Difference Between NPN and PNP?

[Mechatrommer]

Whats the difference between NPN and PNP transistor? when to use NPN? When to use PNP?

[alm]

This should be in any textbook or tutorial about transistor, the short version is that they're opposite polarity.

[Time]

Your base to emitter voltage is positive for conduction in NPN and negative for PNP.

[Mechatrommer]

if i found a circuit... that use PNP, and... i dont have any PNP, can i find a way to modify it using NPN transistor? any example?

[Zero999]

Does it just uses PNP transistors and no NPN?

If so just reverse all other polarised components (capacitors, diodes and LEDs) and reverse the power supply voltage.

It there are NPN transistors too other modifications will probably be required, if it's possible.

Please post a schematic.

[saturation]

Beyond the difference in construction and theoretical operation, they function pretty much the same in terms of performance for equivalent types of BJT.  If you find a good spec'd NPN, chances are there is another equivalent PNP somewhere.

The 'general purpose' matched complimentary BJTs are NPN is 3904 and the PNP version 3906, similarly NPN 2222 and its complimentary PNP 2907.



Yes, you can substitute PNP for NPN by reversing the connections in the schematic, keeping track of the bias source and collector resistors to insure its placed properly.  Make sure the output that feeds the input into the next stage is the proper polarity



or that the design doesn't consist of mix NPN and PNP together, for example push pull amplifiers.




Note, substitution are for general purpose transistor designs, such as using them as simple amplifiers or switches.  Its uncommon today, but NPN or PNP specific designs do exists and are designed to optimize their performances, such circuits are easy to spot because they do not or are very restrictive about what BJT can be substituted into the design.  In the amp sample, Q4 and Q5 are complimentary matched pairs like a 3904 and a 3906, substitute them at your risk.

[tecman]

It truly depends on the circuit you are building.  In many cases there are some clever ways to use a PNP where an NPN might be best, or visa-versa.  Common base vs. common emitter and so on can offer some flexibility.  Easiest is to buy some of each.  Usually simple and cheap.

paul

[alm]

I don't really see the point either, unless the transistor is very exotic. It's not like something like a 2N3906 or 2N2907 (or European or Japanese equivalents) is expensive or hard to find. Although it would certainly be educative to change the circuit, it's probably out of your league if you have to ask about the difference between PNP and NPN.

[Mechatrommer]

i know about the polarity. even i have my own mnemonic for it... in my language female start from letter P, so if i want to remember PNP is the arrow go inside, the other one ("N"PN) is male version where the arrow is going out. o well, sometime ago i stumbled with a circuit that use both PNP n NPN where i only have the NPN. i got p*ssed off and thinking why in the hell they have to use both type? cant it be just using NPN type? i dont know where the circuit is right now, and i already got PNP stock in hand. but the question still a mystery to me.

but i can see some informative explanations here which are good as a starter when i'm facing with the problem again, will get back to this thread for sure. thanx to everybody who contributing the info. so i can see, there is some work to do in order to change the NPN to PNP compatible, not as simple as it might seem. Cheers!

[Zero999]

The circuit probably relies on the opposite polarity of the PNP transistor, gaining an understanding of the schematic should make it obvious why PNP was used, rather than NPN.

[Simon]

its the same as "N" and "P" channel fets/mosfets, in NPN the emiter is negative, in the PNP it is positive, you best bet is find some online tutorial

[jahonen]

The arrow on BJT's emitter indicates the direction of the current, in MOSFETs the arrow points the direction of electrons, logical isn't it

NPN and PNP are not always easily interchangeable, for example in the following image, the PNP produces hard +5v (sources much current), whereas NPN produces hard GND (sinks much current). If it is required that this "inverter" sinks more current than sources, then the PNP one is not usable.



Regards,
Janne