Video using a mosfet driver IC: http://www.youtube.com/watch?v=TkboPrauaOw&feature=plcp
Driving an N Mosfet at higher frequencies requires a buffer / driver circuit to supply ample current to the high capacitance gate of the mosfet. This circuit has been tested to 1 Mhz and works decently well. I’m sure improvements could be made. If you have any suggestions let me know.
Perhaps the simplest buffer requires only 2 transistors in Push Pull (class B) mode. Idea came from Art of Electronics textbook, 2nd ed.
50 ohm square wave generator – 20v peak to peak bipolar square wave.
+15V, 1A power supply
1 FPQ4N50 n type mosfet
1 2N2222A NPN transistor
1 2N2906 PNP transistor
1 220 ohm reisistor
1 390 ohm resistor
1 10 ohm resistor
1 100 ohm resistor
1 4148 or 1N914 diode
1 47 uF tantalum capacitor (power supply bypass capacitor)
I tested this circuit with various input and load resistance values.
Specifically, R1 is best at 390 or below to enable the input square wave to turn off quickly. This was due the the internal capacitance of the mosfet that has to discharge when turned off, but the 1N914 diode before the gate prevents any current from flowing to the ground. When R1 was 1K it caused the input square wave to slope off, like a capacitor discharging between each pulse. The value of R1 should be high enough to not load the signal generator (V1), but not too high to cause the signal to become distorted.
Also, when the load resistor R4 was at 1K, the output wave form was not clean. By lowering it to 220 ohm, the wave form was cleaned up. I am not sure the cause of this, but lower resistances should work, too.
The 2n2904 is not the ideal transistor because it’s Hfe (gain) is not very close to the 2n2222a. It would be better to find a better match to the 2n2222a since it’s gain is higher. I used it here because I already had it in my toolbox.
Use of MC34152 mosfet driver. I’m working on a PCB design for this now