doublet, pole, and zero
Hi,
when ever there are two paths from input to output a zero occurs. One can observe that, when double-ended to single conversion is done using current mirror there are two paths to the ouput. So a zero occurs. Also a pole occurs at the diode connected node, for obvious reasons. It can be proved that the above zero frequency is twice the pole frequency. And this is called pole-zero doublet.
Further reading:
Razavi, Chapter 6, Pg:176
Razavi, Chapter 6, Pg:191-192
Razavi solutions, Prob. 6.15
Regards