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NPN vs PNP, a fundamental or just a mental difference

Like many here, I were in need of a bit of amplification of a weak signal. This signal had "low impedance", I suppose. The supplied circuit diagram shows two different ways of meeting my need.

The left circuit uses a NPN transistor. The right hand circuit uses a PNP transistor. I chose the one with PNP. Quite "arbitrarily". The BP103B is also known as FA300.


Update: labels corrected as Oddbot suggested.


It works. When I shine a laser directly into the phototransistor, my Picaxe picks up a "high" on the digital input. But I have a couple of questions about my choice for the PNP based amplifier.

  1. These two circuits are equivalent, aren't they?
  2. I used them as a digital input, but I suppose they could also serve an analogue input. Correct? Maybe tweak the values a bit?
  3. The big question: does it matter which circuit I choose? Is there a fundamental difference between the two transistor circuits above? Or is it just a difference in fabrication/price/practice?
  4. Perhaps the fact that my photo transistor is itself NPN makes the two circuits substantially different? What if that was not a concern? It could be a high value potentiometer as voltage divider. Does it still (not) matter?

Just to be clear: I am not looking for help with my laser detection (not today anyway). I just want to learn a bit more about this NPN vs PNP difference.


Update 28 nov:

I thought I needed two more circuits to make this picture complete. Two NPN circuits plus Two PNP circuits. One of each inverting, the other one not.

Boy, was I deceived! I ended up (before I gave up) with EIGHT permutations. (eagle file attached)


But while I was drawing all of these, it dawned on me that I do not need to include the photo transistor. The whole NPN vs PNP debate can be pictured with a single transistor. So I drew this one. NPN version only.


This demonstrates the two basic methods of using a transistor, I think. The left circuit has the output on the collector of the transistor. An NPN version of this left circuit will invert the signal (that's what the line above the words means).

The right hand circuit has the output on the emitter of the NPN and will give a normal output.

If you replace the NPN in any of these by a PNP, the function will reverse. 

As I understand now, the emitter-bound output will give slightly better amplification because the resistor is receiving the B-E current in addition to the C-E current.

Any comments, thoughts or aditional questions are still welcomed!


eight permutations: eagle schematic (posing as .dir fil)63.9 KB

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The only real difference, from what I've learned at school, between a PNP and NPN transistor is the chemicals used to make it. I've never really noticed a difference

Quick quote pulled from this site:


I was thinking about this last night and the only other reason I can give about choosing NPN or PNP concerns if you want to invert a signal or not. In fact looking at your schematic above, while the circuit with the PNP will give a high output when hit by the laser your NPN circuit will give a low output when hit by the laser.

I used this in the last update of the Laser RF MkII schematic when I found that while I wanted to run the motor at 10% the laser worked better at 90% and thus changed the laser drive circuit to use a PNP since both laser and motor were driven from a single pin.

Transistors, like impedance are complicated subjects and I tend to ramble sometimes so from this point on I'll just asnswer questions.

If circuit 0 inverts the high input (light) to a low input
and circuit 1 does not
then the two are (functionally) very different!

Sure, in practicality that would not be a problem. But it is a difference.

I will come up with a variation that shows equivalent circuits 2 & 3 tonight.


Idle promises! It took me a few more nights. So many distractions in life on LMR....

This is something I've always been really curious about too. Thanks for posting, I'll be watching the replies.


I don't mean to pick, I was just looking at the labels you've given your resistors in series with the transistors. The connection with the arrow, reguardless of NPN or PNP is the emitter. In your circuit with the NPN transistor it would seem that R0 is also a collector resistor? :\

And please, do pick! With a c, that is. Correction on the way...


Both your circuits are the same and in many cases it may be just a case of what component you have lying about. But in some cases a NPN transistor will out perform a PNP transistor. The same is true of their mosfet equivalents.

Be it a Bi-polar transistor or a Mosfet, the NPN bipolar and the N channel Mosfet do tend to offer better performance in high power applications where the PNP and P channel devices tend to get hotter.

With the transistors I commonly use eg. BC547-BC549 NPN and BC557-BC559 PNP the PNP transistor has slightly less gain and a slightly higher collector-emmiter saturation voltage.

For small signal amplifiers such as your circuits above it usually doesn't matter.