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pnp vs npn?


andrie
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what is the differences between transistor PNP and NPN in the application?

A circuit that uses NPN transistors has a positive supply voltage and the negative of the supply is common or grounded.
A circuit that uses PNP transistors has a negative supply voltage and the positive of the supply is common or grounded.
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Talking in terms of positive and negative voltages is a bit missleading.... talking in terms a potentials isn't because that would apply for all cases... for example, if you had -12V and -24V supplies for a circuit... both NPN's and PNP's could be used, not just PNP's.

If I was to say.... to turn a PNP transitor on, you have to "inject" electrons into the base and to turn a NPN transistor on you have to "pull" electrons out of the base..... does anyone think this is not correct?

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Talking in terms of positive and negative voltages is a bit missleading....

I agree because this exactly what they teach us in schools

guru start by this answer to force andrie to use his mind, guru is a good man


with NPN Tr we connect collector to point more positive than emetter and make the tr ON by aplying a +tive voltage >= 0.7 V 

with PNP Tr we connect collector to point more negative than emetter and make the tr ON by aplying a -tive voltage <= -0.7 V 

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Yeah, but indulis is a lot closer to helping andrie use his mind in the right direction.  (right potential?  ;D )  Both circuits could float, etc.

andrie: as you can see, it is not so basic. Go to the Electronic Resources forum on this site and download the free transistor theory books posted in the messages. This will help you.

MP

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  • 4 weeks later...

This only applies to PNP / NPN Bipolar transistors. (3906 & 3904 examples)

A handy way to remember how they work is to remember what they look like inside.
a PNP is made of 3 regions, Positive, Negative and Positive again. NPN is the same but reversed.

In a diode there are only 2 regions, P and N.  Electron flow is from the N to the P.  (Cathode to Anode)

Basically a PNP is the same as two diodes "Pointing" at each other (Cathodes connected)
and NPN has the Anodes connected "Pointing" Away from each other.

Where the diodes meet is called the Base.

The BASE to EMITTER flow is the same as the diode that makes it up.
n-PN has the electron flow from the Emitter to the base (Base more positive than emitter)
p-NP has the electron flow from the Base to the Emitter (Emitter more positive than Base)

The current flowing between the collector and emitter, in simple DC terms, is the "Hfe" or "Beta" of the transistor times the current between the Base and Emitter.

0.01ma current at Base times a beta(Hfe) of 120 = 1.2ma current through Collector.
(Remember the current at the Emitter is the BASE current PLUS the Collector Current
in this case 1.2ma plus 0.01ma = 1.21ma)

Anyone want to know what a Q point is?

-Mike

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  • 3 weeks later...

Hi indulis,
      Actualy i hv come acros fet's, mosfets etc.. i mean i knw their functionin structure, characteristics etc. But im nt able to figure wts the advantage of a NPN over an PNP transistor. The only benefit i find is that in NPN the base current adds to the output collector current whereas in PNP the base current reduces the output collector current. Also can u plz tel me som examples of applications in which NPN or PNP wud be more suited....

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The only benefit i find is that in NPN the base current adds to the output collector current whereas in PNP the base current reduces the output collector current.

No it doesn't. The base current controls the amount of collector current in both types of transistors. Also in both, the base current adds to the emitter current.
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An NPN or PNP transistor is turned on by giving it base current. You don't add or subtract voltage. You just give positive current to the base of an NPN transistor and negative current to the base of a PNP transitor.

The base current doesn't add or subtract to or from the collector current. The base current is part of the emitter current.

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What does
A simplistic way of looking at it is...
mean to you?

An NPN or PNP transistor is turned on by giving it base current


Remember this from earlier in the thread...
... to turn a PNP transitor on, you have to "inject" electrons into the base and to turn a NPN transistor on you have to "pull" electrons out of the base...

This is fact, which in turn means that in one case current is flowing in, and in the other it's flowing out.
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I know exactly what you mean and what your trying to say, but for the benefit of those that are not "well versed"...

I don't think about electrons when I have a transistor circuit. I see a resistor supplying base current to turn the transistor on and something that stops the base current to turn it off.


Current is electrons and resistors don't supply current, they limit current.
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I know exactly what you mean and what your trying to say, but for the benefit of those that are not "well versed"...



Current is electrons and resistors don't supply current, they limit current.

You are splitting hairs. Audioguru is right.

Base current is controlled by the value of the base bias resistor. Current through the base emitter junction will cause a collector current equal to base current times beta. Both of those currents will flow through the emitter.

It makes no difference whether the device is NPN or PNP.
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Base current is controlled by the value of the base bias resistor. Current through the base emitter junction will cause a collector current equal to base current times beta. Both of those currents will flow through the emitter.

It makes no difference whether the device is NPN or PNP.


Yes, I agree 1000% with that!

This isn't splitting hairs... if you were taking a test, do you think the Professor would let you slide if you were to answer a question by saying... turning transistors on and off it isn't done with electrons, its done with current... ? Do you think that would be an acceptable answer for full credit? For individuals who know, it's no big deal, they understand and make the connection, but for those that are just learning it's a very big deal.
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