sandman wrote on Jan 2nd, 2012, 3:07am:RobG wrote on Dec 30th, 2011, 11:42am:sandman wrote on Dec 30th, 2011, 9:33am:I set out to explore the noise equations for a CM amplifier by hand but this has be expected-ly slow. A couple of papers treat noise in CM's but only through simulation results.
Where is the hangup? The noise source should just be current sources in parallel with the devices in the mirror. The analysis is pretty straight forward from there. As I mentioned earlier, about all you can do to reduce thermal noise is increase the overdrive (i.e. Vgs-Vt).
Apparently he can get the linearity with resistive degeneration so in the worst case he should be able to do far better with a cascode device.
RobG, thanks for your comments. While the treatment of a simple current mirror is straight-forward, I'm using a couple of feedback methods at the input which add their own noise to the input (or output) - hence the added complexity in the details...
I simulated a simple CM alongside a CM with output-stage cascode. The cascode had worse output linearity and the noise was somewhat increased than compared to resistive degeneration. From what I'm familiar with, non-linearity in the CM comes about due to mismatches in the mirror and the transistor non-linearity (non-linear capacitances, deviation from square law, other non-idealities which I'm yet to figure out!). 'Cascoding' the output only compensates for the poor λ. There's a lot more to linearity than just mismatches that you point out.
What are your thoughts on this ?
Source degeneration is one method which works, but that's because it reduces the gain but has a limit to the dynamic range that can be achieved with this method.
My input DC voltage is fixed, so there probably isn't much I could do, to play with the Vgs-Vth than increasing drain current, so (resistive) degeneration seems to be the only solution I can think of.
OK, based on what you said you are losing linearity from things like non-linear capacitance, non-square law etc. That is a hard problem for us to help with since we don't know your topology. Personally, I'm surprised those things affect the mirror accuracy but we all learn new things. I assume these bad effects happen at high frequency and large signal (transient) which always changes what we know.
Try it with the tolerances increased for sure!
As for the noise analysis, you should be able to just do the mirror effects as the noise sources should not influence one another.
Increasing current should help with noise and it sounds like it may be your only option. It will reduce your output impedance which might affect other things.
These are things that generally work. It is entirely possible that there is something special about your circuit that is negating some of these effects. It is hard to help more without seeing your actual circuit.
When I see things like this I usually dive into the equations and try to figure out what is going on taking into account different effects, but I understand that might be too difficult in your case. Starting with ideal devices is also helpful, although these days the model is so complex that it is difficult to do.