pnp npn transistors

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fran1942

Joined Jul 26, 2010
58
Hello, I am putting together a circuit and notice that on the diagram the NPN is drawn with the LED above the transistor and the PNP version on the right is drawn with the LED below the transistor.
Can someone please explain why this is, and is it important that they are configured like that ?

Thanks for any help.
 

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MrChips

Joined Oct 2, 2009
20,914
There are three leads to a bipolar junction transistor (BJT), collector, base and emitter.
The arrow in the transistor symbol represents the emitter.
The direction of the arrow represents the direction of conventional current.
(To make sure that we do not upset the electron current purists, electrons flow in the opposite direction of the arrow).
The arrow points away from the base in a NPN transistor.
The arrow points towards the base in a PNP transistor.
There are three ways a transistor can be configured:
common base
common collector
common emitter

The diode load can be placed on either the collector side or emitter side of the transistor.
When the load is on the collector side, as in both diagrams, this is a common emitter amplifier and this gives the best voltage gain.
Both diagrams are acceptable. What will be important is the voltage applied at the base for the transistor to be turned on.

The important junction to watch is the base-emitter junction. This junction behaves like an ordinary PN diode.
When the voltage drop across the PN junction is below 0.6V the base-emitter current is low and the collector-emitter current path is non-conducting.
When the voltage drop across the PN junction is above 0.7V the base-emitter current rises increasing the flow of electrons and holes across the collector-emitter path. (for silicon transistor)

In the left hand circuit, the base of the NPN transistor must be 0.7v more positive than the emitter voltage for the transistor to conduct.
In the right hand circuit, the base of the PNP transistor must be 0.7V lower than the emitter voltage.

The diode load can be placed on the emitter side for the best current gain. You get no voltage gain in this configuration (common collector a.k.a. emitter follower) because as the current increases in the load, the voltage on the emitter rises and you have to keep raising the base voltage to keep the base at 0.7V above the emitter (with respect to the NPN common collector circuit).
 
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