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Transistor as an amplifier

A device which increases the amplitude of the input signal is called amplifier (Fig. 33).
 
108815.png
Fig. 33
 
A transistor can be used as an amplifier in the following three configuration: (i) CB amplifier, (ii) CE amplifier and CC amplifier.

NPN transistor as CB amplifier

108827.png
Fig. 34
  • 107129.png ib = 5% of ie and iC = 95% of ie
  • VEE < VCC
  • Net collector voltage, VCB = VCCiCRL
     
    When the input signal (signal to be amplified) is fed to the emitter base circuit, it will change the emitter voltage and hence emitter current. This in turn will change the collector current (ic). This will vary the collector voltage VCB. This variation of VCB will appear as an amplified output.
  • Input and output signals are in same phase

NPN transistor as CE amplifier

108850.png
Fig. 35
  • 107135.png ib = 5% of ie, and iC = 95% of ie
  • VCC > VBB
  • Net collector voltage, VCE = VCCiRL
  • Input and output signals are 180° out of phase.




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