Lecture 26OUTLINE•Self-biased current sourcesANNOUNCEMENTS• Homework 12 due Thursday, 12/6EE105 Fall 2007 Lecture 26, Slide 1 Prof. Liu, UC Berkeley•Self-biased current sources– BJT– MOSFET• Guest lecturer Prof. NiknejadReview: 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 BerkeleyReview: 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 BerkeleyReview: Current Mirrors (cont’d)• A: Variations in VCCand temperature cause significant variations in IREF. Consider the following analysis (ignoring base currents and the Early effect):EE105 Fall 2007 Lecture 26, Slide 4 Prof. Liu, UC Berkeley• Thus, a 10 % change in VCCresults in a 11.6 % change in IREF.Base-emitter Reference• Rather than having a source dependent on VCC, why not use some other reference?• For example, a VBEreferenced current source.•Ignoring base currents, we have:EE105 Fall 2007 Lecture 26, Slide 5 Prof. Liu, UC Berkeley•Ignoring base currents, we have:• Q: Why is this less supply dependent?Base-emitter Reference (cont’d)• A: Although IINvaries almost directly with VCC, VBE1won’t vary nearly as much, since the device is exponential. Since IOUTdepends only on VBE1, the output won’t vary much with VCC.• Example:EE105 Fall 2007 Lecture 26, Slide 6 Prof. Liu, UC Berkeley• Thus, a 10 % change in VCCresults in a 0.7 % change in IOUT.Self Biasing• We can do better than the VBEreferenced 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 BerkeleySelf Biasing (cont’d)• Here, we’ve attached a pnp current mirror to force IOUTand IINto match.• There are two stable operating points:– IIN= IOUT= 0 A– Desired operating pointEE105 Fall 2007 Lecture 26, Slide 8 Prof. Liu, UC BerkeleyStart-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 pointEE105 Fall 2007 Lecture 26, Slide 9 Prof. Liu, UC Berkeleydesired operating pointStart-up Circuit (cont’d)EE105 Fall 2007 Lecture 26, Slide 10 Prof. Liu, UC BerkeleyStart-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 D1is four diode drops from ground, meaning D1is on. This drops some EE105 Fall 2007 Lecture 26, Slide 11 Prof. Liu, UC Berkeleymeaning D1is on. This drops some voltage across Rx, forcing current to flow into T1and T2, starting up the circuit.– After the circuit is at the desired operating point, turn D1off by ensuring RxIIN(the drop across Rx) is sufficiently large.MOSFET Current Source• We can build an analogous circuit from MOSFETs as well. Let’s start with a VTHreferenced current source.EE105 Fall 2007 Lecture 26, Slide 12 Prof. Liu, UC Berkeley• If we make Vov1small (by sizing up T1or using small currents), IOUTis controlled primarily by VTHand R2.MOSFET Current Source (cont’d)• Let’s add the current mirror feedback.EE105 Fall 2007 Lecture 26, Slide 13 Prof. Liu, UC BerkeleyMOSFET 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 BerkeleyMOSFET Current Source (cont’d)• Assume IIN= IOUT= 0. This means VGS1= 0, meaning T8is in triode. This turns on T9and forces current to flow into T4and T5.•Once in steady state, we can EE105 Fall 2007 Lecture 26, Slide 15 Prof. Liu, UC Berkeley•Once in steady state, we can size T7to ensure that T9turns off. T7and T8don’t directly affect the circuit themselves, so the start-up circuit has done its job.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 BerkeleyGuest 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 EE105 Fall 2007 Lecture 26, Slide 17 Prof. Liu, UC Berkeleyelectromagnetics (ASITIC), communication systems, and scientific
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