So far I understood, there are two complementary differential input stages (T1, T2 and T3, T4). When a differential signal is applied to the inputs, there is a current disbalance Iq, which is going to be amplified in the next stage.
This stage I do not understand.
It seems for me, that the top voltage source together with the resistors above T5, T6 constitute two current sources of 2I0.
The rest is not clear in detail: What is the idea behind, to put a current mirror T7, T8 at this place?
In particular, in a disbalanced situation with $$ I_q>0 $$, why are currents through T7 and T8 not the same: left we have $$I_0-I_q$$ , on the right $$I_0-3I_q$$ ? I thought the role of a current mirror is to keep both currents the same - here it isn't the case...
I just see, that finally I get an output current $$4I_q$$ to be available for the next stage. But the idea behind the summation circuitry in the middle is a miracle for me.
I appreciate any hints...Thanks!