LNA cascode inductor degeneration

This is a schematic of the cascoded inductor degenerate LNA, so the current flow across M3 is : 1/2 un Cox W/L ( Vdd - Id Rref - Vth)^2 by setting values of rref and W/L , you set the bias current, which is then copied to M1.

But what is Rbias for?

Prevent RF loading (losses) on input. A choke might be better, with less stray shunt substrate capacitance and maybe even less area in an integrated circuit design. Also the M3 mirror will likely degrade noise figure due to noise injection without any filtering.

The Rbias is used in order not to make the signal quantity flow through the biasing network so that the full signal gets appeared at the gate/base of the transistor.Usually the value of Rbias is high so that it provides a high impedance path to signals.

Hope it helps


Santom

Rbias should be a high value resistor to get the best noise figure (higher gm for a given Q).