Saturday, July 30, 2011

Physics Form 5: Chapter 4 - Transistor as an Amplifier

An amplifier circuit

IB = base current
IC = collector current
IE = emitter current
VBB = base voltage 
VCCcollector voltage

The function of resistant RB is to control and limit the base current.

How transistors function as an amplifier:
  • The forward-bias from base to emitter narrows the base-emitter,BE depletion layer.
  • The reverse-bias from base to collector widens the base-collector, BC depletion layer.
  • The free electrons from emitter diffuse easily through the forward-biased BE junction into the p-type region.
  • The base region is lightly doped and very thin, so it has a limited number of holes. Therefore, only a small percentage of the all electrons flowing through BE junction can combine with the available holes in the base region.
  • Most of the electrons do not combine with holes but diffuse into BC depletion layer.
  • Once here, these electrons are pulled through the reverse-biased BC junction by the electric field set up by the force of attraction between the positive and negative ions.
  • Electrons now move through the collector region and into the positive terminal of the collector voltage source.
  • This forms collector current which is larger than the base current.

Click on the diagram below to play!

When there is a small change in the base current, there will be a big change in the collector current. 

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