Ruby, 76 75 71 70 bytes
This feels suspiciously straightforward, so please let me know if I messed up somewhere.
Incidentally, this is the first thing I've ever written in Ruby - I was looking for a language that supported string repetition by multiplying, and Ruby seemed to do the trick.
f=proc{|n|'$$\varphi=1+'+'\cfrac1{1+'*n+'\dd'[0,n+2]+'ots'+'}'*n+'$$'}
To be applied like so:
f.call(0)
$$\varphi=1+\dots$$
$$\varphi=1+\dots$$
f.call(3)
$$\varphi=1+\cfrac1{1+\cfrac1{1+\cfrac1{1+\ddots}}}$$
$$\varphi=1+\cfrac1{1+\cfrac1{1+\cfrac1{1+\ddots}}}$$