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ECE 242 Solid-State Devices & Circuits 15. Current Sources Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois [email protected] ECE 342 – Jose Schutt-Aine 1 Amplifier with Diode-Connected Load Amplifier is single stage M1 driving a load impedance which is that looking into the source of M2 ECE 342 – Jose Schutt-Aine 2 Output Impedance To calculate output impedance, connect voltage supply at source of M2 and measure current vs 2 is 2 g m 2vs 2 g mb 2vs 2 rds 2 ECE 342 – Jose Schutt-Aine 3 Output Impedance vs 2 1 Rs 2 is 2 g m 2 g mb 2 g ds 2 M1 sees this load in parallel with its own output impedance,rds1 Rout 1 rds1 || RS 2 g m 2 g mb 2 g ds 2 g ds1 ECE 342 – Jose Schutt-Aine 4 Incremental model The source to-body voltage changes in M2 use transconductance term gmb2 AMB g m1Rout g m1 g m 2 g mb 2 g ds 2 g ds1 ECE 342 – Jose Schutt-Aine 5 Gain – Diode-Connected Amp AMB g m1 gm2 gm 2nCox (W / L) I D AMB 2nCox (W1 / L1 ) I D 1/ 2 (W1 / L1 ) 2nCox (W2 / L2 ) I D (W2 / L2 ) The magnitude of the voltage gain varies as the square root of the aspect ratio ECE 342 – Jose Schutt-Aine 6 pMOS Diode-Connected Amp • Analog Design Requirements – Load device can be replaced by pMOS transistor – Eliminates body effect of the load device – Increases the resistance nCoxn W1 / L1 pCoxp W2 / L2 1/ 2 AMB 3 W1 / L1 W2 / L2 ECE 342 – Jose Schutt-Aine 1/ 2 7 Integrated Circuits • IC Requirements – Biasing of ICs is based on the use of constant current sources – Use current mirrors – Source circuits are used as loads ECE 342 – Jose Schutt-Aine 8 Integrated Circuits • Analog Design Requirements – Analog ICs may need resistors and capacitors for the design of amplifiers – Resistors and capacitors occupy the space of tens or hundreds of MOS devices – It is important to minimize their use ECE 342 – Jose Schutt-Aine 9 Transistor Biasing ECE 342 – Jose Schutt-Aine 10 Transistor Biasing ECE 342 – Jose Schutt-Aine 11 Current Mirrors A current mirror will reproduce a reference current to the output while allowing the output voltage to assume any value within a specified range. Io=KIin where K is a factor that can be less than or equal or greater than 1 ECE 342 – Jose Schutt-Aine 12 MOS Current Mirror 1 ' W 2 I D1 kn VGS VTn 2 L 1 I D1 I REF VDD VGS R 1 ' W 2 I o I D 2 kn VGS VTn 2 L 2 R is usually external to IC ECE 342 – Jose Schutt-Aine 13 MOS Current Mirror Assuming that the transistors are using the same process Io I REF W / L 2 W2 L1 W / L 1 W1L2 • Can be limited by – Channel length modulation (l) – Threshold voltage mismatch – Imperfect geometrical matching ECE 342 – Jose Schutt-Aine 14 MOS Current Mirror Io I REF • W2 L1 1 l VDS 2 VDSP 2 W1L2 1 l VDS 1 VDSP1 Some Properties 1. MOS current mirrors draw zero control currentbetter than BJT’s 2. Matching of threshold voltages harder than in BJT’s ECE 342 – Jose Schutt-Aine 15 Example A matched pair of MOSFETs are used in a current mirror witl l = 0.032 V-1, Cox=70 A/V2, W/2L =10, and VT = 0.9 V. Find the value of R to create an input current of 100 A. Calculate the output current when Vo = 3 V. Use drain current equation in active region to calculate I D1 CoxW 2L VGS VT 1 lVDS1 2 I D1 100 700VGS 0.9 1 0.032*VGS 2 We can now solve for the value of VGS ECE 342 – Jose Schutt-Aine 16 Example MOS Current Mirror ECE 342 – Jose Schutt-Aine 17 Example VGS = 1.272 V The resistance needed is: 5 VDS 1 5 1.272 R 37.2 k I D1 0.1 The output current is calculated from: I D2 I D2 CoxW VGS VT 1 lVDS1 2 2L 2 7001.272 0.9 1 0.032 3 106 A I D 2 106 A ECE 342 – Jose Schutt-Aine 18 BJT Current Mirror • Characteristics – Base current is not zero – Depends on relative areas of emitter-base junction – Want Io=IREF Io I REF I s 2 Area of EBJ of Q2 I s1 Area of EBJ of Q1 ECE 342 – Jose Schutt-Aine 19 Multiple Output Current Mirror ECE 342 – Jose Schutt-Aine 20