solsopp
Newbie level 4
I am new to analog design, and I am currently attempting to implement the comparator given below.
It is hard to form specific questions regarding this circuit, as I am unsure of where the problem lies. However, I am sure that my methods of designing the circuit are lacking and maybe incorrect. I guess my question is: how do I bias and size the transistors? Where do I start, and how do I go from there? What is the thought process?
I can briefly explain what I've done and how I've thought so far. Feel free to correct me.
1. I started off by deciding (W/L) of the upper current-mirror (M1 & M15). I made sure that the transistors were in saturation, and that the current being mirrored was accurate.
2. I chose (W/L) ratio of the differential pair (M2 & M3) and made sure that they were in saturation.
3. I then chose (W/L) of the cascode transistors (M4 and M5) so that they were in the linear region. I wanted them to be in the linear region to maximise gain.
4. The widths of the cross-coupled pair (M7 & M8) is chosen to be about half of the width of M4 and M5, since nMOS transistors should usually be about a half or a third of the size of pMOS transistor.
So far, I have managed to get the circuit to "function" in terms of switching between high and low states (0 and 1.8 in my case). However, the switching point seems rather arbitrary, since it doesn't switch at the correct value.
Please tell me if more information is needed, or if my descriptions are confusing.
Thanks in advance!
It is hard to form specific questions regarding this circuit, as I am unsure of where the problem lies. However, I am sure that my methods of designing the circuit are lacking and maybe incorrect. I guess my question is: how do I bias and size the transistors? Where do I start, and how do I go from there? What is the thought process?
I can briefly explain what I've done and how I've thought so far. Feel free to correct me.
1. I started off by deciding (W/L) of the upper current-mirror (M1 & M15). I made sure that the transistors were in saturation, and that the current being mirrored was accurate.
2. I chose (W/L) ratio of the differential pair (M2 & M3) and made sure that they were in saturation.
3. I then chose (W/L) of the cascode transistors (M4 and M5) so that they were in the linear region. I wanted them to be in the linear region to maximise gain.
4. The widths of the cross-coupled pair (M7 & M8) is chosen to be about half of the width of M4 and M5, since nMOS transistors should usually be about a half or a third of the size of pMOS transistor.
So far, I have managed to get the circuit to "function" in terms of switching between high and low states (0 and 1.8 in my case). However, the switching point seems rather arbitrary, since it doesn't switch at the correct value.
Please tell me if more information is needed, or if my descriptions are confusing.
Thanks in advance!
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