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Transistor Selection

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krishtriram

New Member
Dear All,

What the constraints that differentiates PNP and NPN transistors.
which the better one (in general and also application based) and in what manner?
 

krishtriram

New Member
I donot specify a particular transistor?

What differentiates the PERFORMANCE of PNP and NPN transistors ?

Which type is generally preferred or used most commonly?
 

Chippie

Member
krishtriram said:
What differentiates the PERFORMANCE of PNP and NPN transistors

Which type is generally preferred or used most commonly?

Can you clarify ..What differentiates the PERFORMANCE of PNP and NPN transistors


There is no rule of thumb as regards preference over NPN/PNP AFAIK....the application of a particular type is dependant upon the circuitry being designed I'd say.....
 

Ravi

Member
Krishtriram, Transistors fall into two main categiries (bipolar and field-effect) and are also classified according to the semiconductor material employed (silicon or germanium) and to their field of application (e.g.general purpose,switching,high frequency etc)

Bipolar transistors generally comprise pnp or npn junctions of either silicon or germainum material.In either case the electrodes are labelled collector,base and emitter. Silicon transistors are superior when compared with germanium transistors in the vast majority of applications (particularly at high tempertures) and thus germanium devices are very rarely encountered.

As 'chippie' said, application of a particular type dependant upon the circuitry being designed.
 

kinjalgp

Active Member
As far as I remember my electronics teacher had told that in PNP transistors current is caused due to flow of Holes (majority carriers) i.e. the lack of electrons creates this flow while in NPN, the flow of electrons causes the collector current which is the actual current flow .
Due to this reason PNP transistors cannot handle very high powers. They are used for small signal or medium power applications.
While, NPN transistor can handle very high currents and thus are used in majority of high power amps apart from small signal and medium power applications.

Although this explanation does not satisfy in most cases, but it was the reply given by my teacher when asked. I could not find any other answer till today.
 

Chippie

Member
I dont wish to go down th eroad of semiconductor theory, holes electron mobility et-al.........suffice to say that, silicon transistors do posses some superior qualities compared with their germanium counter parts....again its a case of selecting the appropriate device for the given design....

Kinjalp...sorry I disagree with the statement,

PNP transistors cannot handle very high powers. They are used for small signal or medium power applications.
While, NPN transistor can handle very high currents and thus are used in majority of high power amps apart from small signal and medium power applications.

Take for instance the device MJ3055, a high power silicon NPN power transistor.......it has a PNP counterpart the MJ2955.......

at the other end of the scale in terms of power ratings, the BC series, 547/557 are npn/pnp devices......
 

kinjalgp

Active Member
Even I disagree with the statement. But it was made by me teacher when I was in school. At the end I have said that it goes wrong for most applications that I have seen using PNP. :)

Also one more personal experience I would like to share is that it is easy to saturate PNP transistor. Even a touch to its base puts it into saturation or atleast take it upto active region. But that is not the case with NPN. It requires much harder base drive.
So a conclusion can be made that PNP are more succeptible to noise than NPN.
 

Chippie

Member
The problem is tho' that if statements like that go unchallenged, people can often end up being given the wrong advice....Hopefully our original poster wil realise this....
 

kinjalgp

Active Member
I think I got the point. If you compare BD139 (NPN) and BD140 (PNP) the maximum power dissipation Ptot is different for both. For BD139 it is 8W max. where as for BD140 it is 12.5 W max.

This I think my teacher was trying to say. The power dissipation of PNP transistor itself is higher which reduces the circuit effeciency.

I am sory if I misguided anyone. :(
 
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