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CURRENT SOURCES, CURRENT SINKS AND CURRENT MIRRORS CURRENT SINKS AND SOURCES Characterization of Current Sinks and Sources: 1. Minimum voltage (VMIN) across sink or source for which the current is no longer constant. 2. Output resistance which is a measure of the flatness of the current sink or source. Increasing the output impedance ROUT of a Current Source vout rout rds2 r1 gm2rds2 gmbs 2rds2 i out Normally gm 10gmbs 100gds giving us rout rgm2rds2 CURRENT MIRRORS AND CURRENT AMPLIFIERS Sources of Errors If the devices are matched io W2 L1 1 VDS 2 i i W1L2 1 VDS 1 If VDS1 = VDS2 then W2 L1 io ii W L 1 2 Matching Accuracy of Current Mirrors Layout Technique to Remove Layout Error vout iout gm vgs gmb vsb rds iout r vout rout rds r gm rds gmb rds 1 gm r rds iout WILSON CURRENT MIRROR rout rds2 gm1 rds1 gm3 rds3 rds1 gm3 rds3 if gm1 gm2 rds2 gm2 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. MODIFIED WILSON CURRENT MIRROR CASCODE CURRENT MIRROR REGULATED CASCODE CURRENT MIRROR If all transistors have the same size, VGS2 = VDS1 = VGS = VDS4, giving us a very good match in mirroring. 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. WIDE RANGE CASCODE CURRENT MIRROR COMPARISON OF VARIOUS CURRENT MIRROR ARCHITECTURE Current Mirror Accuracy Output impedance Vmin Simple Poor rds VON Wilson Poor 2 gm rds Improved Excellent Cascode Excellent Regulated Cascode Cascode Good Excellent 3 gm2 rds 2 gm rds 2 gm rds 2 gm rds 2VON + VTH 2VON + VTH 2VON + VTH 2VON + VTH 2VON