# Calculate input resistance of power amplifier

how can I calculate the input resistance of this circuit?

Thank you!

• WHat's your guestimate formula? and a Darlington is better than a single Q+Diode for current limit slope but Rgts may be too high. Can you also compute the output impedance of the current limiter? – Sunnyskyguy EE75 Jan 27 at 23:05
• I'm thinking about Ube / Ib for DC and r_be * (1+ s*R_e) for AC (input resistance). Is this correct? – Markus Jan 27 at 23:17

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.