2 added 252 characters in body edited Jan 27 at 23:42 Sunnyskyguy EE75 80.7k22 gold badges3030 silver badges116116 bronze badges 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 $$\Zin=h_{fe1}*0.8h_{fe2}(R_E+Z_{load})\$$ until Vbe5 reaches ~ 0.5V where hFE is a function of Iout and can vary 2:1 easily from Ie=100:1 The 80% is from Rbe=100 meaning 20% of the current if 0.7/Rbe = ~10~20% of Ib. The current limiter raises Zout while reducing output current ratio, rapidly. DC Biasing ignored. The negative feedback loop then reduces Zout of Re/Av. The diode provides soft current limiting with 140% short circuit current and replacing single Q and diode with a Darlington squares the current gain feedback so the short circuit to threshold ratio is reduced to ~ 120% approx. Empiracle results ought to be close to this. 1 answered Jan 27 at 23:33 Sunnyskyguy EE75 80.7k22 gold badges3030 silver badges116116 bronze badges Actually it closer to $$\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.