Question about ramsey optimal policy

[lipetty]

I have studying the topics about ramesey optimal policy,there is a big question i havent get through.
In woodfood or Gali(2008),they take the quadratic approximation of the utility function to planner objective which is like the traditional lose function.
but,In Christiano(2008.or 2007notes ),it seems not to take the quadratic approximation of utility function,and if the model with consumption habit, capital accumulation and financial friction,it must be very boriong and cumbersome to take quadratic approxmation. of course ,i also havent get through how to use dynare to solve it? In dynare mamnual, it need a planner_objective model-expression.
so ,would you so kind to help me to explain this? thank you in advance!