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!
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!
