Week_15 - Linear settling time = 1 1 = 3dB ta Folded cascade OPAMP ta = g m1 Current Mirror OPAMP ta = kg m1 CL CL Fully differential OPAMPs

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Linear settling time ta dB βω ω τ 1 3 1 = = Folded cascade OPAMP L m ta C g 1 = Current Mirror OPAMP L m ta C kg 1 = Fully differential OPAMPs differential-in differential-out Use symmetry to reject substrate noise (unwanted coupling) because transistors are non- linear (junction capacitances vs. voltage) Noise injection depends on voltage levels You cannot reject noise 100% + _ in V . . noise Noise injection depends on input signal
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Drawback of fully differential circuits: Æ *CMFB (common-mode feedback circuitry is needed) There is usually feedback on the OPAMP as well as internal CMFB Æ CMFB loop gain is typically not large Æ Just enough to prevent drifts CMFB design Æ Speed must be high in the order of ta ω Æ Continuous-time circuits usually don’t work well for large input signal Æ Switched-capacitor CMFB’s are used: Source of noise Add to the load capacitance *Slow-rate usually smaller than single-ended OPAMPS
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This note was uploaded on 02/19/2012 for the course ECE 455 taught by Professor Mohammadi during the Fall '10 term at Purdue University-West Lafayette.

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Week_15 - Linear settling time = 1 1 = 3dB ta Folded cascade OPAMP ta = g m1 Current Mirror OPAMP ta = kg m1 CL CL Fully differential OPAMPs

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