0
\$\begingroup\$

how can I calculate the input resistance of this circuit?

enter image description here

Thank you!

\$\endgroup\$
  • \$\begingroup\$ 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? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 27 at 23:05
  • \$\begingroup\$ I'm thinking about Ube / Ib for DC and r_be * (1+ s*R_e) for AC (input resistance). Is this correct? \$\endgroup\$ – Markus Jan 27 at 23:17
0
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.