Hey guys a while back i was watching a video about a project and i noticed in the schematic they used a transistor to control a n channel mosfet and i wondered why did not they used the mosfet directly?
P.S i can not find schematic now sorry
Because it was a non-logic-level MOSFET? That could be one reason (level-shifting to 12V)
First of all, a MOSFET IS a transistor. That is what the T at the end stands for. So what you want to say, probably, is that they used a BJT to control a MOSFET.
For the reason, it most probably is as MarkT said. MOSFETs are voltage driven devices and if the controller cannot provide the voltage to switch the MOSFET fully on, a BJT is one possibility to solve this.
Another reason could be to provide more current for faster switching, since the gate of a MOSFET acts as a capacitor and has to be filled to reach the required voltage. The more current, the faster this works.
One transistor gives a 180 degree phase shift in common emitter mode. i.e they may have wanted the mostfet 'on' when the controlling output was 'off'.
Or maybe an NPN transistor drove a p-channel mosfet whose source was connected to a higher level rail to provide a high-side switch...
Without more data it's matter of speculation
regards
Allan