acting as a linear circuit where the output current depends on the input current. This means that as the input current changes, the output current also changes proportionally.

Think of a current amplifier like a water pipe. If you increase the flow of water (input current), the water that comes out of the other end of the pipe (output current) also increases. This relationship shows how a current amplifier works in a similar way.

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 C and, then we do have a DC current at the base which is providing a meaningful bias to the base of the transistor.

In a current amplifier, the core component is often a BJT (Bipolar Junction Transistor). The BJT is biased using a DC voltage and current at its base, allowing it to operate correctly in the desired region. This DC current is essential as it determines how the transistor amplifies the incoming signal. The presence of a bias ensures that the transistor is ready to amplify the input current signal.

Imagine a garden where you need to water the plants (base current) to keep them healthy (the transistor operating correctly). If you don’t provide enough water, the plants will not grow (the transistor won't amplify signals effectively).

So, the main part it is here including the bias. And, all and then if we feed the signal in the form of current at the base as we are seeing here. And, depending on the current here we are expecting this current to the flowing. Note that in simplistic model here we are showing this is signal current, which means that it is average it is 0.

With the DC bias established in the BJT, we can then apply a varying input current signal at the base. The BJT will respond to this signal, allowing a corresponding output current to flow at the collector. In this model, the signal current is characterized as having an average value of zero, indicating that it fluctuates around this average value, rather than being constantly positive or negative.

Think of a seesaw. When children (input current) sit on one side, it goes up and down around the center (average of 0). This motion represents how current signals fluctuate.

So, what we do? 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 observe the output current from a current amplifier, it’s important to isolate the AC (varying) signals from the DC components. This is accomplished by using a DC blocking capacitor at the output. The capacitor allows the AC signal to pass through while blocking any DC component, ensuring that just the varying signal is measured or used in the next stage of an electronic circuit.

Imagine a modern coffee shop that wants to play music (AC signal) but doesn’t want any background noise from outside (DC component) to interfere with the pleasant ambiance. They install soundproofing (the DC blocking capacitor) that allows the music in but keeps the noise out.

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, after multiplying with i_in.

In the current amplifier model, one of the key parameters is the unloaded current gain (A). This gain represents how much the output current increases relative to the input current when there is no load connected at the output. The relation is given by output current being A times the input current, allowing us to understand the amplification effect of the device.

Consider a teacher (the current amplifier) who helps students (input current) understand materials better. If each student can comprehend the lesson faster or more clearly, then they would be able to share that knowledge with their friends (the output current), exemplifying the teacher's amplification of learning.

--