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-   -   help ! how to calculate?????????? (http://www.diyaudio.com/forums/solid-state/72059-help-how-calculate.html)

bogdan_borko 18th January 2006 08:42 PM

help ! how to calculate??????????
 
2 Attachment(s)
how to calculate output impedance of these amp stages?

Tube_Dude 18th January 2006 09:34 PM

Re: help ! how to calculate??????????
 
Quote:

Originally posted by bogdan_borko
how to calculate output impedance of these amp stages?
Hi bogdan_borko

Amp stage A - the output impedance is the value of R4 in parallel with R5

Amp stage B - 1/Gm ( Gm is the transconductance of the mosfet ) in parallel with R9 and R6.

Cheers

ilimzn 18th January 2006 10:05 PM

Re: Re: help ! how to calculate??????????
 
Quote:

Originally posted by Tube_Dude
Amp stage A - the output impedance is the value of R4 in parallel with R5
Amp stage B - 1/Gm ( Gm is the transconductance of the mosfet ) in parallel with R9 and R6.
Actually, assuming R5 in amp A and R6 in amp B are the load, then what you have written is the total load the amp sees, not it's output impedance (this does not include the load). So, for amp A it would be R4, because a MOSFET is a voltage controlled current source, and current sources have very high impedance (ideally infinite), and in amp B it is 1/gm of the MOSFET at the chosen operating point in parallel with R9.

Tube_Dude 18th January 2006 10:15 PM

Re: Re: Re: help ! how to calculate??????????
 
Quote:

Originally posted by ilimzn


Actually, assuming R5 in amp A and R6 in amp B are the load, then what you have written is the total load the amp sees, not it's output impedance (this does not include the load).


Hi ilimzn

But if you assume that R5 and R6 are the DC current path for charging the capacitors C2/C4 and part of the design , the output impedance is calculated as I have said.


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