[SOLVED] Calculation of Poles and Zeros From output impedance

I have a circuit like this

**broken link removed**

By using small signal model i found the 3 current equations

The equations are

I(t)=v(t)*(s*c(l)+g(l))+(v(t)-v)*g(ds(3))+g(m(3))*(-v)
-g(m(2))*v(x)+(v-v(x)*g(ds(2))+v*s*c(x)=-g(m(3))*v+(v(t)-v)*g(ds(3))
-g(m(2))*v(x)+(v-v(x))*g(ds(2))=v(x)*(s*c+g(ds(1)))


By using above equations i got the output impedance v(t)/i(t) =[g(ds(2))+s*c(x)+g(m(3))+g(ds(3))][g(m(2))+g(ds(2))+g(ds(1))+s*c]-g(ds(2))[g(m(2))+g(ds(2))]/[(s*c(l)+g(l))+g(ds(3))][g(ds(2))+s*c(x)+g(m(3))+g(ds(3))][g(m(2))+g(ds(2))+g(ds(1))+s*c]-g(ds(2))[g(m(2))+g(ds(2))]-g(ds(3))[g(m(2))+g(ds(2))+g(ds(1))+s*c][g(m(3))+g(ds(3))]


But i don't know from the above equation how can i found the poles and zeros .


Can anyone suggest how to find the pole and zeros for the above circuit.

Thanks