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Power-adaptive operational
amplifier with positivefeedback self- biasing
Byungsub Kim
Soumyajit Mandal
Rahul Sarpeshkar
Analog VLSI & Biological Systems Group
Research Lab of Electronics
Massachusetts Institute of Technology
{byungsub,soumya,rahuls}@mit.edu
Motivation
Source : http://www-mtl.mit.edu/researchgroups/icsystems/uamps/
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Low-power switched-circuit applications
Need wide dynamic range
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Example: audio
z Large frequency changes 20Hz~20kHz
z Large amplitude changes
Review : switched capacitor
circuits
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Holding mode
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DC steady state
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Strong output current is not
necessary
High gain for feedback.
Î Achievable with small
bias current
Sample mode
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Transient state
Strong output current.
z High gain is not necessary.
Î Needs strong bias current
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Î Control bias current based on
necessity
Idea - bias current control
based on demand
Observe output current i1 with
current replica & rectifier
Compare with a fraction of bias
current (bi2 ,where b < 1)
Vary bias current i2 based on
demand
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Increase i2 if | i1 | > bi2
Decrease i2 if | i1 | < bi2
Problem: positive feedback &
current saturation
Positive feedback for fixed
input voltage vdiff
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increase i2 Æ increase i1 Æ
increase i2 … Æ i2,max
decrease i2 Æ decrease i1 Æ
decrease i2 … Æ i2,min
Bias current saturates to
either i2,max or i2,min
Solution : op-amp in negative
feedback
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Application : op-amp in
negative feedback
Major negative feedback
loop regulates minor
positive feedback loop
Circuit Implementation
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Main op-amp
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Ordinary
transconductance
amplifier (OTA)
Good linearity and
stability (first order)
Current replica
Current rectifier
Current comparator
Circuit Implementation
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Current limiter
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M2 & M5 : Limit maximum
bias current & comparison
current
Sets boundary of operating
range
Minimum current
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M3 & M6 : Provides
minimum bias current &
comparison current
Prevents complete turn-off
Block diagram in voltagefollower configuration
Block diagram: analysis
For small imin & ib
Step response
Performance evaluation
Bias control
ni2 is the total bias current drawn from
the supply
Power consumption for fixed, worst case
biasing
Power saving ratio (PSR)
Modelling current-bounding
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Effects of current limiter
and minimum current
Set by M2, M5, M3, M6
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imax & imin can be varied
Trade-Off
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Settling time
Dynamic range of power
Test chip configuration
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AMI 0.5μm CMOS process
C can be changed using a
NMOS switch
z 1pF, 10pF, 100pF
Expected result
P∝C
P∝ f
P ∝ Vsw
Measurement
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Voltage follower works as expected
100pF, 25kHz, 0.5Vpp
Measured power consumption
Measured power consumption
As expected, we get
P∝C
P∝ f
P ∝ V sw
Conclusion
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Power-adaptive op-amp with positive feedback selfbiasing
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Bias control : positive feedback + external negative
feedback
Simple bias controller
Wide dynamic range
Fast response
Future work
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Improved model for dynamic behaviour