what is difference between npn and pnp transistor

[newone]

what is difference between npn and pnp transistor

and how to meaure them. its good or not

 

[mstechca]

An npn transistor is two diodes with their anodes connected to each other.
A pnp transistor is two diodes with their cathodes connected to each other.

and you can use a multimeter to check them. If the number displayed on the multimeter is close to the transistors HFE (or beta, or gain) rating, then it works.

 

[Nigel Goodwin]

An npn transistor is two diodes with their anodes connected to each other.
A pnp transistor is two diodes with their cathodes connected to each other.

Rather misleading! - a transistor is NOT two diodes connected together, but they can be simply tested as two diodes with a multi-meter. Further tests are required to prove it actually works as a transistor, but a simple O/C, S/C test with a meter is usually enough.

 

[mstechca]

An npn transistor is two diodes with their anodes connected to each other.
A pnp transistor is two diodes with their cathodes connected to each other.

Rather misleading!

Its amazing. Hundreds of sources say what I say, and people like you turn around and say it is misleading. Cmon INTERNET, Get the FACTS straight!!!
I wonder if this is the reason why so many people ask these "why doesnt this work" questions.

- a transistor is NOT two diodes connected together

I must have use the wrong word (is). But I was merely describing how an "ideal" transistor functions.

 

[akg]

he said that it is a misleading information .. that .. one cannot build an NPN with two diodes connected together.

 

[fsahmed]

I must have use the wrong word (is). But I was merely describing how an "ideal" transistor functions.

An ideal transistor cannot function the way two diodes would do back to back, the diode example is just to elucidate the transistor's construction. Technically, an ideal diode has equal doping in P & N regions, while in an ideal transistor, doping levels differ in all regions, emitter being highly doped, base is thinly doped, collector lies somewhere in between. & it is the thin doping of the base that makes a transistor work!

 

[mstechca]

An ideal transistor cannot function the way two diodes would do back to back

Does that mean that no transistor is ideal?

it is the thin doping of the base that makes a transistor work!

then I must be partially right, because I know that I can use the base and emitter junctions and treat them as a proper diode. The collector isn't the same.

I buy my transistors. I don't build them.

Now that you are mentioning doping, why would a common emitter amplifier work if designed properly? I can see it not shorting if the base resistor is high enough.
in an NPN, wouldn't the signal go from base to collector and become amplified?

I never heard that a signal would go from collector to base. this is why I think an NPN is 2 diodes back-to back.

 

[Roff]

I think the point being made here is that a transistor can function as two back-to-back diodes, but two discrete diodes connected back-to-back will never function as a transistor - which I think we all already know.

 

[Nigel Goodwin]

Its amazing. Hundreds of sources say what I say, and people like you turn around and say it is misleading.

In that case 'hundreds of sources' are WRONG! - or you've misread (or misunderstood?) those 'hundreds of sources'.

- a transistor is NOT two diodes connected together

I must have use the wrong word (is). But I was merely describing how an "ideal" transistor functions.

No, you were describing nothing of the kind! - two diodes are NOT a transistor and wouldn't ever work as one (AT ALL!). As already stated by Ron H, a transistor can be used as two diodes, it has the required two NP and PN junctions, but two diodes can't be used as a transistor.

 

[DigiTan]

I have to agree. The "two diodes" analogy implies equal amounts of emitter & collector doping, which wouldn't account for why charge carriers are able to diffuse across the base, rather than just in or out of it.


In response to the original question: the NPN and PNP terms refer to the level of semiconductor doping in emitter, base, and collector structures of the transistor. For NPN transistors, the emitter is more negatively-doped than the collector, so it's really more like an (N+)PN transistor. Likewise, the emitter is more positively-doped than the collector for a PNP device...or (P+)NP if you want to look at it that way. On a diagram, the arrow for NPN is "Not Pointing iN," and for PNP it's "Pointing iN Proudly." A little nmemonic there. 8)

One quick & dirty way to test both kinds is to "saturate" the base pin by connecting it to the emitter (maybe through a button) so that the transistor operates on an on/off basis. To test NPN, connect the emitter+base to positive, , and the collector to negative; and if the trans works, you'll get a large amount of current going out of the collector. For PNP, connect emitter+base to negative, and collector to positive.

If you have a mystery transistor, you can **broken link removed** or purchase a full-featured tester that will return an "NPN/PNP" and "PASS/FAIL" output.

 

[Roff]

One quick & dirty way to test both kinds is to "saturate" the base pin by connecting it to the emitter (maybe through a button) so that the transistor operates on an on/off basis. To test NPN, connect the emitter+base to positive, , and the collector to negative; and if the trans works, you'll get a large amount of current going out of the collector. For PNP, connect emitter+base to negative, and collector to positive.

You made a lot of sense until this statement. Can you explain what you mean? How are you going to turn a transistor on when the base is shorted to the emitter? If you do what you suggested, you simply have a forward-biased base-collector junction. There is no transistor action here.

Perhaps you have transposed collector and emitter?

 

[mstechca]

the reason why I was saying that transistors are two diodes back to back, because it "basically" answered his question.

I know there is more to the transistors than the diode theory. I can also agree with the comment about doping as well.

I also agree that two diodes back-to-back wouldn't make one of today's transistors, because it wouldn't account for half the transistor's parameters.

 

[Nigel Goodwin]

the reason why I was saying that transistors are two diodes back to back, because it "basically" answered his question.

That's the problem, it didn't answer it at all!, the answer was completely wrong - and gave the impression that a transistor was just two diodes.

I also agree that two diodes back-to-back wouldn't make one of today's transistors, because it wouldn't account for half the transistor's parameters.

It won't make one of ANY era's transistors, because it won't make a transistor at all!. If you don't beleive it?, wire two diodes together and build a simple circuit around them!. If you give it a little thought?, the base emitter 'diode' will be forward biased, so the base will be 0.7V higher than the emitter (just like a transistor), but the base collector 'diode' will be reverse biased, so won't pass any current at all. This will result in the 'base' voltage being far lower than normal, as the base bias resistors wil have to provide the 'emitter' current as well. The 'collector' voltage will be at the supply rail, as it's drawing no current at all.

 

[fsahmed]

As I said before, the key to transistor operation is in the fact that the thin doping of the base results in poor recombination of electrons with holes, which leads to huge current passing between the emitter & collector. U cannot imagine that happening with a diode, since that would indicate that a diode has shorted out.

 

[DigiTan]

You made a lot of sense until this statement. Can you explain what you mean? How are you going to turn a transistor on when the base is shorted to the emitter? If you do what you suggested, you simply have a forward-biased base-collector junction. There is no transistor action here.

Perhaps you have transposed collector and emitter?

Yeah, I guess I being unclear on that one. Probably a less convoluted approach would be to take an ohmmeter and test the emitter/base and base/collector junctions both ways. I had a Radio Shack transistor tester than appeared to use a similar method, but it looks like it's been discontinued.