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enter image description here

The picture is showing a normal bjt amplifier.whose output side is converted like this : enter image description here

suppose the amplifier is unloaded. Text books says this amplifier has an output impedance Rc. According to the second image(equivalent to the first image), the output impedance is nothing but an ac voltage source whose terminals are connected by Rc, now, if Rc were in series with the load (which is not present yet) it is clear the impedance should be Rc, but in this case Rc is parallel, so how in this case too the unloaded impedance is Rc?

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According to the second image(equivalent to the first image), the output impedance is nothing but an ac voltage source whose terminals are connected by Rc

Your 2nd circuit is somewhat correct but you misinterpret the transistor as a voltage source when it is in fact a current source. A current source has (theoretically) infinite impedance and, when placed in parallel with Rc, creates no change to the effective impedance of the output: -

enter image description here

The output impedance is still mainly defined by Rc. I say "mainly defined" because, if you look within the BJT itself it has a non zero output admittance (\$h_{oe}\$) in the diagram below: -

enter image description here

Hybrid Parameter model diagram taken from this pdf document.

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  • \$\begingroup\$ great! you said it is "somewhat" can you please explain why its not accurate? so may i assume it is an alternating current source? \$\endgroup\$
    – Sayan
    Dec 21, 2020 at 14:11
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    \$\begingroup\$ Yes you can @Sayan. \$\endgroup\$
    – Andy aka
    Dec 21, 2020 at 14:13
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    \$\begingroup\$ @Sayan "somewhat" means partially correct - there can be confusion over the symbol you used for a voltage source and so I updated my picture showing the old-fashioned symbol for a current source to be less ambiguous. \$\endgroup\$
    – Andy aka
    Dec 21, 2020 at 14:17

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