yvkrishna wrote on Aug 28th, 2015, 1:19am:Thanks for the reply loose-electron,
Well this is not a kind of application where I can use chopper.
I am only trying to understand the strategy for sizing input pair and amount of tail current to use here for minimising this tail noise upconversion to outputs.
Also not sure why I see more upconversion with larger input Frequency for the input pair.
Thanks,
yvkrishna
I've never tackled that particular mixer-type problem, but I have done mirrors. It sounds like you are doing what I would do. As you probably know 1/f is reduced with larger areas devices, but you can't just make the devices wider because the gm goes up as fast as it goes down. You can make it longer to degenerate but you lose headroom. I played with this one time and found that it was best to use a resistor to degenerate the mirror and make the devices as wide/short as possible. Basically the MOS will be in subthreshold and the resistor drop will be as large as headroom allows.
If the noise is coming from the diode connected device you need a larger device, meaning more current in that device. So it boils down to headroom and power.
Usually this is a problem only when mismatches are considered because in an opamp the differential input is very small so noise is steered equally, thus it's common mode. Is this an opamp or are you running a diff pair open loop?