# PSRR of CMOS inverter

M

#### Martin Gruber

Jan 1, 1970
0
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:

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

Martin

R

#### Ralph Barone

Jan 1, 1970
0
Martin Gruber said:
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:

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

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.

Replies
1
Views
2K
L
Replies
3
Views
2K
Helmut Sennewald
H
H
Replies
4
Views
2K
Hammy
H
A
Replies
0
Views
2K
A
Replies
2
Views
193