Marc Murphy wrote on Jan 27th, 2006, 7:40pm:steven wrote on Jan 25th, 2006, 10:38pm:Hi Marc,
Thanks for your comments. I understand what you said, however, I don't quite understand what is going on in this circuit. For exmaple, in the attached figure, bias voltages have been labeled. I didn't draw the bias network and CMFB network. Vb1 and Vb2 are from the bias network. Vbias comes from CMFB. As can see from M1 to M4, the bias is not right, ie. Vds are too small than Vds(sat). I tried to achieve the big "AND" in your post but no avail so far. Something is wrong.
Thanks,
Steven
One nasty problem is that Vsd7 is far too big...it eats up all your voltage.
I'd say you haven't sized the transistors to handle the current sourced by M5, Mb4, and M6. I would need to know the branch currents and transistor sizes to make any useful comments. You'll also need to size Mb5 so that it has Vds5 > Vgs + Vds(sat).
Make sure vbias MOS is 3x to handle all branch currents.
Mb5 should be sized to give the targeted Vgs + Vds across it when you run the branch current through.
Other sizing tips: pick the gm you want, and vgs you want. size each transistor type so that it gives you the desired gm at the vgs you selected. do a vgs sweep with vds set to the target vds when you are trying to get gm
What Vgsn, Vsgp, Vdsn(sat), and Vsdp(sat) are you targeting?
What is Vth for each MOS?
What is v+, v- at?
Hi Cri and Marc,
Thanks for both of you commenting on the circuit. As Marc said, I didn't size it correctly when I posted but tried to illustrate how to get around the Vsd7. I can size them to have open-loop gain at around 85dB but GBW is too small. Still I don't feel comfortable on something. Here are the parameters I used:
V+ and V- are at 1.5.
Vthn is 0.6, Vthp is -0.8. I tried to set |Vds(sat)| around 0.2. Actual Vds(sat) is about 0.3~0.4 (absolute value). Vds(Mb5) now is about 1.2V. It was really hard to set Vds(Mb5)=Vgs+Vds(sat) (I am happy to listen your tricks on doing this).
M10 is in trio mode though, Vds10=0.04 < Vds10(sat)=0.9. I don't if I still need to adjust it or not. The others are at Saturation.
Half branch current is 3.3mA, Id(Mb5) is 31uA. Maybe I should redraw the circuit to show the sizes. Before that, I want to ask these to Marc,
1. When you said make sure vbias MOS is 3x, what is the 3x compared to?
2. How do you do a vgs sweep? This is very useful since I didn't know how to set M3 and M4 efficiently before I posted the original post.
Regards,
Steven