What is transistor saturation?

The transistor is in saturation?

• For the circuit below, VDD = 10V and VOUT = 2V. Assume the transistor is in saturation, VTN = 1.2V, VDSQ = 5? Note there is no more information just what I have written Here is the link for the circuit: http://www7.0zz0.com/2012/03/26/01/464318029.gif Find R1, R2, RD, and RS.

• Answer:

  By saturation I assume the FET is in its current controlled mode Vo = 2 volts = Id*Rs since the drain current flows through Rs in the source Vds = 5 volts Vd = 7 volts = 10- Id*Rd where Vd is the drain voltage Rd = 1.5*Rs since Id*Rs = 2 volts and Id*Rd = 3 volts Vg = 10(R2)/(R1 + R2) 96Ω = R1(R2)/(R1 + R2) If you assume Vg = 5 volts then R1 = R2 and I1 = 0.05(Id) The drain current of a FET is found from: Id = (K/2)(Vg - Vs - Vtn)^2 Id = (K/2)(Vg - 2 - 1.2)^2 Since we don't know any characteristics of the FET, all we can say is that it must have a gate voltage greater than 3.2 volts . Assume Vg is around 4.5 volts 4.5 = 10(.45R)/(0.45R + 0.55R) 96 = (0.45R)(0.55R)/(0.45R + 0.55R) = 0.2475R^2/R R =96/0.2475 = 388, R1 is approximately 213 ohms and R2 = 175 ohms R2 would be 0.45/0.55 of R1, R2 could be a 178 ohm 2% and R1 a 205 ohm 3% resistor. I1 would be around 10/383 = 26 mA Id = 26ma/0.05 = 520 mA Rs = 2/520 ma = 4 ohms Rd = 3/520 ma = 6 ohms With out any more information this is about as good as I can do