I came up with this circuit to control a 12V 6A fan's speed by a 5V PWM from another circuit, not sure if it's gonna work (i'm not very experienced nor knowledgeable in electronics), and i have a few questions:

1- to compensate for Q1's Vce, and turn off Q4 completely and reliably, when Q1 in on and A is low, i added one extra diode in series with Q4's base (total 2), is this a good idea? is it necessary as i thought?

2- are D4, D5 and D12 necessary in this circuit? (i put them there as part of baker clamp for each transistor, to charge/discharge the base faster when switching)

3- is there anything else that you think is unnecessary/lacking/wrong?

 

Wow. Why not just put the PWM signal onto the gate of the MOSFET? Of course use a logic level MOSFET.

 

The scheme is bad.
PNP transistor is always open.
The lower transistor together with the diodes limit the voltage to the voltage at point A.

 

Way too much circuitry to do that job.
Follow wayneh's suggestion.

 

Massively over complicated gate drive circuit. The problem with Wayneh's suggestion is that the gate capacitance will be charging through the 470 ohm resistor. This may not be good enough at 25kHz.

I would use a gate drive IC like the Microchip TC4420. If you can't get one of those use a complementry NPN/PNP pair of transistors between point A and the gate. Point A to the bases of both transistors. join the emitters together connect to the gate. Collectors to 0V and 12V.

This will give better gate drive in the positive direction. Only downside is that the gate is not pulled all the way down to 0V when off (only 0.65V).

Another simple way to drive a MOSFET is to use a 555. Tie the two inputs together as the input. Connect the gate to the output (pin3). It will invert though.