# Stuck on a FET exam review question.

#### student_technician

Joined Dec 28, 2014
18
This is a textbook question:

"In a certain FET circuit, VGS=0V, VDD=15V, IDSS=15mA, and RD=470 ohms. If RD is decreased to 330 ohms, IDSS is

a)19.5mA
b)10.5mA
c)15mA
d)1mA

Is the Answer "c" because IDSS is just the Drain to Source current maximum.

Thanks,

ST

#### MikeML

Joined Oct 2, 2009
5,444
I would say it differently. IDSS is the Zero Gate Voltage Drain Current. Since you are given that Vgs=0, the Drain current will be 15mA unless RD is big enough to limit it...

#### t_n_k

Joined Mar 6, 2009
5,448
If IDSS was strongly dependent upon VDS in the saturation region, then one might expect a significant change in ID when the lower RD presumably gave rise to a higher VDS - with VGS remaining constant. No such strong (or even modest) dependence seems to be indicated in the literature or reported in practice. Hence one can safely assume IDSS is the same with the proposed change in RD.
The usually small non-zero slope of the saturation region characteristic is accounted for by the channel length modulation effect of VDS on ID.
In any event, one can readily argue against options (b) or (d) as the likely increase in VDS would presumably increase IDSS, rather than decrease it. This leaves option (a) as the only other possibility, which is highly unlikely, requiring a very significant change of 30% in IDSS with an increase in VDS of less than 1V (~ 8%) and with VGS held to zero volts.

Last edited:

#### WBahn

Joined Mar 31, 2012
24,684
If IDSS was strongly dependent upon VDS in the saturation region, then one might expect a significant change in ID when the lower RD presumably gave rise to a higher VDS - with VGS remaining constant. No such strong (or even modest) dependence seems to be indicated in the literature or reported in practice. Hence one can safely assume IDSS is the same with the proposed change in RD.
The usually small non-zero slope of the saturation region characteristic is accounted for by the channel length modulation effect of VDS on ID.
In any event, one can readily argue against options (b) or (d) as the likely increase in VDS would presumably increase IDSS, rather than decrease it. This leaves option (a) as the only other possibility, which is highly unlikely, requiring a very significant change of 30% in IDSS with an increase in VDS of less than 1V (~ 8%) and with VGS held to zero volts.
Very good answer. Note how t_n_k used reasoning to narrow down the choices until only one reasonable answer was left -- even if you aren't sure what IDSS even is!