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Lecture 12 ANNOUNCEMENTS • Review session: 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 9.2 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. • 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 temperatureindependent current exist, but require many 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 VT ln VX 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 • Although it provides a path for base currents to flow, this biasing approach is no better than a resistive voltage 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 to placing n unit-sized transistors in 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. I REF VX 3I S exp VT I copy I copy EE105 Fall 2007 VX I S exp VT 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 = 0.05mA and Icopy2 = 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 mirroring error by a factor of . I REF I B , F I C , REF IC ,F I E ,F I C , REF I B,F I copy n I copy n I copy I copy 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 copy2 4 I B,F I copy1 I C , REF 15I copy1 2 I REF 15 4 2 I C , REF I copy1 I copy 2 10I 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 currentsource load for an NPN BJT amplifier stage. EE105 Fall 2007 Lecture 12, Slide 17 Prof. Liu, UC Berkeley Generation of IREF for a PNP-BJT Current Mirror • Neglecting base currents, the currents flowing through QM and QREF2 are the 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, due to IS mismatch. EE105 Fall 2007 Lecture 12, Slide 19 Prof. Liu, UC Berkeley