23.2.1. Prof. Pradip Mandal

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 current signal on the input current signal.

This chunk introduces the concept of current amplifiers. A current amplifier is a type of amplifier that enhances or amplifies the current signal at its output based on the input current signal. This idea parallels that of a voltage amplifier, which amplifies voltage signals. The key takeaway is that both types of amplifiers can be modeled with linear equivalent circuits that define how input signals are transformed into output signals.

Imagine a water pipe system where the input is the flow of water (current) and the output is water flowing out of a different pipe. If you apply pressure (input current) at one end, the output flow increases proportionally, much like a current amplifier increases the output current based on the input current.

As you can see here, what are the things we do have is the BJT is at the center place...as we are seeing here. And, depending on the current here we are expecting this current to the flowing.

In this chunk, we delve into the workings of a current amplifier using a Bipolar Junction Transistor (BJT). The BJT is biased to operate in the active region. The base current provides the necessary bias for the transistor, affecting the collector current, which is the output current of the amplifier. The signal fed into the transistor influences the output current, illustrating how the amplifier operates by transforming input currents to higher output currents based on its configuration.

Think of a speaker system where the input is a small audio signal (like a whisper). The amplifier (BJT) takes this tiny signal and increases its 'volume', allowing it to move the speaker cones (output current) and produce a loud sound. Just as the whisper creates more dynamic sound depending on how it's input, the base current shapes the collector current level.

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.

This section discusses the importance of extracting the output current signal without shorting it to DC ground. To achieve this, a DC blocking (AC coupling) capacitor is used. This allows AC signals (like the varying parts of the collector current) to pass while preventing the DC component from affecting the output. The short-circuit configuration lets the amplifier output current be analyzed, ensuring that only the essential current signal is extracted without interference from the DC biasing voltages.

Imagine trying to listen to a radio through headphones but having a steady annoying hum (DC). If you place a filter in the audio path, it can let the music (AC signal) through while blocking the hum (DC). Thus, you can enjoy clear music without the disturbance, just like the capacitor allows only the desired AC signal from the amplifier.

So, we are basically referring to one equivalent linearize circuit, which is representing the entire circuit to find the input to output relationship, input to output relationship.

This chunk highlights the concept of modeling current amplifiers with linear circuits to understand the relationship between input and output. The model simplifies the circuit into key components: the unloaded current gain, resistances accounting for loading effects, and enables analysis of how signals transform as they pass through the amplifier. Understanding these relationships is crucial for designing and analyzing electronic circuits.

Consider a chef who creates a recipe (the model) from a variety of ingredients (components). The recipe helps the chef know how to mix the ingredients to get the final dish (output) based on the quantities used (input). Similarly, modeling amplifiers helps engineers predict how different parameters affect performance.

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