There are plenty of references you can find in the internet about darlington amplifiers.
You can start from this: "Darlington Circuit("")":https://www.google.com.ph/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0CCkQFjACahUKEwjL0IOt0dnHAhWKj44KHa3QCRc&url=http://www.carlomozetic.net/userfiles/unit_4.pdf&usg=AFQjCNEBcRiTN4fiphO0YpPiPJ4GucmHeQ&sig2=04B28izA0Vw7oxuJceAWpw&bvm=bv.101800829,d.c2E
