- #1
hisotaso
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Homework Statement
I am learning about differential amplifiers, and am having trouble determining the common-mode output resistance. In the text, we have the two sections "Differential-mode gain and input and output resistances" and "Common-mode gain and input resistance." Notice the lack of common mode output resistance.
The only place I can find reference to common-mode output resistance is in the section "Two-port model for differential pairs", and it is given by Roc =~ 2*μf*Ree (in this section, no derivations are shown). At the end of this section is an "Exercise", and it asks for Rod and Roc. Rod = 3.2MΩ and Roc = 4.8GΩ. In other words, Roc is much bigger.
Now, in the homework problem I am working on, VA is given as infinite, which means ro =~ VA/Ic is infinite, which means μf = gm*ro is infinite. However, in the solutions (authors solution manual) given my prof., Roc is given as Rc/2 which is much less than Rod (in this problem Rod =~ 2*Rc = 94kΩ, and and the given solution to Roc = Rc/2 = 13.5kΩ).
So my question is: what is Roc? I have googled around a bit and have not been able to find any reference to it. I would be happy with a link to a derivation (preferable actually) as opposed to just an expression, though at this point any help would be appreciated.