i am trying to simulate an electronic load. in that case i would like to employ a linear MOSFET IXTK22N100 which has Ciss= 7n for having a current at 150khz at output. but in the simulation i couldn’t make it stable and the current is not a pure sinewave at high frequency applications. i am thinking that maybe my op-Amp cannot supply an appropriate current to drive this kind of MOSFET at high frequency but i am not sure that is it true or not? should i use an gate driver stage or using specific gate driver IC between Op-amp and MOSFET? if yes which kind of IC or structure can help me?
Best Answer
Your circuit is commonly used as a current sink and therefore is fundamentally valid.The input C of your linear fet is high and it could form a nasty pole with the Opamps output impedence giving you control system hassles like instability or poor transient response .Putting in some gate resistance in would be wise because the opamp could be unstable into such a big capacitance anyway.An emmiter follower is good because it divides the effective capacitance seen by the Opamp by the gain of the transistors.I havenot applied this circuit or its variants on a expensive linear fet .I would guess that your proposed linear fet would give better stability and transient response than the cheaper more common and less suitable switching type powermosfet.I have used opamps on smaller powerfets in a previous life .I did not have to do too much to the basic circuit to get it stable .