Lecture 12 OUTLINE. Cascode Stage (cont d) Current Mirrors Reading: Chapter 9.2. EE105 Fall 2007 Lecture 12, Slide 1 Prof.

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1 Lecture 12 ANNOUNCEMENTS Review session: 3 5PM 5PMFriday (10/5)in 306Soda (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 OUTLNE Cascode Stage (cont d) supplementary remarks Current Mirrors Reading: Chapter 9.2 EE105 Fall 2007 Lecture 12, Slide 1 Prof. Liu, UC Berkeley

2 Review: Cascode Stage R out The impedance seen looking into the collector can be boosted significantly by using a BJTfor emitter degeneration, with a relatively small reduction in headroom. R [1 + g ( r r )] r + r r R out out g m1 r m O1 O2 ( ) r O2 r π1 π1 O1 O2 π1 EE105 Fall 2007 Lecture 12, Slide 2 Prof. Liu, UC Berkeley

3 Another View of a Cascode Stage nstead of considering a cascode as Q 2 degenerating Q 1, we can also think of it as Q 1 stacked on top of Q 2 (current source) to boost Q 2 s output impedance. EE105 Fall 2007 Lecture 12, Slide 3 Prof. Liu, UC Berkeley

4 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, 1 depends on the temperature as well as the supply voltage, since V T and S depend on temperature. V 1 S e V BE / R + V 2 BE V CC R1 R2 T EE105 Fall 2007 Lecture 12, Slide 4 Prof. Liu, UC Berkeley

5 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

6 Current Mirror Circuitry Diode connected Q produces an output voltage V X that forces copy1 to be equal to, if Q 1 is identical to Q. Current mirror concept Generation of required V BE Current Mirror Circuitry copy 1 S,1 S, V X V T ln V ln copy 1 T S,1 S, EE105 Fall 2007 Lecture 12, Slide 6 Prof. Liu, UC Berkeley

7 Bad Current Mirror Example 1 f the collector and base of Q are not shorted together, there will not be a path for the base currents to flow, so that copy is zero. EE105 Fall 2007 Lecture 12, Slide 7 Prof. Liu, UC Berkeley

8 Bad Current Mirror Example 2 Although h 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

9 Multiple Copies of Multiple copies of can be generated at different locations by applying the current mirror concept to multiple transistors. copy, j S, j S, EE105 Fall 2007 Lecture 12, Slide 9 Prof. Liu, UC Berkeley

10 Current Scaling By scaling the emitter area of Q j by a factor of n with respect to the emitter area of Q, copy,j is scaled by a factor of n with respect to. This is equivalent to placing n unit sized transistors in parallel. n copy, j EE105 Fall 2007 Lecture 12, Slide 10 Prof. Liu, UC Berkeley

11 Example: Scaled Currents EE105 Fall 2007 Lecture 12, Slide 11 Prof. Liu, UC Berkeley

12 Fractional Scaling A fraction of can be created in Q 1 by scaling up the emitter area of Q. V exp V X 3 S T copy V exp V X S T 1 copy 3 EE105 Fall 2007 Lecture 12, Slide 12 Prof. Liu, UC Berkeley

13 Example: Different Mirroring Ratios Using the concept of current scaling and fractional scaling, copy1 0.05mA 05mA and copy2 05mA 0.5mA, derived from a single 0.2mA reference current source ( ). EE105 Fall 2007 Lecture 12, Slide 13 Prof. Liu, UC Berkeley

14 Effect of Base Currents copy C, + + nββ copy β C, copy n copy n n +11 β ( ) EE105 Fall 2007 Lecture 12, Slide 14 Prof. Liu, UC Berkeley

15 mproved Mirroring Accuracy Use Q F (rather than ) to supply the base currents of Q and Q 1, reduce the mirroring error by a factor of β. Q Q 1, g y β B, F + C, copy copy C, F E, F + nβ β C, copy n B, F copy β n copy n n β ( ) EE105 Fall 2007 Lecture 12, Slide 15 Prof. Liu, UC Berkeley

16 Different Mirroring Ratio Accuracy B, F + 4C, copy1 copy2 C, F β β B, F C, β 15 β copy β 2 copy 1 C, copy1 copy β 2 EE105 Fall 2007 Lecture 12, Slide 16 Prof. Liu, UC Berkeley

17 PNP Current Mirror A PNP BJT current mirror can be used as a current source load for an NPN BJT amplifier stage. EE105 Fall 2007 Lecture 12, Slide 17 Prof. Liu, UC Berkeley

18 Generation of for a PNP BJT Current Mirror Neglecting base currents, the currents flowing through Q M and Q 2 are the same. EE105 Fall 2007 Lecture 12, Slide 18 Prof. Liu, UC Berkeley

19 Current Mirror with Discrete BJTs f Q and Q 1 are discrete NPN BJTs, and copy1 can differ dramatically, due to S mismatch. EE105 Fall 2007 Lecture 12, Slide 19 Prof. Liu, UC Berkeley