log
Guest
Sat Jun 04, 2005 4:09 am
You can use NPN transistors for all applications using
PNP transistors, right?? What is then the advantage of
using PNP transistors versus NPN? Or are there situations
when only the PNP can work, what for instance?
log
PeteS
Guest
Sat Jun 04, 2005 9:53 am
<<You can use NPN transistors for all applications using
PNP transistors, right?? >>
No, if you mean you can just replace an NPN with a PNP. It's possible
to have an all NPN solution sometimes, but there are other times when
only an NPN (or a P-channel FET) will do.
<<What is then the advantage of using PNP transistors versus NPN? >>
Well, if you don't much care about switching time differences (PNPs are
typically slower for good physical reasons) and other specs, a circuit
may be much simpler using a PNP.
<<Or are there situations when only the PNP can work, what for
instance?>>
As an example, in power sensitive equipment, some parts are only
powered up when necessary. If it is necessary to power the high side,
then using an NPN would require an extra power supply *above* the power
you are switching, meaning extra circuitry and probably not making it
worthwhile because of the extra inefficiencies of the new supply (plus
all the associated extra components)
If you wanted a simple NPN solution, then you would have to switch the
low side, meaning all pins of the device to be powered would be at V+
while 'off'. In this situation, any devices connected to it would have
to maintain the connected pins at V+. This is often simply not
possible, especially if going across power domains.
Another example if the classic push-pull amplifier. This inherently
symmetrical circuit requires an NPN and a PNP. There are tricks to use
NPNs here, but it would still require a PNP or two to make it work
properly.
There are myriad other examples, of course
Cheers
PeteS
redbelly
Guest
Sun Jun 05, 2005 5:20 pm
For that matter, why must we use electrons? You can just
as well use positrons in all applications, and stop worrying
about "the current flows THIS way, so the charge carriers are
flowing THAT way". With positrons, current and charge travel
in the same direction, making electronics -- or should I
say POSitronics? -- much much easier to understand!
Mark
R.Lewis wrote:
Quote:
You must be right.
The manufacturers are just conning people by offering PNP's and the
'engineers' that are stupidly conned by these manufacturers into using these
useless devices obviously don't know as much as yourself about designing
with electronics.
Spread the word before it is too late.
Please close the door after you
