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CURRENT SOURCES, CURRENT SINKS AND CURRENT MIRRORS

CURRENT SOURCES, CURRENT SINKS AND CURRENT MIRRORS. CURRENT SINKS AND SOURCES Characterization of Current Sinks and Sources: Minimum voltage (V MIN ) across sink or source for which the current is no longer constant.

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CURRENT SOURCES, CURRENT SINKS AND CURRENT MIRRORS

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  1. CURRENT SOURCES, CURRENT SINKS AND CURRENT MIRRORS

  2. CURRENT SINKS AND SOURCES • Characterization of Current Sinks and Sources: • Minimum voltage (VMIN) across sink or source for which the current is no longer constant. • Output resistance which is a measure of the flatness of the current sink or source.

  3. Increasing the output impedance ROUT of a Current Source

  4. Normally gm 10gmbs  100gds giving us

  5. CURRENT MIRRORS AND CURRENT AMPLIFIERS

  6. Sources of Errors

  7. If the devices are matched If VDS1 = VDS2 then

  8. Matching Accuracy of Current Mirrors

  9. Layout Technique to Remove Layout Error

  10. WILSON CURRENT MIRROR

  11. This modification to the Simple Current mirror does ensure an increased output impedance, but does not ensure VDS1 = VDS2. Vmin obtained in this configuration is the sum of the voltage, VDS1(sat) and VDS3(sat), the minimum drain to source voltage required to keep M3 and M1 in saturation. Defining VON = VGS – VTH, and assuming that all transistors have same W/L ratio, we have VDS1 = VGS1 = VON + VTH and VDS3(min) = VON and Vmin = 2VON + VTH.

  12. MODIFIED WILSON CURRENT MIRROR

  13. CASCODE CURRENT MIRROR

  14. REGULATED CASCODE CURRENT MIRROR If all transistors have the same size, VGS2 = VDS1 = VGS = VDS4, giving us a very good match in mirroring.

  15. CURRENT MIRROR WITH LOW Vmin and GOOD MIRRORING PROPERTY We see that in the Cascode Current Mirror, the voltage VGG2 was obtained as 2VGS and hence we had a Vmin of 2VGS – VTH = 2VON +VTH. However if we can obtain VGG2 = 2VON + VTH, Vmin can be reduced to 2VON.

  16. WIDE RANGE CASCODE CURRENT MIRROR

  17. COMPARISON OF VARIOUS CURRENT MIRROR ARCHITECTURE

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