automatic gain control

I am trying to simulate agc in matlab simulink. I am unable to find perfect function for variable gain amplifier. i used Vo = k*Vi* exp(a*Vc) where Vi is the input, Vo is agc output and Vc is the error voltage. but this logic dosen't work and my output changes as input changes. Can someone please suggest me a perfect logic for vga in agc. thank you in advance!

Sudarshan