Negative PSRR of the Two-Stage Op Amp

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eyeweyew
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Homework Statement
Negative PSRR of the Two-Stage Op Amp with VBias Connected to ground vs connected to Vss
Relevant Equations
PSRR- model for Two-Stage Op Amp with VBias Connected to ground
From page 4-9 of this lecture note https://pallen.ece.gatech.edu/Academic/ECE_6412/Spring_2004/L180-PSRR-2UP.pdf, it gives example on how to model a Two-Stage Op Amp to find the PSRR- when VBias is connecting to ground as oppose to when VBias is connecting to Vss. One thing I don't understand is why rds7 is modelled differently given that it is connecting Vout and Vss in both cases?

Also, not sure why Cgd5 is not included in the model? although it doesn't make any difference to the outcome.

Reference:
CMOS analog circuit design. Authors: Phillip E. Allen, Douglas R. Holberg.
pg.307-309
[Link to full PDF copy of textbook redacted by the Mentors]
 
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FAQ: Negative PSRR of the Two-Stage Op Amp

What is Negative PSRR in the context of a two-stage op-amp?

Negative Power Supply Rejection Ratio (PSRR) refers to the op-amp's ability to reject variations in the negative power supply voltage. In other words, it measures how well the op-amp can maintain its output despite fluctuations in the negative supply voltage, typically denoted as V- or VSS.

Why is Negative PSRR important in a two-stage op-amp?

Negative PSRR is crucial because it affects the stability and performance of the op-amp. Poor negative PSRR can lead to increased noise and errors in the output signal, especially in precision applications. Ensuring good negative PSRR helps maintain the integrity of the signal processing in the presence of power supply variations.

How is Negative PSRR typically measured in a two-stage op-amp?

Negative PSRR is typically measured by applying a small AC signal to the negative power supply and observing the resultant change in the output voltage. The ratio of the change in the power supply voltage to the change in the output voltage is then calculated, usually expressed in decibels (dB).

What factors influence the Negative PSRR of a two-stage op-amp?

Several factors can influence the Negative PSRR, including the design of the op-amp, the quality of the components used, the layout of the circuit, and the frequency of the power supply variations. The internal compensation and the architecture of the op-amp also play significant roles.

How can the Negative PSRR of a two-stage op-amp be improved?

Improving Negative PSRR can be achieved through careful design and layout practices. This includes using high-quality components, optimizing the power supply filtering, and ensuring proper grounding. Additionally, feedback mechanisms and compensation techniques within the op-amp design can be optimized to enhance PSRR performance.

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