Analog Electronics, Electronics, Transistor

Common Emitter Amplifier, BJT Transistor Common Emitter Amplifier

In our previous discussions, we learned that a transistor can be configured in three different modes, 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.

 

In this article, you will be able to learn and understand the working of Common Emitter Amplifier, their characteristics, and their applications.

What is the common emitter amplifier?

Common Emitter Amplifier has Emitter terminal as common for both input and output. Input is applied to the Base-Emitter terminal and output is taken from Emitter-Collector terminal.

 common emitter amplifier

The base-emitter junction is forward bias and emitter-collector junction is reverse bias, it is because a transistor must remain in an active region in order to perform amplification.

In order to understand the working of Common Emitter amplifier let’s first understand how does a transistor work as an amplifier?

How Transistor Amplifies?

When a weak AC signal is given to the base of the transistor, a small base current IB starts flowing. Due to transistor action, a much larger (β times the base current) a.c. current flows through the collector load RC. As the value of RC is quite high (usually 4-10 kΩ), therefore, a large voltage appears across RC.

Thus, an applied weak signal at the base circuit appears in amplified form in the output of collector terminal. It is in this way that a transistor acts as an amplifier.

Common Emitter Amplifier Working:

 As shown below a Common Emitter amplifier is made up of voltage divider bias, the input is Base-Emitter terminal and output is Emitter-Collector collector.

During Positive cycle of input, a sinusoidal AC signal is applied at the input terminals of a circuit that cause the forward bias of base-emitter junction hence VBE is increased resulting in an increase in IB.

The collector current Ic is increased by β times with the increase in IB, hence VCE is correspondingly decreased.

common emitter amplifier working
common emitter amplifier working

Calculation:

formula of vo=vcc-icrc

Thus in a Common-Emitter amplifier, a positive going signal is converted into a negative going output signal i.e..180° phase shift is introduced between output and input signal and it is an amplified version of an input signal.

Practical Common Emitter Amplifier Circuit

In order to perform amplification with a common emitter amplifier, we must consider the basing, capacitor and different resistors values. Figure down below shows the circuit of practical common emitter amplifier.

 

common emitter amplifier circuit
common emitter amplifier circuit

Here:

  • C1 and C2: These two capacitors are placed in the input and output of an amplifier; they are used to couple one circuit with another hence they are called as coupling capacitors.
  • CB: This capacitor is known as bypass capacitor which is used to bypass the AC signal to ground. It is very helpful because any noise signal that may be presented in AC signal will be passed out from bypass capacitor.
  • R1 and R2: These are places in between of collector to the base terminal they provide biasing to the transistor hence they are known as biasing resistors.
  • RC: RC is placed in the collector terminal in order produce faithful amplification. It places an important role in the operation of amplification (VC-VCC-ICRC)
  • RE: This RE resistor is placed in the emitter terminal of a transistor, and it is useful to control the gain of an amplifier.

Characteristics of Common Emitter Amplifier:

  • It has Large Voltage and Current Gain.
  • It has hence large power gain.
  • It has input to output phase shift of 180°.
  • It has moderate input and output impedance.

The voltage gain of Common Emitter Amplifier:

The voltage gain of Common Emitter amplifier is the ratio of output voltage to the input voltage.

Here output voltage is referred to as ΔVC and input voltage is referred to as ΔVB.

Av=β Vc/vb.

The current gain of Common Emitter Amplifier:

Current gain in CE amplifier is the ratio of output current to the input current. In CE configuration the current gain is denoted by greek symbol beta.

the output current is referred to as Ic and input current is referred to as Ib.

β=Ic/Ib

Input Impedance of CE Amplifier: 

The input impedance is also an important parameter in CE amplifier, because when a one CE amplifier drive another amplifier circuit then the output of one amplifier is input for another amplifier.

In CE amplifier input impedance is around 1kΩ-2k, input impedance values changes with respect to circuit configuration usually CE amplifier has input impended in between mentioned values.

Zin = R1 || R2 | | Zin (base)

Output Impedance of CE amplifier:

The output impedance of the CE amplifier is the resistance looking in at the collector and is approximately equal to the collector resistor. The output impedance of CE amplifier is around 50k-70k.

Rout = RC

Input characteristics

Input characteristics curve of a common-Emitter amplifier is the curve between IB and VBE whereas VCE is constant.

 input characteristcis curve of common emitter amplifier

Output characteristics

Output characteristics curve of a common-Emitter amplifier is the curve between IC and VCE whereas IB is constant.

output characteristcis curve of common emitter amplifier

Applications of Common Emitter Amplifier:

Common Emitter amplifier configuration is widely used due to its advantage of moderate current and voltage gain.

  • It is used in Audio Amplifiers
  • It is used in Microphones, RADIO, and Music Players
  • It is used in Frequency generation circuit to increase the strength of input signal.
  • It is used to increase the speed of Fans, Motors, and Timer circuits.

Advantages of Common Emitter Amplifier:

Common Emitter Amplifiers is most widely used amplifier than the Common Base amplifier and Common Collector amplifier because:

  • An Ideal amplifier must have very low input impendence, and CE amplifier has very low input impendence.
  • An Ideal amplifier must have very high output impendence, and CE amplifier has very high output impendence.
  • It provides 180° phase shift. Or we can it is inverting amplifier.
  • The current gain and voltage are moderate.

Disadvantages of Common Emitter Amplifier:

  • In the CE amplifier, there is high thermal instability.

Also, read:

  1. Common Base Amplifier, BJT Transistor Common Base Amplifier
  2. Common Collector Amplifier, BJT Transistor Common Collector Amplifier
  3. What is the Difference Between NPN and PNP Transistor
  4. Transistor configurations, Common Emitter, Common Base, Common Collector, and Applications
  5. Transistor Biasing, Self Bias, Emitter Bias, Voltage Divider Bias, and applications
  6. Introduction to BJT Transistor.

 

Conclusion:


In Common Emitter Amplifier, Input is applied to B-E Junction and Output is taken from E-C terminal, here emitter terminal is common for both input and output. It is widely used amplifier circuit because it provides good current gain and good voltage gain and it is also known as inverting amplifier because it gives 180° phase shift from input to output. It is widely used in audio amplification and signal amplification circuits.

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