voltage and current references bandgap references · microsoft powerpoint - ic_slide6.ppt...
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![Page 1: Voltage and Current References Bandgap References · Microsoft PowerPoint - IC_Slide6.ppt [Compatibility Mode] Author: dr shamsi Created Date: 12/25/2017 7:15:49 AM](https://reader033.vdocuments.us/reader033/viewer/2022043016/5f38973b3369d7587b64467f/html5/thumbnails/1.jpg)
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Voltage and Current ReferencesBandgap References
The objective of reference generation is to establish a dc voltage or current with following characteristics:
•Independent of the supply and process.•Having a well-defined behavior with temperature.
•Temperature independent.
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Supply-Independent Biasing
In the design of current sources, we need to a golden current source, IREF. (Figure (a))If instead of IREF we use a resistor, the generated current sources will be supply-dependent. (Figure (b))
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A Simple Circuit to Establish Supply-Independent Current Supply-Independent Biasing
The current is independent of the supply voltage but it is still a function of process and temperature.
=0, =0
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Start-up Problem
Iout=0 is another operating point of the above circuit because it satisfies the following equation. We call this problem as the start-up problem.
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Addition of the Start-up devices
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Temperature-Independent References
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Negative-TC Voltage
We assume that IC is held constant.
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Continued
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Positive-TC Voltage
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Continued
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Bandgap References
2121
CCYoutXout
YX IIRVV
RVVVV
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Continued
3
22321
21
2322
111
RVIIRVVV
IIIRRVV
IRVVBE
CCBEBEBE
CC
CBEout
CBEout
23
22232 1 BEBEBECout VV
RRVIRRV
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Continued
23
22
3
2 ln11 BETBEBEout VnVRRVV
RRV
TVn
qk
RR
TV BEout
2
3
2 ln10
KmVn
qk
RR
5.1ln1
3
2
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Continued
VmVKmVVnV
RRV BETout 2.17503005.1ln1 2
3
2
Assuming VBE2=750mV, we have:
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Compatibility with CMOS Technology
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Modified Bandgap Circuit
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Bandgap Circuit
The drawback of the above circuit is that VDS1VDS2. Therefore ID1 is not accurately identical to ID2.
nVRRVV TEBout ln1
1
22
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Bandgap Circuit (Vout=2.5V)
The drawback of the above circuit is that VDS1VDS2. Therefore ID1 is not accurately identical to ID2.
nVRRVVV TEBEBout ln12
1
243
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PTAT Current Generation
The advantage of the above circuit is that VDS1=VDS2. Therefore ID1 is accurately identical to ID2.
1
lnRnVI T
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Employing PTAT to Generate Temperature-Independent Voltage
The advantage of the above circuit is that VDS1=VDS2. Therefore ID1 is accurately identical to ID2.
1
23lnRnVRVV T
EBout
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Generation of a PTAT Current Using a Simple Amplifier
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Generation of a Temperature-Independent Voltage
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Generation of a Temperature-Independent Voltage
nVRR
VRR
V TEBout ln1
41
2
4
This circuit is useful to generate low reference voltages.
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Generation of a Temperature-Independent Voltage
This circuit is useful to generate low reference voltages.
NVRRV
RRV TEBREF ln
1
22
2
3
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Temperature-independent Current Generation
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Constant-Gm Biasing
The resistor Rs is an off-chip resistor.
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Continued
This opamp utilizes the constant-Gm biasing. Rb is an off-chip resistor.
13
152
2
13
10 ,1112
LWLW
KKR
LWCμ
Ib
oxn
D