Download Low Voltage Low Power constant-gm Rail to

Low Voltage Low Power
constant - gm Rail to Rail CMOS
Op-Amp with Overlapped
Transition Regions
ECEN 5007
9/3/02
Vishwas Ganesan
Motivation
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Low Supply Voltage Operation
Constant gm
Low power consumption making it
suitable for portable applications.
Reduced chip area.
Why this paper came up ?
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Complementary differential pairs operate
in parallel.
This leads to one pair turned on and one
off when input is near rails and both
pairs on at the middle of the input range
leading to gm being twice the value than
the former case
Other Circuit Proposals
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3.
4.
Variation in the tail current in the differential
paper doubling gm when only one pair is
active.
Comparing currents from p and n differential
pairs and the maximum current between
them is selected and processed for gm
constant by maximum-selecting circuitry.
Current bleed circuits
Square root circuit
Difficulties with other circuits
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Increase in slew rate due to increase in
tail current.
Extra circuitry – Signal processing
circuits and many more current mirrors.
All this leads to complication, Power
consumption and more die.
Complementary Input Stage
Gm is constant if
√ (βnIsn) + √ (βpIsp) = constant
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Cutoff : Isn = 0
Vss ≤ Vcm ≤ Vn-
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Transition
Isn = Isn ( Vcm )
Vn- < Vcm ≤ Vn+
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Saturation
Isn = ßMBn ( VGMBn – Vss – Vt n) 2
Vn+ < Vcm ≤ Vdd
General Transition graph
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This shows the
transition region
overlap of a general
n-p differential pair.
Design
Lower boundary of the transition region :
Vn- = Vss + Vtn + VDSMBn
Neglecting VDSMBn we get Vn- ≈ Vss + Vtn
Upper boundary of the transition region VDGMBN = - Vtn
VDGMBn = Vn+ - Vtn - √(Isno/ßM1n) – VGMBn
We get Vn+ = VGMBn + √ (Isno/ ßM1n)
Deriving expression for Isn :
Isn = ßM1n ( Vcm – VDMBN – Vtn ) 2 eqn 1
Isn = ßMBn ( VGMBn – Vss – Vtn – VDMBN – Vss ) ( VDMBn – Vss )
Assuming ßMBn = 2 ßM1n and solving for VDMBn from above
VDMBn =Vcm+VGMBn– Vtn –½√ [2(-Vtn+VGMBn-Vss)2 –(Vcm-VGMBn)2]
Using this in eqn 1
Isn=ßM1n[(Vcm–VGMBn)/2+1/2√(2(-Vtn+VGMBN–Vss)2 –(VCM-VGMBn)2)2
Design
Similarly for Pmos
Vp+ = Vdd + Vtp
Vp-=VGMBp - √Ispo/ßM1p
Isp=ßM1p[(-Vcm+VGMBp)/2+1/2√(2(Vtp–VGMBP+Vdd)2–(VCM-VGMBp)2]2
To overlap the transition regions of the n and p pairs we equate
Vp- = Vn- and Vn+ = Vp+
Also we assume symmetry between the pairs and so
Ispo = Isno , ßM1p = ßM1n , VGMBn = - VGMBp and Vtp = - Vtn
Solving we get
ßM1n = ßM1p = Isno/(Vdd + Vtp – VGMBN)2
Complementary Input Stage with DC
Level Shifter
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Use a dc level shifter to shift the
p-transition curve leftward to overlap the
n-transition curve.
Vshift small  gm exceeds normal limit
Vshift large gm drops below limit
∆ Voptimal yields constant Gm
2VGMBN<∆Voptimal< 2VGMBN+ √(Isno/βM1)
Plot Of gm vs Vcm
OPAMP Circuit
OPAMP Implementation
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DC level shifters implemented by 2 pairs of
PMOS source followers MS1-MS4
Three stages
Complementary input stage
Folded cascoded stage M21-M28 provides
high gain
Class AB output stage M30-M33
MB1 and MB11 are biasing transistors.
Frequency Response
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Amplitudes and
phase plots show
unstability due to
varying gm.
My simulation results
Input offset voltage
30 mV
Power dissipation
.36 mW
Output Voltage swing
1.1 V to -1V
Simulation plots
Paper Results
Input offset voltage
3 mV
Power dissipation
.31 mW
OpAmp area
.12 mm2
Output Voltage swing
Vss + 0.04 to Vdd –
0.07 V
Conclusions
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DC level shifters can be used to overlap
transition regions to obtain constant gm.
Low Power dissipation was achieved
Low supply voltages .
There is considerable gain achieved
leading to CMRR improvement.