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8/10/2019 Current mode bandgap reference
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A Novel Temperature Stable Current Mode Bandgap
For Wide Range of Supply Voltage Variation
Sovan Ghosh
Department of Electrical Engineering, IIT Madras
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Voltage Reference
Analog circuits incorporate the voltage and current
references extensively . Such references are dc quantity that
exhibit-
A minimum dependence on the supply and processparameters.
It has a well defined dependence on the temperature
(PTAT, constant Gm, orTemperature Independent)We will analyze the operation of a temperature independent
voltage reference.
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BANDGAP REFERENCE
One representative reference satisfying these keyparameters is the bandgap voltage reference.
The bandgap output voltage is realized by adding
a voltage that is complementary-to-absolute-temperature (CTAT) to another voltage which is
proportional-to-absolute-temperature (PTAT) to
yield a first-order temperature-compensatedvoltage.
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First Order Bandgap CircuitNow if we assume that the Op-Amp
is working perfectly in its negativefeedback configuration then current
I2is given by
I2=(VBE1-VBE2)/R3
VBE1=VTln (I1/I0)
VBE2=VTln (I2/NI0)V1=V2
I1R1=I2R2
I2=(VBE1-VBE2)/R3=VTln (R2*N/R1)/R3
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Temperature stability of first order Bandgap
Several solutions to improve the temperature behavior exist. But They require
precision matching of current mirrors or a pre-regulated supply voltage and
Sometime special process also.
Fig: Variation Of Output voltage of a first order Bandgap with temperature.
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Low Voltage Bandgap
Current Mode References Voltage Mode References
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Voltage Mode References(VMR)
This method uses a reverse
bandgap voltage principle.
Instead of adding a VBEvoltage
to a scaled VTvoltage, voltage-
mode references add a VTvoltage to an attenuated VBE
voltage.
Main draw back of this
implementation is that it need
special process (TwinWell/BICMOS) to get high
quality BJT.
It needs separate bias current.
.
Fig: Conventional Low Voltage VMR
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Drawbacks
Transistors Q1 and Q2 are fabricated in low voltage twin well
process. Figure below shows a Low voltage twin-well CMOS
process.
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Prev. Slide Continued.
When the NPN device is fabricated, a parasitic PNP device is
formed from the base and collector of the NPN to the p-typesubstrate. When the NPN device is saturated, the parasitic PNPdevice begins operating in the forward-active region since theemitter-base voltage (VEB) of the PNP is equal to VBCof the NPN.This means we cant neglect the base current anymore and asmall change in VCE1 will change the IB1significantly.
Twin Well or BICMOS processes are costlier.
Another disadvantage is that the circuit requires a separate biascurrent source for proper operation instead of using a feedbacksystem to control the current of the reference core. The use of aseparate current source causes the currents inside the naturallogarithm to rely on temperature-dependent parameters insteadof ratios of resistors as or ratios of transistor sizes . Hence, itcomplicates the calculation of the scale factor. This currentsource also degrades the PSRR.
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Current Mode Reference(CMR)
Though it has high flat bandand 1/f output noise problem
due to pMOS current mirror.
But this types of reference can
operate in low supply voltage
and can be port to different
process. Minimum Supply Voltage is
limited by the common mode
of the amplifier.
Fig: Conventional Low Voltage CMR
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Detail Analysis Of the New Current Mode
Reference
Circuit Diagram
Amplifier/Op-Amp Architecture
Operational Details Performance Result
Comparative Analysis
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Circuit Diagram
Fig: Circuit Diagram of The Proposed Bandgap Reference.
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Amplifier Circuit
Amplifier/Op-Amp Circuit Bias_genarator for the amplifier
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Analysis
Both the nMOS and pMOS input pair of the amplifier is biased at sub
threshold region of operation. The biasing circuit ensured that tail current of
nMOS input pair Intail= Ibias- Iptailirrespective of supply and common mode
voltage. So Iptail+ Intailis also constant. Now the DC gain of the amplifier is
given by Gm*R where Gmis input effective trans conductance of the
amplifiers input pairs and R is the impedance seen by looking into thecircuit from node PBIAS. Now R can be approximately written 1/(*IMp7)
where is a process dependent constant and IMp7 is the source to drain
current of Mp7. As Intail+ Iptailis constant and Inbiasis constant so the biasing
current of the output transistor IMp7is also constant; i.e. R is constant. Now
the Gmis given by gmn+gmpwhere gmnis nMOS input pairs trans
conductance and gmpis pMOS input pairs trans conductance. Nowgmp=(iptail/2)/Vtand gmn= (intail/2)/Vt. Where iptailand intailare biasing current as
shown in fig. 5 and Vtis thermal voltage. So Gm= gmn+gmp= ibias/(2*Vt)
which is independent of supply and common mode voltage.
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Performance Of the Amplifier
0.4 0.6 0.8 1.0 1.2 1.467
68
6970
71
72
73
74
75
76
77
78
79
80
81
82
83
Amplifier'sDCGainin
dB
Input Common Mode Voltage in volt
For 220 nM
For 180 nM
Fig: Variation of Amplifier gain with input Common Mode Voltage
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Performance Of the BG
-60 -40 -20 0 20 40 60 80 100 120 1400.71640
0.71645
0.71650
0.71655
0.71660
0.71665
0.71670
0.71675
0.71680
B
A
B
andgapOutputVoltageinVolt
Temperature(.C)
A- For 180 nm
B- For 220 nm
Fig: Variation of Bandgap Output Voltage with Temperature
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Continued.
0.000 0.005 0.010 0.015 0.020
0.7164
0.7165
0.7166
0.7167
0.7168
0.7169
1.50
1.75
2.00
2.25
2.50
2.75
3.00
3.25
B
C
D
Amplitude(V)
Time (s)
D-Supply Voltage
C-Bandgap output for 180 nm
B-Bandgap Output for 220 nm
Fig: Variation In Bandgap Output Voltage with Supply Voltage Variation
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Continued.
100
101
102
103
104
105
106
107
108
109
-90
-80
-70
-60
-50
-40
-30
-20
-10
PSRR
(dB)
Frequency (Hz)
For 220 nm
For 180 nm
Fig: Variation of PSRR of proposed Bandgap with frequency.
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Comparative Analysis
Parameters [12] [9] [10] [11] This work
Supply
Voltage (V)
2 1.2 2 2.5 1.5 / 3
Supply Current (A) 25 40 23 38 11.2
Ref.
Voltage (V)
1 .487 1.14 .617 .716
Temp.
Coefficient
(ppm/C)
3.68 8.9 5.3 3.9
to
13.7
2.7
Temperature
Range
-40C to
150C
-40C to
110C
0C
to100C
-50C to
150C
-55C
to125C
Line Regulation (%
/V)
- .24 .286 .039 .028
CMOS
Technology (m)
0.35 0.5 0.6 0.35 0.22 / 0.18
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References
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