Lecture 26 ANNOUNCEMENTS Homework 12 due Thursday 12 6 OUTLINE Self biased current sources BJT MOSFET Guest lecturer Prof Niknejad EE105 Fall 2007 Lecture 26 Slide 1 Prof Liu UC Berkeley Review Current Mirrors The current mirrors we discussed require a golden current source IREF to copy EE105 Fall 2007 Lecture 26 Slide 2 Prof Liu UC Berkeley Review Current Mirrors cont d In lab 6 and lab 10 you used a resistor as your current source Q What are some problems associated with this method EE105 Fall 2007 Lecture 26 Slide 3 Prof Liu UC Berkeley Review Current Mirrors cont d A Variations in VCC and temperature cause significant variations in IREF Consider the following analysis ignoring base currents and the Early effect Thus a 10 change in VCC results in a 11 6 change in IREF EE105 Fall 2007 Lecture 26 Slide 4 Prof Liu UC Berkeley Base emitter Reference Rather than having a source dependent on VCC why not use some other reference For example a VBE referenced current source Ignoring base currents we have Q Why is this less supply dependent EE105 Fall 2007 Lecture 26 Slide 5 Prof Liu UC Berkeley Base emitter Reference cont d A Although IIN varies almost directly with VCC VBE1 won t vary nearly as much since the device is exponential Since IOUT depends only on VBE1 the output won t vary much with VCC Example Thus a 10 change in VCC results in a 0 7 change in IOUT EE105 Fall 2007 Lecture 26 Slide 6 Prof Liu UC Berkeley Self Biasing We can do better than the VBE referenced source using feedback What if our source had a current mirror attached that fed back the output current to act as the input current EE105 Fall 2007 Lecture 26 Slide 7 Prof Liu UC Berkeley Self Biasing cont d Here we ve attached a pnp current mirror to force IOUT and IIN to match There are two stable operating points IIN IOUT 0 A Desired operating point EE105 Fall 2007 Lecture 26 Slide 8 Prof Liu UC Berkeley Start up Circuit Need a way to start up the circuit like a car starter starts up your car Requirements Must keep the circuit out of the undesired operating point Must not interfere with the circuit once it reaches the desired operating point EE105 Fall 2007 Lecture 26 Slide 9 Prof Liu UC Berkeley Start up Circuit cont d EE105 Fall 2007 Lecture 26 Slide 10 Prof Liu UC Berkeley Start up Circuit cont d Let s ensure this works Assume IIN IOUT 0 This means approximately that VBE1 VBE2 0 However note that the left side of D1 is four diode drops from ground meaning D1 is on This drops some voltage across Rx forcing current to flow into T1 and T2 starting up the circuit After the circuit is at the desired operating point turn D1 off by ensuring RxIIN the drop across Rx is sufficiently large EE105 Fall 2007 Lecture 26 Slide 11 Prof Liu UC Berkeley MOSFET Current Source We can build an analogous circuit from MOSFETs as well Let s start with a VTH referenced current source If we make Vov1 small by sizing up T1 or using small currents IOUT is controlled primarily by VTH and R2 EE105 Fall 2007 Lecture 26 Slide 12 Prof Liu UC Berkeley MOSFET Current Source cont d Let s add the current mirror feedback EE105 Fall 2007 Lecture 26 Slide 13 Prof Liu UC Berkeley MOSFET Current Source cont d Finally the start up circuitry It s more typical to use more MOSFETs in MOS technologies rather than diodes EE105 Fall 2007 Lecture 26 Slide 14 Prof Liu UC Berkeley MOSFET Current Source cont d Assume IIN IOUT 0 This means VGS1 0 meaning T8 is in triode This turns on T9 and forces current to flow into T4 and T5 Once in steady state we can size T7 to ensure that T9 turns off T7 and T8 don t directly affect the circuit themselves so the start up circuit has done its job EE105 Fall 2007 Lecture 26 Slide 15 Prof Liu UC Berkeley References Material and figures largely from Analysis and Design of Analog Integrated Circuits Fourth Edition by Gray Hurst Lewis and Meyer EE105 Fall 2007 Lecture 26 Slide 16 Prof Liu UC Berkeley Guest Lecturer Prof Ali Niknejad Faculty director of the Berkeley Wireless Research Center BWRC Primary research interests include analog integrated circuits mm wave CMOS RF and microwave circuits device modeling BSIM electromagnetics ASITIC communication systems and scientific computing EE105 Fall 2007 Lecture 26 Slide 17 Prof Liu UC Berkeley
View Full Document
Unlocking...