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Using FET transistor as votage controled resistor

Question asked by rdastyle on Jun 26, 2009
Latest reply on Jun 30, 2009 by rdastyle

I need some help for my issue : I try to make a variable resistor using a transistor FET which reference is " cf_hm_HMF06100d200_19940325" (found in library of ADS). I used the "Fet_Curve_tracer" to obtain the DC caracteristics.

I want to obtain a resistor of around 70.25 ohms.
That's why I took VGS = - 2 V and VDS = 2,75 V which gives  I = 39 mA so Rds = 2.75/(39e-03) = 70.51 Ohms.

Here my trouble : I put my transistor FET in a schematic, with  1 V_DC on VDS and 1 other on VGS,  using a pi network attenuator, where the first resistor is the FET transistor and the 2 others resistors are simply "true resistors".

Actually, I made a simulation with 3 resistors in a pi network --> my results look like what I want. With a FET used as resistor, I lost my previous results. I mean I've tried to move cursors in "Tuning" but it has no effects.
I run simulation in "Transcient" because I send a pulse. Is it possible that ADS does not understand it needs to put V_DC on VGS and VDS and in the same time calculate the Transcient simulation ? I don't think so but I feel confused about this.

May you help me ?