Concerning the specral plots of DC bias signals:

Lower amplitudes will lead to limit-cycles of lower frequency (ie. 'pulses' less often).
This may lead to spectral components leaking into the signal band of interest.
To combat this problem, one can counter-intuitively increase the bias voltage in order to be certain 
that limit-cycles are outside of the band of interest , where they can filtered (along with other noise) at a later stage.