Скачать или смотреть Mismatch and CMRR in Differential Amplifiers: Common-Mode Gain and Tail Source Imperfections

Real differential amplifiers are never perfectly symmetric—device mismatch, resistor variation and non-ideal tail current sources all degrade performance.
In this lecture, we study how mismatch creates finite common-mode gain, how this affects CMRR, and how tail-source imperfections convert small imbalances into differential output errors.

We begin by revisiting the ideal differential pair, then introduce mismatch in load resistors and transistor gm values.
Using small-signal analysis and superposition, we derive expressions for common-mode gain and differential gain and show how non-ideal tail impedance leads to finite CMRR.

We also build intuition for how device layout practices, matching strategies and current-source design improve CMRR in real analog ICs.

Topics covered:
• Load resistor mismatch and its impact on output balance
• gm mismatch and current steering inequality
• Common-mode gain and why it becomes non-zero
• Differential gain review
• CMRR definition and practical interpretation
• Role of tail-source impedance (RSS)
• How non-ideal current sources degrade CMRR
• How to improve CMRR in real designs (matching, cascoding, layout symmetry)

This lecture is ideal for:
• Students learning precision analog design
• Engineers preparing for mixed-signal interviews
• Anyone wanting deeper intuition behind differential amplifier robustness

By the end, you will understand exactly how mismatch creates common-mode errors and how CMRR quantifies the quality of a differential stage.