Lecture 12 ANNOUNCEMENTS Review session 3 5PM 3 5PM Friday 10 5 in 306 Soda HP Auditorium Midterm 1 Thursday 10 11 3 30PM 5 00PM location 106 Stanley Hall Students with last names starting with A L 306 Soda Hall Students with last names starting with M Z OUTLINE Cascode Stage cont d supplementary remarks Current Mirrors Reading Chapter 9 9 2 2 EE105 Fall 2007 Lecture 12 Slide 1 Prof Liu UC Berkeley Review Cascode Stage Rout The impedance seen looking into the collector can be boosted significantly by using a BJT for emitter degeneration with a relatively small reduction in headroom Rout 1 g m rO 2 r 1 rO1 rO 2 r 1 Rout g m1rO1 rO 2 r 1 EE105 Fall 2007 Lecture 12 Slide 2 Prof Liu UC Berkeley Another View of a Cascode Stage Instead of considering a cascode as Q2 degenerating Q1 we can also think of it as Q1 stacked on top of Q2 current source to boost Q2 s output impedance EE105 Fall 2007 Lecture 12 Slide 3 Prof Liu UC Berkeley Temperature and Supply Voltage Dependence of Bias Current Circuits should be designed to operate properly over a range of supply voltages and temperatures temperatures For the biasing scheme shown below I1 depends on the temperature as well as the supply voltage since VT and IS depend on temperature I1 I S e VBE VT VBE EE105 Fall 2007 R2 VCC R1 R2 Lecture 12 Slide 4 Prof Liu UC Berkeley Concept of a Current Mirror Circuit designs to provide a supply and temperature independent p current exist but require q manyy transistors to implement golden current source A current mirror is used to replicate the current from a golden current source to other locations EE105 Fall 2007 Lecture 12 Slide 5 Prof Liu UC Berkeley Current Mirror Circuitry Diode connected QREF produces an output voltage VX that forces Icopy1 to be equal to IREF if Q1 is identical to QREF Current mirror concept Generation of required VBE I copy1 EE105 Fall 2007 I S 1 I S REF Current Mirror Circuitry I copy1 I REF VX VT ln VT ln I I S 1 S REF I REF Lecture 12 Slide 6 Prof Liu UC Berkeley Bad Current Mirror Example 1 If the collector and base of QREF are not shorted together there will not be a path for the base currents to flow so that Icopy is zero EE105 Fall 2007 Lecture 12 Slide 7 Prof Liu UC Berkeley Bad Current Mirror Example 2 Alth Although h it provides id a path th for f base b currents t tto flow fl this biasing approach is no better than a resistive voltage divider divider EE105 Fall 2007 Lecture 12 Slide 8 Prof Liu UC Berkeley Multiple Copies of IREF Multiple copies of IREF can be generated at different locations by applying the current mirror concept to multiple transistors I copy j EE105 Fall 2007 IS j I S REF Lecture 12 Slide 9 I REF Prof Liu UC Berkeley Current Scaling By scaling the emitter area of Qj by a factor of n with respect to the emitter area of QREF Icopy j is scaled by a factor of n with respect to IREF This is equivalent q to p placingg n unit sized transistors in p parallel I copy j nI REF EE105 Fall 2007 Lecture 12 Slide 10 Prof Liu UC Berkeley Example Scaled Currents EE105 Fall 2007 Lecture 12 Slide 11 Prof Liu UC Berkeley Fractional Scaling A fraction of IREF can be created in Q1 by scaling up the emitter area of QREF V I REF 3I S exp X VT V I copy I S exp X VT I copy EE105 Fall 2007 1 I REF 3 Lecture 12 Slide 12 Prof Liu UC Berkeley Example Different Mirroring Ratios Using the concept of current scaling and fractional scaling Icopy1 0 scaling 0 05mA 05mA and Icopy2 0 5mA 0 5mA derived from a single 0 2mA reference current source IREF EE105 Fall 2007 Lecture 12 Slide 13 Prof Liu UC Berkeley Effect of Base Currents I REF I C REF I C REF I copy n I copy I copy n I copy nI REF 1 1 n 1 EE105 Fall 2007 Lecture 12 Slide 14 Prof Liu UC Berkeley Improved Mirroring Accuracy Use QF rather than IREF to supply the base currents of QREF and Q1 reduce the mirroringg error byy a factor of I REF I B F I C REF I C F I E F I C REF I B F I copy n I copy n I copy I copy py 1 2 1 n I copy nI REF 1 1 2 n 1 EE105 Fall 2007 Lecture 12 Slide 15 Prof Liu UC Berkeley Different Mirroring Ratio Accuracy I REF I B F 4 I C REF IC F I copy1 I copy 2 4 I B F I copy1 I C REF 15 I copy1 2 I REF 15 4 2 I C REF I copy1 I copy 2 10 I REF 15 4 2 EE105 Fall 2007 Lecture 12 Slide 16 Prof Liu UC Berkeley PNP Current Mirror A PNP BJT current mirror can be used as a current source load for an NPN BJT amplifier stage stage EE105 Fall 2007 Lecture 12 Slide 17 Prof Liu UC Berkeley Generation of IREF for a PNP BJT Current Mirror PNP BJT Neglecting base currents the currents flowing through QM and QREF2 are the same same EE105 Fall 2007 Lecture 12 Slide 18 Prof Liu UC Berkeley Current Mirror with Discrete BJTs If QREF and Q1 are discrete NPN BJTs IREF and Icopy1 can differ dramatically dramatically due to IS mismatch mismatch EE105 Fall 2007 Lecture 12 Slide 19 Prof Liu UC Berkeley
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