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Actually it closer to \$Zin=h_{fe1}*0.8h_{fe2}(R_E+Z_{load})\$ until Vbe5 reaches ~ 0.5V where hFE is a function of Iout and can vary 2:1 easily from Ie=100:1

The 80% is from Rbe=100 meaning 20% of the current if 0.7/Rbe = ~10~20% of Ib. The current limiter raises Zout while reducing Zinoutput current ratio, rapidly.  

DC Biasing ignored.

The negative feedback loop then reduces Zout of Re/Av. The diode provides soft current limiting with 140% short circuit current and replacing single Q and diode with a Darlington squares the current gain feedback so the short circuit to threshold ratio is reduced to ~ 120% approx.

Empiracle results ought to be close to this.

Actually it closer to \$Zin=h_{fe1}*0.8h_{fe2}(R_E+Z_{load})\$ until Vbe5 reaches ~ 0.5V where hFE is a function of Iout and can vary 2:1 easily from Ie=100:1

The 80% is from Rbe=100 meaning 20% of the current if 0.7/Rbe = ~10~20% of Ib. The current limiter raises Zout while reducing Zin.  

DC Biasing ignored.

The negative feedback loop then reduces Zout of Re/Av.

Empiracle results ought to be close to this.

Actually it closer to \$Zin=h_{fe1}*0.8h_{fe2}(R_E+Z_{load})\$ until Vbe5 reaches ~ 0.5V where hFE is a function of Iout and can vary 2:1 easily from Ie=100:1

The 80% is from Rbe=100 meaning 20% of the current if 0.7/Rbe = ~10~20% of Ib. The current limiter raises Zout while reducing output current ratio, rapidly.

DC Biasing ignored.

The negative feedback loop then reduces Zout of Re/Av. The diode provides soft current limiting with 140% short circuit current and replacing single Q and diode with a Darlington squares the current gain feedback so the short circuit to threshold ratio is reduced to ~ 120% approx.

Empiracle results ought to be close to this.

1
source | link

Actually it closer to \$Zin=h_{fe1}*0.8h_{fe2}(R_E+Z_{load})\$ until Vbe5 reaches ~ 0.5V where hFE is a function of Iout and can vary 2:1 easily from Ie=100:1

The 80% is from Rbe=100 meaning 20% of the current if 0.7/Rbe = ~10~20% of Ib. The current limiter raises Zout while reducing Zin.

DC Biasing ignored.

The negative feedback loop then reduces Zout of Re/Av.

Empiracle results ought to be close to this.