Please, can you explain the expression Ae ~ -(0.9Rl/hib ?I think we are looking at the same thing on two different levels, with yours being the more technically correct. My level is what I will call first-order voltage gain of a common emitter stage where "Ae ~ -(0.9Rl/hib), the term hib may be considered as a small resistor in series with the emitter load". As a technician, I never needed to go much deeper than that and most circuits I worked on used an op-amp for gain and transistors for switches, current sources and current buffers and hfe was about all you worried about.
What is the background of the factor 0.9 ?