Hello all,
Do the load need to be at this position exactly? In the case of a NPN transistor, why I must put the load before the transistor, why can't I put it after the transistor? Someone can explain?
You don't put the load before the transistor in any case.
The load is in the collector in both cases. This is known as a common emitter configuration.
One is used for current sourcing and the other for current sinking.
Read up on transistor theory to see the differance between NPN and PNP.
You can have a voltage follower, not much of a load, but voltage.
http://www.google.com/imgres?imgurl=http://www.circuitstoday.com/wp-content/uploads/2011/11/voltage-follower-using-transistor.png&imgrefurl=http://www.circuitstoday.com/current-amplifier-and-buffers&h=315&w=264&sz=6&tbnid=AcBReAAGr13-4M:&tbnh=90&tbnw=75&zoom=1&usg=__Az_iCylWU5NLWikhv1W_8CGZKaM=&docid=CJzjtuTuJXorvM&sa=X&ei=MfckUuPLPMLY2AXqhID4BQ&ved=0CEAQ9QEwAg&dur=69006
Ok, thanks all 
If you put the load on the emitter side ("emitter-follower" circuit) then you'll get less voltage across the load and more
across the transistor (it cannot saturate). If you are switching power this is usually undesirable. You also have to
make the base resistor smaller to reduce the voltage loss across it. Remember the emitter voltage (for NPN) has to be
about 0.7V below the base, whereas the collector-emitter voltage can saturate down to less than 0.1V
There is one benefit to the emitter-follower, it should switch faster (because it doesn't saturate and because
there is no "Miller effect"). Usually this isn't important and power/voltage efficiency is wanted.
guix:
Do the load need to be at this position exactly?
Yes.
guix:
Someone can explain?
Because transistors work when there's a difference between base and emitter. If the emitter of an NPN isn't at zero (ground) then the voltage you apply to the base isn't going to be what you think it is.