23.3. Lecture - 23

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.

A current amplifier is a type of amplifier that specifically takes an input signal in the form of current and amplifies it to produce a larger output current. Just like a voltage amplifier, which amplifies voltage signals, a current amplifier operates under a similar principle but focuses on current. The key here is that the output current is directly dependent on the input current, establishing a linear relationship between them to represent this dependency mathematically.

Think of a current amplifier like a water pump. If you have a smaller pipe (input current) feeding water into the pump, the pump (current amplifier) can push out a larger stream of water through a larger pipe (output current). The more water you push into the pump, the more it pumps out, illustrating the amplification of the input signal.

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.

The example provided involves a Bipolar Junction Transistor (BJT), which is used as the primary component in a current amplifier. The BJT is biased with a DC current to operate in an active region, allowing it to amplify the input current to provide a corresponding output current. This biasing setup ensures that the transistor is ready to respond to input changes and amplify the current accordingly.

Imagine the BJT like a sturdy lever that lifts weights. When you press down on one end of the lever with a little weight (input current), the other end lifts a much heavier weight (output current), showing how the small input can control or move a much larger output.

Depending on this I and beta of the transistor we may having a if the device it is in active region of operation, we may be having this collector current DC collector current.

The collector current in a BJT is defined by the base current (I) and the current gain (beta). When the transistor operates in its active region, the output collector current can be calculated based on these parameters. Keeping the transistor in this region is necessary for correct amplification, which is influenced by the applied bias.

Think of the base current as a small gust of wind that prompts a large wind turbine to spin faster (the collector current). The stronger the gust (higher base current), the more energy is produced by the turbine (the greater output current), showing how a small input can effectively control a larger output.

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.

At the base of the transistor, the base current consists of two components: a constant DC part and a varying AC part. The DC part is essential for initial biasing, while the AC part represents the actual signal we want to amplify. When these two components combine, they create the overall base current that drives the collector current.

Imagine controlling a faucet. The constant flow of water represents the DC part, keeping the faucet open, while adjusting the knobs (AC part) changes how much water is allowed to flow through. The overall flow is a combination of both the steady supply and the adjustments made.

And hence we need to put one DC blocking capacitor or it is referred as AC coupling capacitor.

An AC coupling capacitor is placed in the circuit to block any DC component while allowing the AC signal to pass through. This is crucial for ensuring that only the varying part of the current is amplified, which helps maintain the integrity of the signal being processed by the amplifier.

This can be compared to a filter in water purification. A filter lets clean water (the AC component) pass, while blocking impurities (the DC component) that could contaminate the final output, ensuring only the purest form reaches the end user.

So, the output I current unloaded current should be A times the input current i.

The unloaded current gain (A) of the current amplifier indicates how much the input current (i) is amplified to produce the output current (I). It is defined as a ratio that does not account for load effects, allowing for theoretical analysis of the amplifier's performance in ideal conditions.

Consider this like a sound amplifier, where a small input sound signal (like a whisper) is transformed into a much louder sound (like a shout). The gain describes the relationship between the whisper (input) and the shout (output), providing an idea of how much louder the output can be compared to the input.

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