aryajur
Joined: 23 Oct 2004
Posts: 675
Helped: 69
Location: Sunnyvale, USA
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 07 Dec 2004 22:40 What is the input Common Mode Range??? |
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Hello,
I have attached an image of a Folded Cascode amplifier. If I want to calculate the input Common Mode range of the amplifier, the normal way is as follows. The highest value of Vin,cm will be Vdd - Vss - Vgs1 where Vss is the drop required across Iss, which may be a overdrive voltage of another PMOS.
Now the lowest value of Vin,cm is Vdsat5 - |Vth1| but I have a problem analysing this qualitatively. When Vin,cm starts to decrease it tends to increase Vgs1 hence the current through M1 but the voltage at the Gate of M1 also falls to counteract this to make the current practically the same. If Iss is implemented by a PMOS device then this means that the |Vds| of that PMOS is increasing thus the current should increase by a small amount. Now considering M5, if the current increases by a small amount since its Vgs5 is fixed therefore Vds5 should increase. On the contrary we assume Vds5 to be of its minimum value = overdrive voltage of 5 = Vdsat5 when calculating the minimum value of Vin,cm
Could anyone explain this a bit better to me??
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