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Question

Which device acts like an NPN and a PNP transistor connected base-to-base and emitter-to-collector?

This question was previously asked in
RRB ALP 2018 CBT 2 Fitter Question Paper (21-Jan-2019) (Shift 3)
The correct answer is
SCR

SCR Equivalence to Transistors

A Silicon Controlled Rectifier (SCR) is a semiconductor device primarily used for controlling high power loads.

Its internal structure can be understood as comprising two complementary transistors: one NPN and one PNP.

These transistors are interconnected in a specific feedback configuration:

  • The collector terminal of the NPN transistor is linked to the base terminal of the PNP transistor.
  • The collector terminal of the PNP transistor is linked to the base terminal of the NPN transistor.
  • The emitter of the NPN transistor corresponds to the SCR's Anode.
  • The emitter of the PNP transistor corresponds to the SCR's Cathode.

This arrangement, where the bases are effectively cross-connected via the collectors, forms the fundamental operational principle of the SCR. This structure is conceptually similar to the description of NPN and PNP transistors connected base-to-base.

Among the given options, the SCR is the device that accurately represents this NPN and PNP transistor combination.

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