Hi,
I will try to answer. In a passive resistor there is -approximately- only one physics noise source: the thermal motion of electrons.
In an active load the noise is due to many factors: the thermal motion of electrons in channel, the dangling bounds on the oxide-silicon surface and -in very short channel MOS- we can see also shot noise.
Morover, in active loads the R value is not constant.

This is my own answer to why active loads have more noise then passive loads.

I hope this is clear.
Best Regards,
MC

raja.cedt wrote on Sep 12^th, 2009, 6:50am:



Hi,
     An active load is a voltage-dependent resistance which introduces non-linearity effects into the circuit ( Total Harmoic Distortion increases ). also as in one of the replies above, an active device introduces thermal noise whose magnitude is proportional to γ - a parameter that scales based on channel lengths. [ nearly 2/3 for long channel , nearly 1 for short channel ] Also, flicker corners vary with different processes, that also decides the total thermal noise contribution. Accounting for all these factors, total noise from an active load should generally be more than its passive counterpart.

Hope this extends the above post by MarcoC & explains your query,

thanks,
Mayank.