Venus,

  Could you provide more information about your design? In
general, the issue when stacking cascoded devices is the Vds
not the Vt. There are techniques for designing low overhead
current mirrors, see Johns and Martin's book for more
details. It was not mentioned, are the input devices included
in the cascode stack? It may also help to separate the input
stage from the gain stage, for example use a differential
input stage with a folded cascode gain stage.

                                                        Sheldon

Hi,

   Still can't quite understand the circuit architecture,
so it is difficult to answer in detail. One comment,  is
that open loop gain is relatively insensitive to bias
current.  For example, for a bipolar transistor:
transconductance is gm = Ic/Vt and output resistance is
ro= Va/Ic so the gain is Av = Va / Vt.  To increase gain,
alternative architectures are required. For example, using
a cascode increases the output resistance, ro~Bo*Va/ Ic.
So gain becomes Av = Bo * Va / Vt. MOS transistors have
similar characterisitics, that is, increasing bias current
does not usually increase gain.  Since you are already
using cascodes, the next step is to use regulated
cascodes(gain boosting).

                                               Sheldon