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I am trying to simulate the following cmos comparator circuit using cadence virtuoso spectre.enter image description here

Here's my schematic on cadence: enter image description here I am using the following test bench to simulate the circuit with Ibias = 5 microAmp, VDD = 5 V, VSS = 0. VN is grounded and VP is a sinewave.

enter image description here

This is the result of transient simulation. As can be seen the output on the bottom does not react to the input at all. I expected to see a square wave but I just get Vdd the entire time. Any feedback on this?enter image description here

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1 Answer 1

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You have not biased the comparator properly. An NMOS input pair comparator will need the reference voltage to be higher than VGS(M1)+VDS(M5). Since you've connected the reference to 0V, the comparator will not work.

To get this to work, you need to figure out a value of VGS(M1)+VDS(M5) such that the transistors are all in saturation. Then, you can feed this as your comparator reference voltage. You can then feed a 0-5V sine wave on the other comparator terminal.

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  • \$\begingroup\$ Your suggestion worked. I had to change reference voltage from 0 to a value such that the M1 is saturated. If I were insistent on having 0 V as the reference voltage, how should I modify the circuit? \$\endgroup\$
    – ah286
    Commented Nov 1, 2023 at 5:31
  • \$\begingroup\$ You could try a pmos input pair based comparator. \$\endgroup\$
    – sai
    Commented Nov 1, 2023 at 6:35

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