EE105 Fall 2007Lecture 12, Slide 1Prof. Liu, UC Berkeley Lecture 12 OUTLINE Cascode Stage (cont’d) – supplementary remarks Current Mirrors Reading: Chapter.

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EE105 Fall 2007Lecture 12, Slide 1Prof. Liu, UC Berkeley Lecture 12 OUTLINE Cascode Stage (cont’d) – supplementary remarks Current Mirrors Reading: Chapter 9.2 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

EE105 Fall 2007Lecture 12, Slide 2Prof. Liu, UC Berkeley Review: Cascode Stage R out 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.

EE105 Fall 2007Lecture 12, Slide 3Prof. Liu, UC Berkeley Another View of a Cascode Stage Instead 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 2007Lecture 12, Slide 4Prof. 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, I 1 depends on the temperature as well as the supply voltage, since V T and I S depend on temperature.

EE105 Fall 2007Lecture 12, Slide 5Prof. Liu, UC Berkeley Concept of a Current Mirror Circuit designs to provide a supply- and temperature- independent 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 2007Lecture 12, Slide 6Prof. Liu, UC Berkeley Current Mirror Circuitry Diode-connected Q REF produces an output voltage V X that forces I copy1 to be equal to I REF, if Q 1 is identical to Q REF. Current mirror conceptGeneration of required V BE Current Mirror Circuitry

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

EE105 Fall 2007Lecture 12, Slide 8Prof. 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 2007Lecture 12, Slide 9Prof. Liu, UC Berkeley Multiple Copies of I REF Multiple copies of I REF can be generated at different locations by applying the current mirror concept to multiple transistors.

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

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

EE105 Fall 2007Lecture 12, Slide 12Prof. Liu, UC Berkeley Fractional Scaling A fraction of I REF can be created in Q 1 by scaling up the emitter area of Q REF.

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

EE105 Fall 2007Lecture 12, Slide 14Prof. Liu, UC Berkeley Effect of Base Currents

EE105 Fall 2007Lecture 12, Slide 15Prof. Liu, UC Berkeley Improved Mirroring Accuracy Use Q F (rather than I REF ) to supply the base currents of Q REF and Q 1, reduce the mirroring error by a factor of .

EE105 Fall 2007Lecture 12, Slide 16Prof. Liu, UC Berkeley Different Mirroring Ratio Accuracy

EE105 Fall 2007Lecture 12, Slide 17Prof. 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.

EE105 Fall 2007Lecture 12, Slide 18Prof. Liu, UC Berkeley Generation of I REF for a PNP-BJT Current Mirror Neglecting base currents, the currents flowing through Q M and Q REF2 are the same.

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