rahulloveselectronics
Junior Member level 1
Hi,
How can we define quantitatively the variation of Id and Gm versus the input voltage for a common source with source degeneration
resistance.
My assumptions are as follows:
Variation of Gm versus Vin
Until Vth since there is no channel and hence current flow, the Gm should be Zero
Above Vth ( small Vin slightly above the threshold), and Since the current is very small, we can say that the gm = ( sqrt (2UnCox W/L Id ) should be small, hence 1/gm will be greater than Rs. Thus Gm= gm. Now gm = Un Cox W/L (Vgs - Vth), the current is very small, hence we can assume that Vin is almost equally available across Vgs, thus gm ~= Un Cox W/L (Vin - Vth ). Hence Gm varies linearly with Vin, until 1/gm = Rs. Then Gm = 1/Rs, which will be a constant value henceforth for increasing Vin.
Variation of Id versus Vin
Until Vth, there is no channel and hence the current flow is zero
Above Vth, how do I find the variation?
I felt it should be Id = Un Cox/2 . (W/L) . (Vin - Id.Rs - Vth )^2.
Can you please help me understand the concept.
How can we define quantitatively the variation of Id and Gm versus the input voltage for a common source with source degeneration
resistance.
My assumptions are as follows:
Variation of Gm versus Vin
Until Vth since there is no channel and hence current flow, the Gm should be Zero
Above Vth ( small Vin slightly above the threshold), and Since the current is very small, we can say that the gm = ( sqrt (2UnCox W/L Id ) should be small, hence 1/gm will be greater than Rs. Thus Gm= gm. Now gm = Un Cox W/L (Vgs - Vth), the current is very small, hence we can assume that Vin is almost equally available across Vgs, thus gm ~= Un Cox W/L (Vin - Vth ). Hence Gm varies linearly with Vin, until 1/gm = Rs. Then Gm = 1/Rs, which will be a constant value henceforth for increasing Vin.
Variation of Id versus Vin
Until Vth, there is no channel and hence the current flow is zero
Above Vth, how do I find the variation?
I felt it should be Id = Un Cox/2 . (W/L) . (Vin - Id.Rs - Vth )^2.
Can you please help me understand the concept.
