Question

Why BJT is used as an amplifier?

Answer 1

A BJT (Bipolar Junction Transistor) is used as an amplifier because it can control the flow of a large current using a much smaller input current, which is the fundamental principle behind amplification.

Here’s why BJTs are useful as amplifiers:

1. Current Amplification: A BJT amplifies the input current in the base to produce a much larger current in the collector. The current gain (denoted as β) allows small input signals to control large output currents. This is the essence of amplification: a small change in the input results in a much larger change in the output.
   

1. Voltage Gain: When used in the right configuration, BJTs can also amplify voltage. For example, when a small voltage signal is applied to the base of a BJT, it can control a larger voltage signal across the collector-emitter terminals. This is useful for amplifying audio signals, radio signals, and other electronic signals.
   

1. Linear Behavior: BJTs have a linear region of operation, especially in the active region, where the relationship between the input (base current) and output (collector current) is linear. This linearity is important because it allows for faithful amplification without distortion.
   

1. Wide Frequency Response: BJTs can amplify signals across a wide range of frequencies, which makes them suitable for a variety of applications like audio amplification, RF (radio frequency) amplification, and signal processing.
   

1. High Power Handling: BJTs can handle relatively high power compared to other small-signal devices like FETs (Field Effect Transistors), making them useful in power amplification applications.
   

How it works in an amplifier circuit:

1. Input Signal: The small input current or voltage is applied to the base of the BJT.
   

1. Amplification: The transistor’s active region allows a larger current to flow between the collector and emitter, based on the small input signal.
   

1. Output Signal: The amplified current or voltage is then taken from the collector and sent to the next stage of the circuit or to the load.
   

So, in short, BJTs are used as amplifiers because they can take a small input signal and amplify it into a much larger output signal, both in terms of current and voltage.