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enter image description here

I try to solve Zout for this BJT small-signal model but I find

RC || (ro + RE)

because Zout is calculated with Ib = 0 => βIb = 0.

From this link too Vin was calculated.

But the expression for Zo given in the book by Boylestad and Nashelsky is:

Zo = RC || (ro + β (ro + re) / (1 + β re / RE)])

Can you explain how to derive this?

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  • \$\begingroup\$ Put the schematic in your question. \$\endgroup\$
    – Andy aka
    Jun 14, 2020 at 19:12
  • \$\begingroup\$ Just a question for you. What happens to \$Z_o\$ when \$R_\text{E}=0\:\Omega\$ from your last equation? Is it sensible? \$\endgroup\$
    – jonk
    Jun 14, 2020 at 21:05
  • \$\begingroup\$ If RE=0 Ω (short circuit) from the last equation Zo = RC || [ ro + β (ro + re) / (1 + infinity)] = Zo = RC || [ ro ] = RC . It is sensible but considering ib = 0 \$\endgroup\$
    – user7058377
    Jun 14, 2020 at 21:35
  • \$\begingroup\$ Vi = 0 not => ib = 0 \$\endgroup\$
    – user7058377
    Jun 14, 2020 at 22:28
  • \$\begingroup\$ But what do You want to show me by thinking about "RE= 0 Ω" jonk ? \$\endgroup\$
    – user7058377
    Jun 14, 2020 at 23:39

2 Answers 2

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Solution : Vi = 0 not => ib = 0 enter image description here

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If you put the equations correctly:

point C :   io vo ve ib
point E :      vo ve ib
ib def  :         ve ib

3 unknowns : (vo OR io) AND ve AND ib

to get vo = function(io) OR io = function(vo)

enter image description here

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