# Output Resistance of this circuit - Cascode

Please see images of question, my working out and the correct solution. I seem to be incorrect for some reason. This is without resorting to the small-signal model.

• 1) You cannot determine Rout without using the small signal equivalent circuit. 2) the right circuit in the picture with Rg grounded and the gate of M2 grounded makes no sense 3) Start with Rg and M2 and determine the impedance (to ground) of Rg and M2. Then add M1 and recalculate. Commented May 3, 2016 at 17:04
• Oh. The reason why I grounded them, is that I assumed infinite impedance at the gate terminal hence no current flow. Commented May 3, 2016 at 17:26
• Also, why has Rg been totally neglected in the final answer? Commented May 3, 2016 at 17:27
• Rg can be neglected because no current can flow through it since the gate is basically an open (infinite impedance). But that does not mean Rg does nothing ! The gate of M2 has the same voltage as the drain of M2 because.... Rg provides a connection. So if you ground Rg (like you did) that connection is no longer present. Changing the behaviour of M2 ! Commented May 3, 2016 at 18:47
• Oh. So would that mean the Vgate(M2) = Vdrain(M2) = Vsource(M1) ? Commented May 4, 2016 at 9:10