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Transistor as an Amplifier

An amplifier is a electronic circuit that increases the strength of a low signal and thus acts as an amplifier. We have covered transistor basics such as what is BJT transistor, Biasing of Transistor, Different Configurations of Transistor in our previous articles. Today in this article we are going to explain working of transistor as an amplifier.

Transistor as an Amplifier

The transistor increases the strength of a weak signal and hence acts an amplifier. The transistor amplifier circuit is shown in the figure down below. The transistor has three terminals namely base, emitter, and collector. In order to operate transistor as an amplifier, the Base-Emitter junction of the transistor are connected in forward biased and the collector-base junction is in reverse bias.

The low signal is applied in the input of Base-Emitter junction and output is taken across the load resistor (across collector resistor). In order to achieve good amplification, the input Base-Emitter circuit should always remain forward biased. To do so, an external D.C voltage (VCC) is applied in the Base-Emitter circuit in addition to the signal as shown given below.

This D.C voltage VCC is known as the bias voltage and its magnitude is such that it always keeps the input B-E junction forward biased. As the input B-E circuit has low resistance, therefore, a small change in input signal voltage causes a noticeable change in emitter current. This causes the *same change in collector current (IE=IB+IC).

In the collector circuit, RC is connected of high value, when collector current flows through the load resistance it produces a large voltage drop across it. Thus, a low signal applied in the input B-E circuit appears in the amplified form in the output collector circuit. Hence in this way, a transistor acts as an amplifier.

Given Fig. below shows the basic circuit of a transistor as an amplifier.

transistor as an amplifier

common emitter amplifier circuit

Practical Example of Transistor as an Amplifier:

The action of  transistor as an amplifier can be made very simple. if we consider typical circuit values. Suppose collector load resistance RC = 5 kΩ. Let us further assume that a change of 0.1V in signal voltage produces a change of 2 mA in emitter current ( IE≈IC=βIB).

Obviously, the change in collector current IC would also be approximately 2mA. This collector current flowing through collector load RC would produce a voltage = 5 kΩ × 2 mA = 10 V. Thus, a change of 0.1 V in the signal has caused a change of 10 V in the output circuit. 

In other words, the transistor has been able to raise the voltage level of the signal from 0.1 V to 10 V  with a voltage amplification factor (β) of  100. (β=output voltage/input voltage) 

Extra Info:

In this article, we learned that how transistor works as an amplifier, but a BJT transistor can be used in three different configuration such as Common base, Common Emitter, and Common Collector configuration and in each configuration a transistor can be worked as an amplifier, but  the most widely used transistor configuration is Common-Emitter Configuration, and this is due to the reason, that a common emitter configuration provides good voltage and current gain.

For More Information visit:

  1. Common Emitter Amplifier
  2. Common Base Amplifier
  3. Common Collector Amplifier
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  1. Ataulhaq

    Great work sir. You have really explained it in very simple words.
    Really appreciate. Thanks for explaining.

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