# Min. Common Mode Input MOS Diff Pair with Current Mirror

#### jegues

Joined Sep 13, 2010
733
See first figure attached for problem statement.

I've solved everything in this circuit, I just have some confusion about solving $$V_{CMmin}$$ when I have a current mirror acting as my current source connected to the sources of Q1 and Q2.

Usually when I have this setup with an ideal current source instead of a current mirror I can solve for $$V_{CMmin}$$ by writing a KVL from the common mode input down to the -Vss.

Like so,

$$V_{CMmin} = V_{GS} + V_{CS} - V_{SS}$$

Where $$V_{CS}$$ is the minimum voltage required across the current source.

(See 2nd figure attached for example)

How do I do this now with my current mirror in place?

It looks as though my $$"V_{CS}"$$ is going to be replaced by the voltage $$V_{DS3}.$$

Computing $$V_{DS3}$$,

First note that,

$$V_{GS} = -V_{S}$$

Where $$V_{S}$$ is the voltage at the source of Q1 and Q2.

$$-V_{S} - V_{t} = V_{ov}$$

$$V_{S} = -V_{ov} - V_{t} = -0.7V$$

Thus,

$$V_{DS3} = V_{S} + V_{SS} = 0.5V$$

Now writing my KVL,

$$V_{CMmin} = V_{GS} + V_{DS3} - V_{SS} = 0V$$

I'm not entirely sure if my reasoning is correct so if somebody could point out any mistakes or confusions I'm having it would be greatly appreciated.

Thanks again!

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