sandman wrote on Dec 24th, 2011, 4:12pm:Hi,
I've tried some basic topologies (e.g. source degeneration) which give me decent results for linearity (> +30dBm), but it burns up a lot of current and is noisy. Could anyone suggest a very linear (>30dBm) transconductance or a means to further linearise a current mirror and yet achieve <2nV/√Hz (<-160dBm/Hz) output noise floor with some small gain ? Or even any seminal papers or interesting designs/topologies ?
For noise, I suggest starting out with the equations and substituting 2*I/(Vgs-Vt) for gm in your final equations. This will give you a physical feel for the trade-offs that you have. Also, you should be able to look at the simulation results and determine the contribution of each device in the circuit after doing your noise analysis.
Current mirrors are pretty basic: the lower the gm, the lower the noise they produce. If you lower the gm by decreasing the width, the mirror will be faster. If you lower gm by increasing the length, the mirror will be slower. Physically, lowering gm means making Vgs-Vt as large as possible
for a given current so a low noise current mirror will need lots of headroom.
Alternatively, resistive degeneration accomplishes the same thing, and is even better for 1/f noise. Drop as much voltage as you can stand across the resistor. I do not know why this didn't work for you, but you must be misinterpreting the results or describing the circuit incorrectly to us. Furthermore, your current consumption should stay the same - I suspect you are doing something wrong as the current consumption increase should reduce the noise,
assuming you kept the gain the same.The noise could also be generated before the mirror: garbage in, garbage out.
I assume finite output impedance of the mirror is causing your linearity problem. Resistive degeneration will help as you found out, as will a cascode. The former will reduce noise,; the latter will add negligible noise for a given operating point, but the bigger impact of the cascode will be loss in headroom which will limit how much vgs-vt you can have. In other words, you will lose headroom by adding the cascode, forcing you to reduce vgs-vt of the mirror (or the IR drop if degenerated). The loss in headroom will force a design with higher noise.
rg