Hi Baab,

Note that when they talk about the short circuit current they are referring to the terminals of the noiseless circuit. When the input of the noiseless circuit is shorted then the current that flows through the short is due to three sources

i_s , i_n and v_nxY_s

i_s and i_n are two completely independent noise sources that are uncorrelated.

v_n is uncorrelated from the thermal source noise, but it can be partially correlated to the circuit input noise current i_n, so a correlation admittance Y_c is used to represent the correlation between i_n and v_n.

Just ask if there is something confusing about this.