R12, R13, Q8, Q9 form wilder current source. Ic(Q8)*R12 = Vt*Ln( I(R13)/Ic(Q8)).
DC input at base of Q1, Q2 raise from 0 to 100V, current at wilder current source varies little.
DC voltage at output almost remain constant. (High voltage protection by active load).
Vout = Vc(Q4) = Vc(Q3) ~ V(R10) ~ V(R9 or R11) = I(R9)*R9 ~ (Ic(Q8)/2)*R9.
Current output from wilder current source varies little => Vout varies little.
When we delete the feed back loop between active load and the wilder current source, we lose over voltage protection.
Ib(Q3) + Ib(Q4) + Ic(Q3) + Ic(Q4) ~ IC(Q8). If Ic increase, Ib will decrease, which in turn result in Ic decrease. The feedback loop of the active load keep system stable.
also the current source becomes unstable.
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