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How to Calculate Output impedance in V-V feedback?

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J

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In Razavi's "Design of Analog CMOS IC" chapter 8 , Figure 8.18, calculating the output impedance.

He neglected the drawn current by the feedback network !

But why?

Why he want to get the result, Ro'=Ro/(1+A*F) ?
 

The network isn't part of the output impedance, it's an external circuit to the amplifier, as the load.
You can easily calculate the total impedance, if you like to.
 

FvM said:
The network isn't part of the output impedance, it's an external circuit to the amplifier, as the load.
You can easily calculate the total impedance, if you like to.
Click to expand...

You mean that the output impedance is different form the impedance at the output point ?

And I think now that the author maybe supposed Rout,open << R1+R2 ?

If so, I doubt if necessary to make Rout,open << R1+R2 when designing .
 

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