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Whilst reading about differences between PNP and NPN transistors I came across the following circuit diagrams: http://www.learningaboutelectronics.com/Articles/Difference-between-a-NPN-and-a-PNP-transistor

The uppermost circuit diagram, illustrating the function of the NPN transistor, shows the load positioned before the transistor. The lowermost circuit diagram, illustrating the function of the PNP shows the load positioned after the transistor.

Would anybody be kind enough to advise if this is significant and a function of the type of transistor. My understanding would be that the behaviour of the load would remain the same, regardless of its position before or after the transistor.

Thanks

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  • \$\begingroup\$ Have you written out the transistor equations? \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 13 '15 at 6:06
  • \$\begingroup\$ Hints: In the linked article loads are at the collector in both cases. Vbe can be thought of as constant in both devices. \$\endgroup\$ – HKOB Feb 13 '15 at 6:09
  • \$\begingroup\$ No - apologies my understanding is not this deep yet. Would you be kind enough to identify these specific equations if you feel it would help me answer my own question. \$\endgroup\$ – James Izzard Feb 13 '15 at 6:12
  • \$\begingroup\$ I'm not sure about "before" and "after", but notice that both diagrams have the load connected to the collector pin of the transistor. \$\endgroup\$ – The Photon Feb 13 '15 at 6:15
  • \$\begingroup\$ Thankyou. I appreciate the differences you are pointing out between the diagrams now. I believe this: electronics.stackexchange.com/questions/38285/… now answers my question. \$\endgroup\$ – James Izzard Feb 13 '15 at 6:19
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What do you mean by "before the transistor" and "after the transistor"?

In both cases shown, the load is connected to the collector of the transistor. The circuits may look different, because an NPN and PNP transistors work with opposite polarity power supplies.

An NPN transistor works with the emitter as the most negative terminal, and the collector positive, while a PNP transistor wants the emitter to be the most positive terminal, and the collector most negative.

If you have a simple NPN circuit, you can change the transistor to PNP and invert the power supply polariity, and the circuit should still work.

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  • \$\begingroup\$ "invert the power supply" ... and any other polarised components - diodes, electrolytic capacitors, etc "and the circuit should still work". \$\endgroup\$ – Brian Drummond Feb 13 '15 at 12:37
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This is independent of transistor-type. Either one may be used in multiple cases. The important thing to notice in the diagrams is the polarity of the base/gate.

With a "PNP"-type, you must drain current from the gate or the transistor will not conduct.
With an "NPN"-type, you must supply current to the gate or the transistor will not conduct.

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    \$\begingroup\$ These are bipolar transistors, not FETs. The middle terminal is the base, not the gate. \$\endgroup\$ – Peter Bennett Feb 13 '15 at 6:19
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The load stay the same with both type of transistors

you can tray this circuit it will work very fine

i tried the two circuit and it work very fine

1/NPN circuit enter image description here

the switch in position 1 the transistor conduct and the LED switch on, glow

the switch in position 2 the transistor switch off and the LED switch off

2/PNP circuit enter image description here

The first step we invert the power supply and the LED

the switch in position 1 the transistor conduct and the LED switch on, glow

the switch in position 2 the transistor switch off and the LED switch off

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