High Gain Comparator

Discussion in 'Homework Help' started by menewbie, Mar 12, 2006.

  1. menewbie

    Thread Starter Active Member

    Jan 31, 2006
    34
    0
    Hi, can somebody show me how to design a comparator with a gain that is larger than 100000? The one I have now has the gain about 2000 which is not good enough. The comparator I have now consists 3 stages: the differential amplifier with active loads, hysteresis, and a complementary CMOS differential amplifier. I tried to add gain stage, which is a common-gate configuration. But that is still not good enough. Therefore, I think I need to try a different type of architecture.

    Finally, Can someone recommend some A/D books to me? So far, I have "Principles of Data Conversion System Design" by Razavi and "CMOS Circuit Design, Layout and Simulation" by Baker, Li and Boyce.
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    It would be helpful if you could post the schematic of your circuit.

    hgmjr
     
  3. menewbie

    Thread Starter Active Member

    Jan 31, 2006
    34
    0
    Thank you for the reply. I took this comparator out of a book. What I want to know is how to increase the gain of the whole circuit?

    For the first stage, the pre-amp stage, I tried to increase size the area of current mirror (The nmos transistor on the right has larger area than the one left and the output current is more than the input current)

    For the second stage, the hysteresis, I just dont have any idea how to increase the gain. I know how it acts as a hysteresis but I dont see why it has a positive feedback loop.

    For the last stage, it is a complementary CMOS differential amp with an inverter connected. I know this circuit is self-biased, has a negative feedback loop to make it stable and has a wide rage of output swing. To increase the gain, I tried to increase the transconductance. But it seems that at some point that the gain will stop increasing, or rather the comparator stops working like a comparator if I increase the transconductance too much.

    So there are two solutions that I am thinking: one is to add another gain stage and the second one is to use a different comparator. And I dont know how to do either one of them....
     
  4. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    I can't see your circuit, but is there any reason why you cannot use a comparator IC? You'll probabably get about x 200,000 gain.
     
  5. menewbie

    Thread Starter Active Member

    Jan 31, 2006
    34
    0
    Yeah...I accidentally posted the same thing twice and so I was trying to figure out how to delete the extra post...But I accidentally deleted all the circuits that I have posted. Sorry about that. Anyway, I am designing the comparator on a chip level. Using a comparator IC is a great idea and I wish I could use it. But I cant because I am doing this thing on a chip level....
     
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