Its a MOSFET, so not much current is needed.
You're really just protecting the driver from the FET's input gate capacitance.
With 1590 pF input capacitance, not much protection is needed.
74LS138 can only source 0.4mA of current, so I would add pullup resistors to turn the FET off fast/
It can sink 8mA, so size the pullup to not exceed that.
5V/1K =5mA, should provide good performance.
Thank you for the response. If I read this right you are recommending a 1K pullup between the 138 pin and the base of the MOSFET, not a base resistor. I was thinking that a base resistor might be necessary like a normal transistor. There is some disagreement out there on this. For example:http://arduino.cc/forum/index.php?topic=80026.0
Grumpy Mike and johnwasser seem to disagree here. This article:http://talkingelectronics.com/projects/MOSFET/MOSFET.html
Says the following:
"However the 10k resistor on the gate of the MOSFET is not needed. Providing the voltage (up to 18v) on the gate rises and falls quickly, the MOSFET will not get hot. The critical period of time is the 0v to 3v section of the waveform as this is when the MOSFET is turning on."
Which seems to indicate to me that you can get away with not having a base resistor, but good design principles might indicate having one just to keep power usage the lowest possible by reducing dissipation of the MOSFET while in the ohmic region, no matter how short that is.
So, am I right on not needing a base transistor? Just go with the 1K pullup and be done with it? That works for me.