t10+folded+cascode simone
TRANSCRIPT
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
EECS 240Analog Integrated Circuits
Topic 10:
Folded Cascode OTA
Bernhard E. Boser, Ali M. Niknejad,S.Gambini
Department of Electrical Engineering and Computer Sciences
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Folded Cascode Good general purpose single-stage OTA
Analysis
Bias
Gain
Input capacitance
Frequency response Feedback, stability
Settling time
Noise
Common-Mode Feedback
Design
Specifications
Circuit parameters
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Schematic
Many more transistors, but overall characteristics very similar to common-source stage.
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Bias Subcircuit
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
DC Gain, Bias
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Bias Differential Inputs
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Low Frequency Gain, Adm
Gain / Output Range tradeoff
Adm=120!!Vod= 4.3V
Adm=200!!Vod= 3.6V
(nominal parameter)
Increasing Adm:
Moderate:
Increase L
Affects V*, phase margin
Substantial:
Double Cascode
Gain boosting
Multi-stage amplifier
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Increased Adm
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Output Resistance
Beware of imbalance between NMOS and PMOS current source
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Gm, CodTest Circuit
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Transconductance
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Differential Output Capacitance
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Differential Input Capacitance
CGS1= 2/3 CoxW L
= 117 fF
Miller effect
significantly
increases Cidat low
frequency
Cascoding M1, M2
helps (at the
expense of reduced
input common-
mode range)
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Frequency Response (Adm)
Bandwidth Strong function of CL
Increasing fu?
Feedback Stability
Phase margin
Increasing "m
Increasing CLincreases "min
single-stage
amplifier (costly)
"m
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Common Mode Feedback
(CMFB) (See G&M, Ch. 13)
Essential in all fully-differential amplifiersHow to implement it?Main choices:
Continuous-time CMFB!Resistors (mostly academic)!Triode Devices
!Double Differential Pair
Discrete-time CMFB
Switched capacitor CMFBIssues: Bandwidth, rejection
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Ideal CMFB
+
-
-
+
vop
von
!cmc A
Issue:How do we sense the output common-mode without disturbing theamplifier? (See next)
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Resistive CMFBConsider Folded Cascode AmplifierGain=GmRout=70dB (~3000 V/V)Rout=2.5M"Rsense> 2.5M"to avoid gain degradation-impractical
+
-
-
+
vop
von
cmcRsense
Rsense
What about using transistors?
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Triode Device CMFBWhat about using transistors?
Advantage:Simple
Problem:one side shuts off for large signal->this become DMFB (differential mode
feedback)
vdd
biasPbiasP
vss
vopvon
Vref Vref
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Triode Device CMFBWhat about using transistors?
Advantage:Simple
Problem:one side shuts off for large signal->this become DMFB (differential mode
feedback)
Similar version with double diff pair and
same problems
vdd
biasPbiasP
vss
vopvon
Vref Vref
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Switched Capacitor CMFBWhat about using capacitors?
Problem:initialization
+
-
-
+
vop
von
cmcCsense
Csense
Solution: Switched Capacitor
(see next)
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad & S.Gambini
Switched Capacitor CMFB
+
-
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+
vop
von
cmc
Csense
Solution: Switched Capacitor
#1
#1
#1
Csense
#2
#2
#2
Vcmref
Vcmref
Vbias