Amninder
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Amninder said:Hi,
I am new to this filed on analog design. I am reading the book by Behzad Razavi on "Design of Analog CMOS Integradted Circuits"
While reading on MOSFETs, I encountered an equation for transconductance which says that : gm = 2 * Id / Vgs - Vth .Its explanation states that Transcondutance decreses with overdrive (Vgs - Vth) when Id is constant. I did not undestand that how Id can be constant when we are varying the overdrive.A change in Vgs will change the drain current even in saturation region.
Can anybody explain this contradiction? It would be of great help.
Objection! Unfortunately you are misunderstanding the formula in the same way as the original poster and many contributors did. In equation 2.19 (Razavi page 21) "Id is constant" implies that transistor geometry W/L is varied. Transistor is staying in saturation.I am explaining in case a new user arrive and find what exactly happening in this equation:
(...)
Since current cannot change in the branch , so transistor come out of saturation to follow KCL at output node. Since transistor is no longer in sat mode and entered into triode region, we know gm is always less in this region.
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