Input Common-Mode Resistance MOS Diff. Pair

Discussion in 'Homework Help' started by jegues, Oct 14, 2011.

1. jegues Thread Starter Well-Known Member

Sep 13, 2010
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45
How did they come up with that expression for $R_{icm}$?

Here's what I came up with,

$R_{icm} = \frac{v_{id}}{i_{b}}$

$i_{e} = \frac{v_{id}}{\frac{R_{E} + r_{e}}{2} + R_{EE}}$

$i_{e} = (\beta + 1)i_{b}$

$\Rightarrow R_{icm} = (\beta + 1)( \frac{R_{E} + r_{e}}{2} + R_{EE})$

Am I doing this incorrectly?

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2. jegues Thread Starter Well-Known Member

Sep 13, 2010
735
45
Still looking for clarification on this one.

3. The Electrician AAC Fanatic!

Oct 9, 2007
2,501
393
The expression from the book has Rc and ro involved, and you don't.

Your work involves re, which the book doesn't.

4. jegues Thread Starter Well-Known Member

Sep 13, 2010
735
45
Is mine correct if we were to neglect ro?

Also where does ro sit? Across Rc or across the collector to emitter?

5. The Electrician AAC Fanatic!

Oct 9, 2007
2,501
393
ro is across the collector-emitter.

The expression in the book appears to be correct with one exception; where they have β they should have β+1. That being the case, I don't think your expression is correct.

You also have RE and REE; what is RE?

Last edited: Oct 18, 2011