Beginner: How can I calculate Vds

[princerock]

Consider a simple common source with active load circuit as shown below:

Assuming all the transistors work in saturation and assign Vov=Vgs-Vt

The output swing should be: Vdd-Vov1-Vov2

If I want a 2V output swing and Vdd=2.5v, then I can get Vov1+Vov2<0.5v

Then according to other specifications (such as dc gain) I can do raw calculation about the size of M1 and M2 and let them meet the requirement Vov1+Vov2<0.5.

But in simulation, Vds2 is much larger than Vds1 and thus M1 can't work in saturation(Vds1<Vgs1-Vtn). This is because I assume Vds=Vov when I did the calculation but in fact Vds>Vov also makes transistors saturated. Obviously I can see that Vds1+Vds2=Vdd, but how can I calculate Vds1 and Vds2 and make ture Vds>Vov for both transistors?

 

[analoger]

Make sure your input bias voltage is correct.

 

[flushrat]

take care of bias voltage of net12

 

[sankudey]

You also need to take care of the bias currents (DC) flowing thru two branchs....as the Wp/Wl is around 1, Vov1+Vov2<0.5 should be distributed with their majority carrier velocities which is generally 2.5.....you can check once more the calculation u performed for the same....also the input swing is of important concern

 

[rockycheng]

Vov2=Vov3, which is fixed. Make sure Vov1<0.5-Vov3.

 

[happy2005]

Common source amplifier with active load has very high and sharp gain. As can be shown in the figure, you should bias Q point. But it is not easy. In the Q point, VGS voltage of M1 should be equal to VGS1 voltage. more information in the book Sedra&Smith Fig 5.45 in 4th edition.