I came up with this circuit to control a 12V 6A fan's speed, with a 5V PWM from another circuit. I'm not sure if it's going to work (I'm not very experienced nor knowledgeable in electronics) and I have a few questions:
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?
Are D4, D5 and D12 necessary in this circuit? (I put them there as part of the Baker clamp for each transistor, to charge/discharge the base faster when switching.)
Is there anything else that you think is unnecessary/lacking/wrong?