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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, Cod Test 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 CapacitanceCGS1 = 2/3 Cox W L = 117 fFMiller
effect significantly increases Cid at 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 CL
increases m in 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 CMFBResistors (mostly academic)Triode
DevicesDouble Differential Pair
Discrete-time CMFBSwitched capacitor CMFB
Issues: Bandwidth, rejection
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad
& S.Gambini
Ideal CMFB+
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+
vop
von
cmc A
Issue: How do we sense the output common-mode without disturbing
the amplifier? (See next)
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EECS 240 Topic 10: Folded Cascode 2011 B. Boser,A. Niknejad
& S.Gambini
Resistive CMFBConsider Folded Cascode AmplifierGain=Gm Rout=70dB
(~3000 V/V)Rout=2.5MRsense> 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:SimpleProblem:one side shuts off for large
signal-> this become DMFB (differential mode feedback)
vddbiasPbiasP
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:SimpleProblem:one side shuts off for large
signal-> this become DMFB (differential mode feedback)Similar
version with double diff pair and same problems
vddbiasPbiasP
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
+
-
-
+
vop
von
cmc
Csense
Solution: Switched Capacitor
1
1
1
Csense
2
2
2
Vcmref
Vcmref
Vbias
