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A Low Voltage CMOS Bandgap Reference with Current Feedback

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A Low Voltage CMOS Bandgap Reference with Current Feedback

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    1. A Low Voltage CMOS Bandgap Reference with Current Feedback Tao Li Bhaskar Mitra Kabir Udeshi

    2. Operation & Utility Generates a DC Voltage independent of supply, temperature and process. Needed to bias circuits like a differential pair and ADC.

    3. Principle of Operation Base Emitter voltage of a BJT changes inversely with Temperature (CTAT) Add this voltage to a voltage proportional to temperature. (PTAT).

    4. Block Diagram Supply Independent Current Source Feedback to maintain same nodal voltage Startup Circuit Measure and add VBE and ?VBE

    5. Implementation Amplifier adjusts gate voltage to ensure constant current Iss independent of supply voltage. Ratio of resistors R1 and R2 determine multiplication constant.

    6. Amplifier Simple two stage opamp design Input Common mode ~0.7V so PMOS input stage used Output drives PMOS current sources so a second common source stage needed at output Current mirror input is driven by the amplifier output so there is some parasitic feedback which reduces gain

    7. Startup Circuit Circuit has two stable states, so we need a startup Initially M7 is off, M4 and M5, and M6 turn on This pulls the gate of the Current sources down, so they begin conducting When circuit starts up M7 turns on and M5 and M6 turn off M4 and M5 are weak devices so that they do not consume to much current and circuits starts up properly

    8. Results-Temperature Varaition Bandgap Voltage of 1.1570 is obtained Optimized to have minimun slope at 27C Very stable with temperature (0.5% over temperature) Differs from theoretical value(1.26V) due to nature of transistors and temperature dependence of resistors

    9. Results-Supply Rejection Shows excellent Vdd Rejection (50dB) Two Most of the rejection is in the range 2.25V-2.40V Better rejection (~70dB) in 2.40-2.75V

    10. Layout

    11. References Behzad Razavi, Design of analog CMOS Integrated circuits, Chapter 11, pgs 377-400 T. Brooks and A.C Westwisk, A low power differential CMOS bandgap Reference, ISSCC Dig. Of Tech. Papers, pp 248-249, Feb 1994 K. Lasanen, et. Al, Design of a 1-V low power bandgap reference based on Resistive Subdivision, Proceedings of the 45th IEEE Midwest Symposium on Circuits and Systems, Tulsa, Oklahoma, USA, August 2002

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