MOSFET GOT XTREMELY HOT

You've made a power oscillator. Reduce the gate resistor to something like 100R. Add a 1K resistor on the inverting input. Connect 10n from the output to inverting input.

Your MOSFET will still get extremely hot (maybe even hotter) but it won't oscillate.

Best regards, Spehro Pefhany

finally make it look like this->

noise is lower than simply putting a smaller resistor in series with MOSFET gate.

Still don't know how to get 1k and 104 capacitor.

I see unstable has already been mentioned.

Even in stable operation it's still a bit of heat. The IRF640 will waste 9.6W @8A. Hopefully heat sinked ok.

D from BC British Columbia Canada

It all depends on what you are trying to do. If you're making a 8A current source then the mosfet will be operating in the linear reagoin and will get hot.

Mosfet only run cool whne fully saturated or off.

The suggetions on making connection a cap from the output to to the - input and a resistor in series with the -input is to allow direct feedback at high frequencies to the OPAMP. You are getting oscillations because at those frequensies you have significant phase shift between the 10K resistor and the MoSFETS transfer function to cause positive fedback instead of the desired negative feedback.

Doing this will slow the outputs response to the input command. you can reduce the capacitance until it starts to honk again the increase it some to get into a safe reagon of operation (FOR THAT MOSFET).

Oops.. stuck in smps mode.. :P

This is a linear app. I'll pick some arbitrary numbers.. Let's say Vds = 12V and Id =8A That's 96Watts.. Toasty!

D from BC British Columbia Canada