Hi! I’m working on an SAR ADC, and for the comparator, I’ve used the following circuit. Everything works fine when I use a constant current source. However, I need to implement it at the transistor level. When I try using a current mirror, I encounter issues where the current supplied by the mirror is very low and not the required bias current for the circuit. Any suggestions on what to do for the constant current part? I've inserted my xschem file of the comparator as well.

When you are using a current mirror for Ibias, are you using a sufficient high common mode voltage at the comparator inputs? Your current mirror output device may be pushed into triode?

@Boris Murmann ah i see since its below the vth therefore the transistor will not turn on. if so do you think it will work if i add a voltage source of 900mV to increase the input? using a summing amplifier?

No need for a summing amplifier. To get your simulation to work, put back the current mirror, and change the existing input voltage sources from 0V and 5mV, to 900mV and 905mV, respectively.