What is the necessity of both p-n-p and n-p-n transistors?

Dear student,

 

NPN transistors are used when you want to sink a current; i.e. current flows into the collector. PNP transistors are used when you want to source a current; i.e current flows out of the collector. As a result of this, NPN transistors usually have their emitter connected to the low side of the supply. And conversely PNP transistors usually have their emitter connected to the high side.

The circuit diagram below is, a circuit for making simple solar powered toys. I picked this as an example because it includes one of each, 1 PNP, 1 NPN, both wired in the manner that I claim these things are usually wired.

For the PNP: Ic+Ib flows into the emitter, then they split: Ib flows out the base and Ic flows out the collector.

For the NPN: Ib flows into the base. Ic flows into the collector. Then Ib and Ic sort of merge together and Ib+Ic flows out the emitter.

Hope this helps.

Thanking you

Team

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