#### sk.shawn

##### Junior Member level 3

Hi,

Inserted is a basic diff pair circuit and I am trying to analyze its common-mode behaviour. But before that, I realized that my thoughts have already been trapped without knowing how the source coupled voltages (Vs) of M1 & M2 vary at all. This is really painful as

1) Without analytically knowing how Vs varies, I cannot be entirely certain if M1 & M2 are on despite driving their gates with a reasonably high common-gate voltage (Vin,com) alone as in real fact the common gate-source voltages (Vgs) of M1 & M2 are not completely known due to the unknown state of Vs.

2) Without knowing the gate-source voltages (Vgs) of M1 & M2 now, how will the branch currents of M1 & M2 vary with an increase in the common-gate voltage (Vin,com) since the branch currents are a function of their respective gate-source voltages (Vgs) and not Vin,com alone?

3. Without knowing Vs analytically, how can I be certain about the operating region of M3 since Vs is also the drain-source voltage of M3?

So, it seems to me that the root cause of my self-trapping thoughts is not exactly knowing the state of Vs analytically which is somehow driving me nuts and I hope you will be able to shed some lighton this as I've tried searching online for answers but couldn't find anything to straighten out my thinking. Really look forward to hear from you soon. Really appreciate it and thanks.

Regards,

Shawn

Inserted is a basic diff pair circuit and I am trying to analyze its common-mode behaviour. But before that, I realized that my thoughts have already been trapped without knowing how the source coupled voltages (Vs) of M1 & M2 vary at all. This is really painful as

1) Without analytically knowing how Vs varies, I cannot be entirely certain if M1 & M2 are on despite driving their gates with a reasonably high common-gate voltage (Vin,com) alone as in real fact the common gate-source voltages (Vgs) of M1 & M2 are not completely known due to the unknown state of Vs.

2) Without knowing the gate-source voltages (Vgs) of M1 & M2 now, how will the branch currents of M1 & M2 vary with an increase in the common-gate voltage (Vin,com) since the branch currents are a function of their respective gate-source voltages (Vgs) and not Vin,com alone?

3. Without knowing Vs analytically, how can I be certain about the operating region of M3 since Vs is also the drain-source voltage of M3?

So, it seems to me that the root cause of my self-trapping thoughts is not exactly knowing the state of Vs analytically which is somehow driving me nuts and I hope you will be able to shed some lighton this as I've tried searching online for answers but couldn't find anything to straighten out my thinking. Really look forward to hear from you soon. Really appreciate it and thanks.

Regards,

Shawn