Linearity performance comparison of cascode current source and single-device current source IDPs; analyses, simulations and measurements

Khayrollah Hadidi, Makoto Morimoto, Keita Futami, Takeshi Oue, Mitsuyoshi Ito, Masahiro Sasaki, Abdollah Khoei, Takashi Matsumoto

Research output: Contribution to journalArticle


The input differential pair (IDP) is usually a major source of nonlinear distortion in any op-amp. This is especially true if the input signal has a large common-mode component, as is the case when an op-amp functions as a unity-gain buffer or as part of a single-ended sample-hold (S/H) circuit. In this paper, we analyse the distortion of the commonly used cascode current source IDP structure and explain the sources of its nonlinear behaviour. Next, a special design technique is proposed which enhances the linearity of IDPs. The circuit uses a single device current source that has the same channel length while its width is double those of IDP devices. Theoretical analysis, as well as simulation and experimental results, is given to confirm the improved linearity of a unity gain buffer. Simulations predict improvements up to 20 dB. 15 dB total harmonic distortion (THD) reduction was also achieved for a 15 MHz input signal based on measurement of a test chip. The method is valuable as power supply voltages shrink, and the design offers extra voltage headroom at input.

Original languageEnglish
Pages (from-to)341-353
Number of pages13
JournalInternational Journal of Electronics
Issue number5
Publication statusPublished - 2003 May 1


ASJC Scopus subject areas

  • Electrical and Electronic Engineering

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