PSRR of CMOS inverter

[Martin Gruber]

How to calculate the PSRR for a CMOS inverter?
I'm struggling a bit because I do not get meaningful result values.

Let explain what I have. I have the following transistor parameters from a simulation result.

High-side PMOS:
rds2 = 11.67k
gm2 = 879.4 uS

Low-side NMOS:
rds1= 20.35k
gm1 = 1.659 mS

With that I want to calculate the PSRR. I created the small signal equivalent as shown in the image:
http://s7.directupload.net/images/140102/pu4dzkw6.jpg

With that I calculate the PSRR:

PSRR = dVout / dVdd

For doing that I took Kirchhoff's law.

dVout = UR1 = IR1 * R1

IR2 - IR1 - gm2 * Vgs2

In small signal equivalent Vgs2 is dVDD.

IR1 = IR2 - gm2 * dVdd

IR2 = (dVdd - UR1) / R2
IR2 = (dVdd - dVout) / R2

This can be inserted in the equation before:

dVout = ((dVdd - dVout) / R2 - gm2 * dVdd) * R1

dVout/dVdd = (R1/R2 - gm2 * R1) / (1 + R1/R2)

Inserting now the number values from above unfortunately yields a negative result which can't be true.

PSSR = -5.8859

Can somebody please help me to do it the right way?

Thanks in advance!
Martin

 

[Ralph Barone]

How to calculate the PSRR for a CMOS inverter?
I'm struggling a bit because I do not get meaningful result values.

Let explain what I have. I have the following transistor parameters from
a simulation result.

High-side PMOS:
rds2 = 11.67k
gm2 = 879.4 uS

Low-side NMOS:
rds1= 20.35k
gm1 = 1.659 mS

With that I want to calculate the PSRR. I created the small signal
equivalent as shown in the image:
http://s7.directupload.net/images/140102/pu4dzkw6.jpg

With that I calculate the PSRR:

PSRR = dVout / dVdd

For doing that I took Kirchhoff's law.

dVout = UR1 = IR1 * R1

IR2 - IR1 - gm2 * Vgs2

In small signal equivalent Vgs2 is dVDD.

IR1 = IR2 - gm2 * dVdd

IR2 = (dVdd - UR1) / R2
IR2 = (dVdd - dVout) / R2

This can be inserted in the equation before:

dVout = ((dVdd - dVout) / R2 - gm2 * dVdd) * R1

dVout/dVdd = (R1/R2 - gm2 * R1) / (1 + R1/R2)

Inserting now the number values from above unfortunately yields a
negative result which can't be true.

PSSR = -5.8859

Can somebody please help me to do it the right way?

Thanks in advance!
Martin

Why can't that be true? It's an inverter with gain, so if dVdd produces
something which could be considered a positive voltage excursion at the
input, then you should get a negative excursion at the output.