MOS Rail-to-Rail Op-Amp (2): Input Common-Mode Range

MOS Rail-to-Rail Op-Amp (2): Input Common-Mode Range

by Vincent Chang

Formal & Physical Sciences Engineering & Technology TJFElectronics engineeringTJFCElectronics: circuits and components
🎙 Vincent Chang 👥 2K 📅 August 1, 2021 ⏱ 15 min 👁 692 📄 tutorial 🧭 2026-08-17
Available in: English (current) Français

Keywords

input common-mode rangerail-to-railCMOS op-ampdifferential pairtransconductance

Summary

This lecture continues the analysis of a CMOS rail-to-rail operational amplifier, focusing on determining its input common-mode range (ICMR). The instructor, Vincent Chang, begins by reviewing the circuit topology, which consists of an NMOS input differential stage on the left and a PMOS input differential stage on the right. He sets up the problem by assuming all transistors have equal overdrive voltage (0.3 V) and threshold voltage (0.7 V), and that the current source transistors are in saturation. He then calculates the common-mode range for each input stage separately. For the NMOS stage, the range is from -1.2 V to +2.9 V; for the PMOS stage, it is from -2.9 V to +1.2 V. The overall ICMR is the union of these ranges, from -2.9 V to +2.9 V. However, the region where both stages operate simultaneously is only from -1.2 V to +1.2 V. The instructor emphasizes that the small-signal gain expression derived in the previous lecture, which assumes both stages are active, is only valid within this overlapping region. He clarifies the relationship between the DC common-mode input voltage and the small-signal transconductance, addressing a common confusion among engineers. The lecture concludes by summarizing the three operating zones and their implications for the amplifier’s gain.

207 words

Critical Evaluation

Value of the Information & Strength of the Argument

The lecture provides a clear, step-by-step derivation of the input common-mode range for a rail-to-rail op-amp, which is a fundamental concept in analog IC design. The instructor uses a concrete example with specific values, making the calculations easy to follow. He also connects the DC bias conditions to the small-signal gain, which is a valuable insight that helps bridge the gap between DC analysis and AC analysis. The argumentation is logical and builds on previous lectures, reinforcing the learning. However, the lecture is purely instructional and does not present new research or compare different design approaches. It is a tutorial that explains known principles, but it does so effectively.

Scientific Rigor, Source Quality, Title Accuracy

The lecture is scientifically rigorous in its methodology, following standard textbook approaches for analyzing differential amplifiers. The instructor clearly states his assumptions and derives the results from first principles. However, no external sources are cited, and the content is based on established knowledge in the field. The title accurately describes the content, which is focused on the input common-mode range. The instructor’s credentials lend credibility to the material, but the lack of citations means that viewers cannot easily verify the information or explore further references. Overall, the lecture is reliable for educational purposes, but it is not a research presentation.

224 words

Title / Content Match

The title accurately reflects the content, which focuses on the input common-mode range of a CMOS rail-to-rail op-amp.

Quality & Reliability

8/10

The lecture is based on established analog IC design principles, with clear assumptions and step-by-step calculations. The instructor is an experienced educator with 30 years in the field. However, no external sources are cited, and the content is presented as a tutorial without peer review.

Key Moments

Markers derived by PSI from the transcript: the creator did not define chapters.

Contribution & Novelties

This lecture provides a clear pedagogical explanation of the input common-mode range for a rail-to-rail CMOS op-amp, emphasizing the conditions under which both input stages are active and how this affects the small-signal gain. It bridges the gap between DC bias analysis and AC small-signal behavior, which is often a source of confusion for engineers. The lecture is part of a series that builds a strong foundation in analog IC design.

Pour aller plus loin :

  • Rail-to-rail operational amplifier — Overview of rail-to-rail op-amp design and challenges.
  • Common-mode rejection ratio — Related concept of common-mode behavior in differential amplifiers.
  • Operational amplifier — General background on op-amp operation and parameters.

109 words

Radar Profile

The radar profile shows high scores across all dimensions, indicating a well-rounded educational video. The technical depth is appropriate for an intermediate audience, and the information is presented clearly and reliably.

Reliability 8/10