Hi,I'm new here.
I simulate the differential amp, when I connect the substrate of the M1 and M2  to the source terminal,there comes a strange current at the tail current source M3 when the input is 0,and until reaching the saturation region is  the sum of the two mosfet  equal to the tail current.But if I connect the substrate to the ground, everything turns normal.I don't know why, so I come here asking for help.Thank you! The *.sp file is as follow:

.lib 'cmos90.sp' tt
*the lib is the 90nm cmos lib from UCB
m1 out1 in pp pp nmos l=1u w=2u
m2 out2 in pp pp nmos l=1u w=2u
m3 pp bb gnd gnd nmos l=1u w=4u



rd1 dd out1 10k
rd2 dd out2 10k

vdd dd gnd dc 3
vbb bb gnd dc 0.6
vin in gnd

.op
.dc vin 0 3 10m
.print  lx4(m1) lx4(m3) par("lx4(m1)+lx4(m2)")
.end

It doesn't work...I don't think it is the ground that causes the problem...