I have this equation but I cannot understand it because I can't find what V_inCM represents.

V_SD = V_DD - (V_inCM + V_OV+ V_tp)

V_SD = source drain voltage
V_DD = voltage at drain
V_OV = overdrive voltage
V_tp = threshold voltage

EDIT: I know it's to do with common mode voltages but my question is more about getting an intuition for what (input) common mode voltage is.

 

I don't know how well the following blog entry survived the port from the old to the new forum software.

https://forum.allaboutcircuits.com/blog/analysis-of-the-basic-differential-amplifier-topology.576/

But if you do a Google Search for "common mode voltage" you will get no shortage of hits.

Since any voltage is the measure of the voltage at one point relative to the signal at another point, all voltages signals involve two points. The common-mode voltage is the voltage that they have in common and the differential-mode voltage is the voltage they don't.

Say you have V1(t) at one point and V2(t) at the other. The common mode is the average of the two.

Vcm = V1(t) + V2(t).

While the differential signal is the difference

Vdm = V1(t) - V2(t)

 

I don't know how well the following blog entry survived the port from the old to the new forum software.

https://forum.allaboutcircuits.com/blog/analysis-of-the-basic-differential-amplifier-topology.576/

But if you do a Google Search for "common mode voltage" you will get no shortage of hits.

Since any voltage is the measure of the voltage at one point relative to the signal at another point, all voltages signals involve two points. The common-mode voltage is the voltage that they have in common and the differential-mode voltage is the voltage they don't.

Say you have V1(t) at one point and V2(t) at the other. The common mode is the average of the two.

Vcm = V1(t) + V2(t).

While the differential signal is the difference

Vdm = V1(t) - V2(t)

Thanks for your response. In the case of a CMOS what is common mode? I get that it's the common voltage between two points but in a CMOS it seems to be something to do with the gate voltage but common with what?

 

Thanks for your response. In the case of a CMOS what is common mode? I get that it's the common voltage between two points but in a CMOS it seems to be something to do with the gate voltage but common with what?

Common-mode voltage is a generic concept. If you want to discuss it in terms of a specific circuit, then you need to present a circuit about which to have a discussion.