16/11/2020

Amplifier Terminologies : Gain , Voltage gain, Current gain and Bandwidth

 Gain of the Amplifier

  • The ratio of the output quantity to the input quantity of the amplifier is called as Gain of the Amplifier.

Gain of Multi Stage Amplifier

  • Let us consider three amplifiers having respective gain of G1, G2 and G3. The total gain of the amplifier is

       G = G1 × G2 × G3

  • The gain of the multistage amplifier G is less than produce of G1 × G2 × G3 due to loading of next stages.

Decibel Gain

  • The gain of the amplifier is very large therefore it is represented by common logarithm ( base 10 ). The unit of gain is bel or db.

Power gain

  • The ratio of output power to the input power in log to the base 10 is called as power gain of the amplifier.

       Power Gain = Log10 ( POUT / PIN ) bel

                  1 bel = 10 db

                          = 10 Log10 ( POUT / PIN ) db

Voltage Gain

  • The ratio of the output voltage to the input voltage in log of base 10 is called as voltage gain of the amplifier.

       Let POUT = VOUT2 / R

                PIN = VIN2 / R

  • Voltage gain = 10 Log10 ( POUT / PIN ) db

                            = 10 Log10 [ VOUT2 / R ] / [ VIN2 / R ] db

                            =  20 Log10 ( VOUT / VIN ) db

Current Gain

  • The ratio of the output current to the input current in log of base 10 is called as voltage gain of the amplifier.

       Let POUT = IOUT2R

                PIN = IIN2R

  • Current Gain = 10 Log10 ( POUT / PIN ) db

                              = 10 Log10 [ IOUT2R ] / [ IIN2R ] db

                             = 20 Log10 ( IOUT / IIN )

Finally

Power Gain

Log10 ( POUT / PIN ) bel

10 Log10 ( POUT / PIN ) db

Current Gain

Log10 ( IOUT / IIN ) bel

20 Log10 ( IOUT / IIN ) db

Voltage Gain

Log10 ( VOUT / VIN ) bel

20 Log10 ( VOUT / VIN ) db

 

Bandwidth

  • It is range of frequencies over which the voltage gain is equal to or greater than 70.7% of the maximum gain.
  • The f1 and f2 is range of frequencies over which the gain is equal / greater than 70.7% of the maximum gain.

       Where f1 = Lower cut off frequency

                  f2 = Higher cut off frequency

  • Bandwidth = f2 – f1
  • It is important that the signal frequencies lie between f1 and f2 for distortion less amplification.

Bandwidth in terms of db

  • Let us consider that the maximum gain of the amplifier is 100.
  • Fall in voltage gain from 100 to 70.7%

         = 20 Log10 ( 100 ) – 20 Log10 ( 70.7 ) db

         = 20 Log10 ( 100 / 70.7 ) db 

         = 20 Log10 ( 1.414 ) db 

         = 3db

The bandwidth of the amplifier is defined as the range of frequency over which the voltage gain fall by 3 db from the maximum gain.

  • The frequency f1 or f2 is called as half power frequency or 3 db frequency.


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