Advertisment:

Wednesday, 1 July 2015

Electronics Year 2 - Comparison of FETs with BJTs

FET component:




Pins:
(D)rain - Electrical current enters here
(G)ate - This pin controls the size of the FET "gate", how much current is able to pass through.
(S)ource - Electrical current exits here

Comparison of BJT and FET:

As an analogy, the FET transistor can be compared to the BJT
BJT:

  • (B)ase pin acts as input
  • Current flows from (C)ollector to (E)mitter
  • Input voltage V_b must be above 0.2V for the device to work in an operable region (a.k.a active region)

FET:

  • (G)ate pin acts as input
  • Current flows from (G)ate to (S)ource
  • Input voltage V_GS must be above a certain threshold, V_TO for the device to work in an operable region (a.k.a saturation region)

Comparison of voltage graphs:

Note that the FET voltage graph is not unlike that of the BJT
BJT

FET

Note that while the output current of the BJT (emitter) is dependant on the input current, the output current of the FET (source) is dependant on the input voltage. This is an important point to consider when analysing the small signal properties of the transistors.

No comments:

Post a Comment