Hi
I'm building a circuit like so:

8-bit DAC > OP-AMP(as a buffer) > MOSFET > 555 circuit > Servo

I use the MOSFET to control current which in turn controls timing.
Since the MOSFET is only useful from 4V (threshold voltage) to 5V and the DAC puts out 0V to 5V, I lose 204 degrees of accuracy. How could I solve this problem?

-The DAC is home made so I could modify it, it's a R-2R setup.
-It's a power MOSFET, is it possible to get low threshold voltage FET?

It seems like there should be a simple solution, but I just can't think of it!
I'm interested in learning so if this whole thing seems like a stupid approach (maybe a MOSFET is not the right tool) please tell me.

 

Can you post the schematic of the circuit? It's really hard to guess what is going on.

 

All I really need to know is how I can change a 0V-5V signal to a 4V-5V signal. I'm using the MOSFET like a variable resistor. Let's see how my drawing skills are:

 

Tim,
Create/save your image in .PNG format. .jpg format is too lossy. MS Paint can save in .png.

Make sure the image is under 100k before uploading it.

 

Sorry it looked fine on my screen. Here is the PNG image:

Is it better? I could draw it in LTspice if that's better

 

Add these two resistors to your OpAmp.

 

Add these two resistors to your OpAmp.
Attached Thumbnails

... and, if interested, here is how I calculated the resistors:

Solving the Summing Amplifier (http://masteringelectronicsdesign.com/solving-the-summing-amplifier/)http://theartofelectronicsdesign.com/?p=339

 

Geezz, Of coarse! Lol. I don't know why I couldn't think of that!
Hey thanks alot eh, I really appreciate it.