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What practical applications do PNP and NPN transistors have? Answered

I read about NPN and PNP transistors and I was wondering what type of applications are right for each type of transistors. I understand that depending on the type of transistor the emitter should be connected to either an anode or cathode.


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 one of Mark Tilden's famous "solar engines", 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.


This response confuses me a good bit, but I barely know anything about electronics in practice, so feel free to make me understand this better: To my understanding, the idea of current sourcing and current sinking is defunct outside of the context of DC circuits due to the intrinsic nature of AC current, well....alternating. Does this mean that NPN and PNP transistors are only used in DC circuits in practice?

DC and AC can exist in the same circuit simultaneously.

Or maybe a better way to say that, is to say any current or voltage signal can be thought of as the sum of two parts, a DC part and an AC part. Mathematically, the DC part is constant in time, and the AC part is, usually, sinusoidal. For example,

V(t) = 3 + 2*cos(w*t)

where VDC = 3, is the DC part,
and VAC = 2*cos(w*t), is the AC part.

This page,
shows an amplifier built around a single NPN BJT, and the trick to analyzing this circuit is to think about it two ways.

First consider what the circuit does when the only currents are DC currents; i.e. when the AC input (and output) is zero; i.e. the quiescent (another word for "quiet") case.

Second consider the AC signals. These are sort of there in the circuit, on top of, as a small addition to the DC signals.

PNP and NPN transistors are opposites. When you apply current to the base of an NPN, it allows more power to flow through the transistor. It is good for amplifiers. The PNP does the opposite. When you apply current to the base it shuts off. Both transistors work like a valve. Turn it one was and the water (electricity) is able to flow, turn it the other way and it does not. Both of the transistors make up the basic components of Logic Gates for processing digital signals (in computers and stuff) Anyway, hope that helped. Best of Luck! P.S. look up the datasheet for each transistor to know exactly where to apply current to make it amplify or shut off (just google the part number and it should pop up)

Are you saying that PNP transistors can't be used for amplifiers then ?

I think you have got your BJTs confused with your FETs.

If you replace the word "NPN" with "enhancement mode FET" and replace the word "PNP" with "depletion mode FET", then what you said above would be correct.

Its hard to say, you can make circuits using NPN, you can make analagous ones in PNP. Generally, PNP transistors are a little slower than NPN.

In some classic amplifier circuits called "Push-Pull" amplifiers, using NPN and PNP in complimentary pairs makes for a neat circuit.


From my understanding, they're pretty much the same except the polarities are reversed.  Wikipedia lists the applications, I think they apply to either type.