Lecture 20-Slewing

# Lecture 20-Slewing - Lecture 20 Slewing Boris Murmann...

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EE 214 Lecture 20 (HO#28) B. Murmann 1 Lecture 20 Slewing Boris Murmann Stanford University murmann@stanford.edu Copyright © 2004 by Boris Murmann

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EE 214 Lecture 20 (HO#28) B. Murmann 2 Overview Reading – 9.6.1, 9.6.2, 9.6.5 (Slew Rate) Introduction – Today we'll complete our discussion of transient behavior in OTA circuits with capacitive feedback. Aside from the feedforward artifact we've discovered last time, there exists another effect called "slewing". Whenever the differential input pair is driven outside its "linear range", the differential output current is limited by the available tail bias. In typical switched capacitor circuits, slewing can cause a significant speed reduction, and is therefore an important effect to consider.
EE 214 Lecture 20 (HO#28) B. Murmann 3 Simulation Example from Last Lecture Using simple single stage OTA (See lecture 13, slide 9) Parameters –C s =C f =500fF, C L =10pF, β =0.48, G m =1mS, G m R o =85 10gig C L C L C f C f V id V ic Diffdrive Diffsense C s C s V od V oc 0 V idstep

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EE 214 Lecture 20 (HO#28) B. Murmann 4 Another Simulation •S e t V idstep =-1V (C L =10pF insignificant feedforward to output) What causes this discrepancy ? 0 50 100 150 200 0 400 600 800 1000 Time [ns] Voltage [mV] -V id V od (simulation) V (theory?)
EE 214 Lecture 20 (HO#28) B. Murmann 5 Capacitive Divider at OTA Input Half circuit during initial transient: V im V op C f C s C in V idstep /2 C L V odstep/2 10pF 500fF 500fF 40fF V xdstep V xd mV 480 fF 500 fF 40 fF 500 fF 500 V 1 C C C C C C C V V L f L f in s s idstep xdstep = + + + + + = Initially -480mV across differential pair input!

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EE 214 Lecture 20 (HO#28) B. Murmann 6 Differential Pair Characteristics Differential output current begins to saturate ~|V id | > 1.4·2/(g m /I D )
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## Lecture 20-Slewing - Lecture 20 Slewing Boris Murmann...

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