When deriving Zout_G1 - it is assumed that driving source of block G1 is of low impedance (such as opamp output stage), analysis will be different if this is not the case
Let’s define Gideal = G1*G2, and Gactual is the actual G1, G2 overall gain that factor in inter-stage loading. Then Gerr = 100% * (Gideal - Gactual)/Gideal
From the graph, it can be seen that:
1. As driving stage (G1 in this case) output impedance becomes much lower than the input impedance of loading stage (G2 in this case), each block becomes more and more independent of each other.
2. For system design, it is critical to have each block having high input impedance and low output impedance, so that we can design each stage by itself and still be able to cascade them together