according to data sheets it is constant , however according to caculations it also depends upon the value of 'R'

like,

Vo = kt/q * ln (vi/R*Is)

if we now arrange this equation for 'Is' the final equation is so weird :/ i want know what exactly the value of Is should i use for my calculations ???

i am designing simple Natural Logarithm Opamp in which diode is a feedback factor