input to amplifier at any time is vin(actual)=vin-vout
therefore
vout=H(s)*v(actual)
or vout=H(s)[vin-vout]
vout+vout*H(s)=vin*H(s)
vout[1+H(s)]=vin*H(s)
therefore, vout/vin = H(s)/{1+H(s)}
which is the said transfer function...
you can read this theory in Behzad Razavi's analog CMOS design book.