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

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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

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.

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?

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.

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?

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 %.

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?

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

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

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

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.

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.

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

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.

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.

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.

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.