ECE 342 – Jose Schutt-Aine 1 ECE 242 Solid-State Devices & Circuits 15. Current Sources Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois

ECE 342 – Jose Schutt-Aine 2 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 3 Output Impedance To calculate output impedance, connect voltage supply at source of M2 and measure current

ECE 342 – Jose Schutt-Aine 4 Output Impedance M1 sees this load in parallel with its own output impedance,r ds1

ECE 342 – Jose Schutt-Aine 5 Incremental model The source to-body voltage changes in M2  use transconductance term g mb2

ECE 342 – Jose Schutt-Aine 6 Gain – Diode-Connected Amp The magnitude of the voltage gain varies as the square root of the aspect ratio

ECE 342 – Jose Schutt-Aine 7 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

ECE 342 – Jose Schutt-Aine 8 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 9 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 Integrated Circuits

ECE 342 – Jose Schutt-Aine 10 Transistor Biasing

ECE 342 – Jose Schutt-Aine 11 Transistor Biasing

ECE 342 – Jose Schutt-Aine 12 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. I o =KI in where K is a factor that can be less than or equal or greater than 1

ECE 342 – Jose Schutt-Aine 13 MOS Current Mirror R is usually external to IC

ECE 342 – Jose Schutt-Aine 14 MOS Current Mirror Assuming that the transistors are using the same process Can be limited by –Channel length modulation ( ) –Threshold voltage mismatch –Imperfect geometrical matching

ECE 342 – Jose Schutt-Aine 15 MOS Current Mirror Some Properties 1.MOS current mirrors draw zero control current  better than BJT’s 2.Matching of threshold voltages harder than in BJT’s

ECE 342 – Jose Schutt-Aine 16 Example A matched pair of MOSFETs are used in a current mirror witl  = V -1,  C ox =70  A/V 2, W/2L =10, and V T = 0.9 V. Find the value of R to create an input current of 100  A. Calculate the output current when V o = 3 V. Use drain current equation in active region to calculate We can now solve for the value of V GS

ECE 342 – Jose Schutt-Aine 17 Example MOS Current Mirror

ECE 342 – Jose Schutt-Aine 18 Example The resistance needed is: The output current is calculated from: V GS = V

ECE 342 – Jose Schutt-Aine 19 BJT Current Mirror Characteristics –Base current is not zero –Depends on relative areas of emitter-base junction –Want I o =I REF

ECE 342 – Jose Schutt-Aine 20 Multiple Output Current Mirror