DC operating point of CMOS single end two stage op amp

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simbaliya

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I was asked this question during a interview, they asked me to draw a CMOS single end two stage op amp, and then told me for all transistors Vdsat=0.1V and Vth=0.3V, besides, the inerviewer also provide Vdd=1.8V and Vout=0.9V, I was asked to tell the DC operating point from node 1 to node 5, can anybody teach me how to solve this? Please refer to attached figure for node positon.

 

I would have a few questions to verify that I am solving the right thing but I will give it a shot. I assume node 5 is connected to both top gates.

Node 1 is just the drop across the transistor from the 1.8V source. Assuming all of them are the same transistor, I would also say this point is 0.9V because of the same drop they have shown in parallel. Node 5 would be the voltage required for a 0.9V drop across the transistor (the voltage to turn the transistor on). We can see the Vth is 0.3V so anything greater than that puts the transistor in active mode. Maybe not the best answer but just gave it a shot. Let me know what you think or any other feedback that they gave you to call this right or wrong?
 

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