Why do we neglect Cgd's contribution to the output current?

[eesurgeon]

For a MOSFET, while deriving the transit frequency wT (defined as where the current gain is unity), it is common to neglect the current fed forward through Cgd (i.e. the output current io = gm*vgs and not gm*vgs-jw*Cgd).

What is the reason for doing so?

 

[LvW]

For a MOSFET, while deriving the transit frequency wT (defined as where the current gain is unity), it is common to neglect the current fed forward through Cgd (i.e. the output current io = gm*vgs and not gm*vgs-jw*Cgd).

What is the reason for doing so?

No - that`s not true. That is the only input current that does exist.

 

[eesurgeon]

No - that`s not true. That is the only input current that does exist.

LvW, thanks for your response though I do not understand what you have written.

To be more clear, I am referring to the derivation by Grey and Meyer in "Analysis and Design of Analog Integrated Circuits 5e", section 1.6.8 MOS Transistor Frequency Response. As repeated here, the input current is s*(Cgs+Cgb+Cgd)*vgs, while the output current io is approximated as gm*vgs.

Thomas Lee's "The Design of CMOS Radio-Frequency Integrated Circuits 2e" states that "the gate-to-drain capacitance is considered only in the computation of the input impedance; its feedforward contribution to output current is neglected."

It is not clear to me the reason or justification for making this approximation.

 

[LvW]

My reply was related to your statement that - for finding the transition frequency wT - it would be "common to neglect the current fed forward through Cgd" .
I think, in this case, we are not allowed to neglect this input current because it is the only current through the input terminal (gate).

Regarding Thomas Lee`s book, does he speak about "normal" amplifying operation of the device or about definition of wT? Both cases are different!

 

[eesurgeon]

My reply was related to your statement that - for finding the transition frequency wT - it would be "common to neglect the current fed forward through Cgd" .
I think, in this case, we are not allowed to neglect this input current because it is the only current through the input terminal (gate).

Regarding Thomas Lee`s book, does he speak about "normal" amplifying operation of the device or about definition of wT? Both cases are different!

I agree with the statements in your first paragraph, I used a poor choice of words. If the output of a common-source amplifier is short-circuited to ac ground and an input test current is applied, the input current is iin = s(Cgs+Cgb+Cgd)*vgs.

The current resulting from sCgd*vgs cannot be neglected which is what I believe you are saying.

And the output current io = gm*vgs-s(Cgd)*vgs.

However, the second term -s(Cgd)*vgs is dropped in derivations for transit frequencies found in many Analog IC textbooks including Gray and Meyer, resulting in the expression |io/iin| = gm/(s(Cgs+Cgb+Cgd)) so that wT = gm/(Cgs+Cgb+Cgd).

I believe that without neglecting the second term -s(Cgd)*vgs, |io/iin| = (gm-sCgd)/(s(Cgs+Cgb+Cgd)) and therefore wT = gm/(Cgs+Cgb+2Cgd).

Lee is referring to the wT figure of merit.

So my question still remains, why neglect the current -sCgd*vgs from the output current?

 

[AMS012]

If you choose not to neglect Cgd, then isn't Wt=gm/(Cgs+Cgd)?
Anyway, I think, Cgd is neglected only due to the fact that Cgd will be smaller in comparison with Cgs of the MOSFET when operated in saturation.