MOS Cascode Current Mirror (2): Output Resistance

MOS Cascode Current Mirror (2): Output Resistance

by Vincent Chang

Formal & Physical Sciences Engineering & Technology TJFElectronics engineeringTJFCElectronics: circuits and components
🎙 Vincent Chang 👥 2K 📅 November 11, 2020 ⏱ 13 min 👁 920 📄 tutorial 🧭 2026-08-17
Available in: English (current) Français

Keywords

cascodecurrent mirroroutput resistanceMOSFETsmall-signal analysis

Summary

This lecture video, part of a series on MOS cascode current mirrors, focuses on calculating the output resistance of the cascode configuration. The instructor, Vincent Chang, begins by revisiting a question from the previous video about whether the output resistance is simply the series combination of the two transistor output resistances. He explains that this is incorrect because the cascode is a three-terminal device, not a two-terminal one. He then demonstrates a method to simplify the circuit by replacing the MOS transistors with their small-signal models, specifically using the 1/gm resistance for the diode-connected transistor. He shows that the floating gate of the upper transistor results in no current through the 1/gm resistance, allowing it to be shorted. The analysis then reduces to a single transistor with a source resistance, for which he provides a formula: Rout = ro + (1 + mu) * Rs, where mu = gm * ro. He emphasizes the ‘reflection’ of the source resistance to the drain multiplied by (1 + mu), similar to the BJT case where the emitter resistance is reflected to the base multiplied by (1 + beta). He also explains why the simple series combination (ro + Rs) is wrong: because the source is not grounded, vgs is not zero, and the voltage-controlled current source has a multiplication effect. The video concludes by highlighting the high output resistance of the cascode mirror, making it a high-performance current source.

237 words

Critical Evaluation

Value of the Information & Strength of the Argument

The video provides valuable insights into the analysis of cascode current mirrors, offering a clear and intuitive method for calculating output resistance. The argumentation is solid, building logically from previous lessons and connecting MOS and BJT concepts. The instructor emphasizes understanding over rote memorization, encouraging students to use ‘analysis by inspection’ and to see the big picture. The explanation of why the simple series combination is wrong is particularly instructive, highlighting the importance of considering the voltage-controlled current source. The video effectively demonstrates the ‘reflection’ technique, which is a powerful tool for circuit analysis.

Scientific Rigor, Source Quality, Title Accuracy

The video is scientifically rigorous, with accurate technical content and a logical progression. The instructor has extensive experience and the teaching is clear. However, no external sources are cited, and the video relies solely on the instructor’s expertise. The title accurately reflects the content, focusing on the output resistance of the cascode current mirror. The video is part of a series, and it references previous lessons, which helps in building a coherent understanding. The production quality is basic, but the content is well-delivered.

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Title / Content Match

The title accurately reflects the content: the video focuses on deriving the output resistance of a MOS cascode current mirror, specifically the second part of a series.

Quality & Reliability

8/10

The video is a clear, well-structured tutorial by an experienced educator (30 years in semiconductor microelectronics). The reasoning is methodical, building on previous lessons and connecting MOS and BJT concepts. The technical content is accurate and the pedagogical approach is effective. However, the video lacks explicit citations to external sources, and the production quality is basic (screen recording with handwritten notes).

Key Moments

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

Contribution & Novelties

The video offers a clear and intuitive method for analyzing the output resistance of a cascode current mirror, emphasizing the ‘reflection’ technique and the importance of considering the voltage-controlled current source. It bridges the gap between MOS and BJT analyses, providing a unified perspective. The pedagogical approach encourages ‘analysis by inspection’ and understanding the underlying concepts.

Pour aller plus loin :

82 words

Radar Profile

The radar profile shows high scores in quality, technical level, and reliability, with a slightly lower score in quantity of information due to the focused scope. This indicates a technically deep and reliable tutorial, though it may not cover a broad range of topics.

Reliability 8/10