Biasing a FET (March 13, 2005)
So your building some sort of a N-MOSFET as a switch circuit, you wire the FET between the load and the ground (or source), then apply the source voltage to the gate, and the FET switches on a bit. Congratulations, your FET is incorrectly biased.
The problem is actually beautifully simple; to turn all the way on, the FET needs a greater voltage between its gate and source terminals then between its drain and source terminals. How much voltage? The definitive answer seems to be some.
The real question is how do you get more voltage then +ve?.
Among the builders of battle-bots, Maxim's MAX622 IC is a popular answer; it provides 25mA at 11 volts above +ve. As wonderful a little packet of joy this is, it has 2 short fallings: a)It's not always easy to find(1), and when it is, it's somewhat pricey (between $5 and $15) and b)it only puts out 25mA of current.
Any sort of a step up converter you could envision would be equally good, but, again, they usually have little head room in the current department.
Another (not so) common technique is to add a fair sized resistor below the FET (and before -ve) so that less voltage appears across the FET. (Making the +5v apear higher). The Mircrowave Encylopedia advocates this method.
Unfortunately, we can't always acquire nifty little pieces of silicon like the MAX622 or waste power/limit the voltage range of the FET on a biasing resistor. Conveniently, there exist common devices that provide predefined amounts of voltage rise at acceptable current rates and that are dirt cheap to boot. They're little known devices called batteries. Good old rectangle shaped 9V batteries are 4 for a dollar at the local corner store, and, when paired with a suitable 8¢ clip, provide 9v of rise to any low draw circuit.
For my 6v, IRF530 based H-Bridge circuit, biasing the gates with a 9v battery between ground and the gates provided roughly 5v across the bridge when driving a 2amp load. Compare this with only 2.4v when the gates were fed from the regular 6v supply. For voltages bigger then 6V you'd likely want to run the 9v battery between then +ve rail and the gates giving you 9v of rise, instead of between -ve and the gates. If you feel so inclined, a diode between the +ve rail and the -ve of the 9v battery might be a nice touch.
(1) Futurlec has MAX622s on for $3.90USD right now.
FET (3) Howto (9) Transistors (4)
Posted by spiffed at March 13, 2005 1:55 PM