Need help on gm measurement

[richloo]

Need help from u guyz... i have simulated a gm cell, but the problem is
which point of gm should i take from the plot?
i mean this is a large differential input signal, should i take at the common mode voltage which is 0.9V?

i m using 1.8V single supply.

input:
inp= DC sweep 0->1.8V
inn=0V (sounds like common mode at 0V... pls clarify me if i m wrong)

output:
gm=deriv(IS("/V4/PLUS"))


thanks in advance!

 

[Vamsi Mocherla]

gm=deriv(IS("/V4/PLUS"))

I really do not understand how it could be gm. Well, when you are running the gm measurement. You have to keep one of the inputs at VMID value which is VDD+VSS/2. And then sweep the input voltage on the other terminal from VSS to VDD. Usually, it should be more or less constant in some range and many times coincides with the VMID voltage at the input. Hence your design can be used for that range of values.

Your setup seems fine. The output should also be at VMID level and is at ac ground.

Hence gm =d(iout)/dVin

 

[richloo]

gm=deriv(IS("/V4/PLUS")) is to plot the derivative of the output current vs input voltage sweep. Do u mean that gm is constant around VMID,so i need to sweep input voltage in small step around VMID?

 

[Vamsi Mocherla]

Well, keep one of the inputs at VMID and sweep the other. You will see that there is a constant slope of your output current curve in a region. When you find that slope out, you will see it is gm