BJT Common-Base Amplifier (3): Derivation Explained

BJT Common-Base Amplifier (3): Derivation Explained

by Vincent Chang

Formal & Physical Sciences Engineering & Technology TJFElectronics engineeringTJFCElectronics: circuits and components
🎙 Vincent Chang 👥 2K 📅 December 4, 2022 ⏱ 11 min 👁 253 📄 tutorial 🧭 2026-08-17
Available in: English (current) Français

Keywords

common-baseBJTsmall-signal modelvoltage gaininput resistance

Summary

This lecture concludes the series on the BJT common-base amplifier by deriving its small-signal parameters. The instructor, Vincent Chang, begins by reviewing the T-model equivalent circuit from previous lectures. He then derives the input resistance as the parallel combination of the emitter resistance (re) and the external resistor (RE). The voltage gain is derived step-by-step, showing that it equals gm times the parallel combination of RC and RL, multiplied by a voltage divider factor involving the input resistance and source resistance. The output resistance is shown to be simply RC, with a detailed explanation of why the dependent current source gmVpi becomes an open circuit when the input is shorted. The lecture emphasizes analysis by inspection, encouraging students to derive results directly from the circuit without drawing the small-signal model. A practical example is provided: an integrated circuit common-base amplifier biased with a 0.25 mA current source, yielding a voltage gain of 15. The lecture concludes by summarizing the three BJT amplifier configurations (common-emitter, emitter follower, common-base) and previewing MOS transistor amplifiers.

172 words

Critical Evaluation

Value of the Information & Strength of the Argument

The video provides a clear and logical derivation of the common-base amplifier parameters, building on previous lectures. The instructor emphasizes intuitive understanding through ‘analysis by inspection’ and reinforces the concepts with a worked example. The argumentation is solid, with each step justified by circuit theory. The practical example helps solidify the theory. However, the presentation is somewhat informal, with occasional verbal slips and a few moments of confusion (e.g., when the instructor accidentally touches buttons). The value lies in its pedagogical approach, making complex derivations accessible.

Scientific Rigor, Source Quality, Title Accuracy

The scientific rigor is high: the derivations follow standard small-signal analysis techniques (T-model, hybrid-pi) and are consistent with textbook treatments. The instructor demonstrates deep expertise. However, no external sources are cited within the video or in the description, which limits the ability to verify claims independently. The title accurately reflects the content, and the video fulfills its promise of explaining the derivation. The description provides background on the instructor’s credentials, adding credibility.

173 words

Title / Content Match

The title accurately reflects the content: the video focuses on deriving the key parameters (input resistance, voltage gain, output resistance) of the common-base amplifier.

Quality & Reliability

8/10

The content is a clear, step-by-step derivation of the common-base amplifier parameters, based on established small-signal models. The instructor demonstrates deep expertise and provides a practical example. However, the video lacks explicit citations to external sources, and the presentation is somewhat informal with occasional verbal slips.

Key Moments

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

Contribution & Novelties

The video provides a clear, step-by-step derivation of the common-base amplifier parameters, emphasizing intuitive analysis. It bridges the gap between theoretical small-signal models and practical circuit analysis. The worked example demonstrates application in integrated circuits.

Pour aller plus loin :

  • Hybrid-pi model — The small-signal model used in the video; essential for understanding BJT amplifiers.
  • Common base — Overview of the common-base amplifier configuration, including characteristics and applications.
  • Bipolar junction transistor — Background on BJT operation and models.

78 words

Radar Profile

The radar profile shows high scores in technical level and information quality, indicating a technically dense and accurate tutorial. The quantity of information is moderate, as the video focuses on a specific derivation. Overall, the video is a reliable educational resource for electronics students.

Reliability 8/10