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EE105 Fall 2007Lecture 26, Slide 1Prof. Liu, UC Berkeley Lecture 26 OUTLINE Self-biased current sources – BJT – MOSFET Guest lecturer Prof. Niknejad ANNOUNCEMENTS Homework 12 due Thursday, 12/6

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EE105 Fall 2007Lecture 26, Slide 2Prof. Liu, UC Berkeley Review: Current Mirrors The current mirrors we discussed require a “golden” current source, I REF, to copy.

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EE105 Fall 2007Lecture 26, Slide 3Prof. Liu, UC Berkeley Review: 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?

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EE105 Fall 2007Lecture 26, Slide 4Prof. Liu, UC Berkeley Review: Current Mirrors (cont’d) A: Variations in V CC and temperature cause significant variations in I REF. Consider the following analysis (ignoring base currents and the Early effect): Thus, a 10 % change in V CC results in a 11.6 % change in I REF.

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EE105 Fall 2007Lecture 26, Slide 5Prof. Liu, UC Berkeley Base-emitter Reference Rather than having a source dependent on V CC, why not use some other reference? For example, a V BE referenced current source. Ignoring base currents, we have: Q: Why is this less supply dependent?

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EE105 Fall 2007Lecture 26, Slide 6Prof. Liu, UC Berkeley Base-emitter Reference (cont’d) A: Although I IN varies almost directly with V CC, V BE1 won’t vary nearly as much, since the device is exponential. Since I OUT depends only on V BE1, the output won’t vary much with V CC. Example: Assuming V BE1 = 0.7 V, a change in input current by 10X causes an output current change of 8.7 %.

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EE105 Fall 2007Lecture 26, Slide 7Prof. Liu, UC Berkeley Self Biasing We can do better than the V BE referenced source using feedback. What if our source had a current mirror attached that fed back the output current to act as the input current?

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EE105 Fall 2007Lecture 26, Slide 8Prof. Liu, UC Berkeley Self Biasing (cont’d) Here, we’ve attached a pnp current mirror to force I OUT and I IN to match. There are two stable operating points: – I IN = I OUT = 0 A – Desired operating point

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EE105 Fall 2007Lecture 26, Slide 9Prof. Liu, UC Berkeley Start-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 point

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EE105 Fall 2007Lecture 26, Slide 10Prof. Liu, UC Berkeley Start-up Circuit (cont’d)

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EE105 Fall 2007Lecture 26, Slide 11Prof. Liu, UC Berkeley Start-up Circuit (cont’d) Let’s ensure this works: – Assume I IN = I OUT = 0. This means approximately that V BE1 = V BE2 = 0. However, note that the left side of D 1 is four diode drops from ground, meaning D 1 is on. This drops some voltage across R x, forcing current to flow into T 1 and T 2, starting up the circuit. – After the circuit is at the desired operating point, turn D 1 off by ensuring R x I IN (the drop across R x ) is sufficiently large.

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EE105 Fall 2007Lecture 26, Slide 12Prof. Liu, UC Berkeley MOSFET Current Source We can build an analogous circuit from MOSFETs as well. Let’s start with a V TH referenced current source. If we make V ov1 small (by sizing up T 1 or using small currents), I OUT is controlled primarily by V TH and R 2.

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EE105 Fall 2007Lecture 26, Slide 13Prof. Liu, UC Berkeley MOSFET Current Source (cont’d) Let’s add the current mirror feedback.

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EE105 Fall 2007Lecture 26, Slide 14Prof. Liu, UC Berkeley MOSFET Current Source (cont’d) Finally, the start-up circuitry. It’s more typical to use more MOSFETs in MOS technologies rather than diodes.

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EE105 Fall 2007Lecture 26, Slide 15Prof. Liu, UC Berkeley MOSFET Current Source (cont’d) Assume I IN = I OUT = 0. This means V GS1 = 0, meaning T 8 is in triode. This turns on T 9 and forces current to flow into T 4 and T 5. Once in steady state, we can size T 7 to ensure that T 9 turns off. T 7 and T 8 don’t directly affect the circuit themselves, so the start-up circuit has done its job.

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EE105 Fall 2007Lecture 26, Slide 16Prof. Liu, UC Berkeley References Material and figures largely from Analysis and Design of Analog Integrated Circuits, Fourth Edition by Gray, Hurst, Lewis, and Meyer.

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EE105 Fall 2007Lecture 26, Slide 17Prof. Liu, UC Berkeley Guest 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 systems, and scientific computing.

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