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Simple and strange, looking at datasheet of BSS138 (Graphs) for this circuit below which is always ON Saturation mode Mosfet, Rds-ON = 3.5 Ω, VDS-ON = 0.1 V to 0.2 V.

Id should be 16.66 mA but simulation is 12.5 mA. Why? The resistance in simulation doesn't change; it's 300 Ω fixed. To get 12.5 mA RDS-On should be around 85 Ω, which is not the case.

In other words I see VDS drop of 1.27 V (Vn002 in graph) , to get this voltage drop, again resistance should be in the range of 300 Ω, which doesn't make any sense to me.

Note: the model is correct and part number is BSSS138, Mosfet Circuit

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You have the drain and gate connected together. This configuration is called "diode connected". That is not the configuration under which Rds(on) is specified. The Rds(on) specification is valid when the gate is driven high with respect to the source and is not connected to the drain. The drain will then be free to drop to a much lower voltage.

You should disconnect the gate from the drain, and directly connect the gate to 5V in order to achieve low Rds(on).

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  • \$\begingroup\$ What are you talking, if you observe carefully, all i am doing is making gate high by connecting it to 5V. this will drive mosfet into saturation mode, LT spice doesnt come with the model, i downloaded from Mosfet website. \$\endgroup\$ Dec 15 '20 at 22:05
  • \$\begingroup\$ In your schematic, pin 3 and pin 4 are connected together. Please take a look at your schematic and tell me if you agree or disagree. According to the legend you put on the schematic, 3 is the drain and 4 is the gate. Please let me know if that is not correct. In a normal BSS138, pin 1 is the gate, pin 2 is the source, and pin 3 is the drain. \$\endgroup\$
    – mkeith
    Dec 15 '20 at 22:14
  • \$\begingroup\$ ah.. sorry but discussion is not about that, i confirm the model is correct, i dont see what are you seeing wrong here. do not compare this model with datasheet, i can call any name or number to the pins in LT spice model. \$\endgroup\$ Dec 15 '20 at 22:22
  • \$\begingroup\$ Please disconnect drain and gate, then connect the gate directly to the 5V supply. In order to have low Vds, you must have high Vgs. Therefore, g and d cannot be connected together. The pin numbers don't matter I was just checking my assumptions. That your legend is accurate. \$\endgroup\$
    – mkeith
    Dec 15 '20 at 22:23
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    \$\begingroup\$ Ah now i understood what you mean, sorry for the misunderstanding. and i totally agree with you. the gate was not getting full voltage and i missed to see this. Thanks \$\endgroup\$ Dec 15 '20 at 23:21

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