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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.
2
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
4
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.
Addition of the Start-up devices
Temperature-Independent References
Negative-TC Voltage
We assume that IC is held constant.
Continued
Positive-TC Voltage
Continued
Bandgap References
2121
CCYoutXout
YX IIRVV
RVVVV
Continued
3
22321
21
2322
111
RVIIRVVV
IIIRRVV
IRVVBE
CCBEBEBE
CC
CBEout
CBEout
23
22232 1 BEBEBECout VV
RRVIRRV
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
Continued
VmVKmVVnV
RRV BETout 2.17503005.1ln1 2
3
2
Assuming VBE2=750mV, we have:
Compatibility with CMOS Technology
Modified Bandgap Circuit
Bandgap Circuit
The drawback of the above circuit is that VDS1VDS2. Therefore ID1 is not accurately identical to ID2.
nVRRVV TEBout ln1
1
22
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
PTAT Current Generation
The advantage of the above circuit is that VDS1=VDS2. Therefore ID1 is accurately identical to ID2.
1
lnRnVI T
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
Generation of a PTAT Current Using a Simple Amplifier
Generation of a Temperature-Independent Voltage
Generation of a Temperature-Independent Voltage
nVRR
VRR
V TEBout ln1
41
2
4
This circuit is useful to generate low reference voltages.
Generation of a Temperature-Independent Voltage
This circuit is useful to generate low reference voltages.
NVRRV
RRV TEBREF ln
1
22
2
3
Temperature-independent Current Generation
Constant-Gm Biasing
The resistor Rs is an off-chip resistor.
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