Date post: | 15-Jan-2016 |
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Voltage or Current References
Requirements of a Reference Circuit: • Should be independent of power supply• Should be independent of temperature • Should be independent of processing variations• Should be independent of noise
Voltage or Current References
Requirements of a Reference Circuit: • Should be independent of power supply• Should be independent of temperature • Should be independent of processing variations• Should be independent of noise
S = ( Vref / Vref ) / ( Vdd / Vdd )Vdd
Vref
Sensitivity:
Voltage or Current References
Requirements of a Reference Circuit: • Should be independent of power supply• Should be independent of temperature • Should be independent of processing variations• Should be independent of noise
S = ( Vref / Vref ) / ( Vdd / Vdd )Vdd
Vref
Sensitivity:
If sensitivity = 1, then 10% change in Vdd results in a 10% in Vref
Simple Bias Circuits
Vout
Vdd
GND
Typically usinga potentiometer
Simple Bias Circuits
Vout
Vdd
GND
Typically usinga potentiometer
S = 1Vdd
Vref
Simple Bias Circuits
Vout
Vdd
GND
Typically usinga potentiometer
GND
Vout
Vdd
Transistor-basedresistive divider
S = 1Vdd
Vref
Simple Bias Circuits
Vout
Vdd
GND
Typically usinga potentiometer
GND
Vout
Vdd
Transistor-basedresistive divider
S = 1Vdd
Vref
If KN = KP and VTN = |VTP|
S = 1Vdd
Vref
Simple Bias Circuits
Vout
Vdd
GND
Typically usinga potentiometer
Typically poor Power Supply Rejection (low PSRR)
GND
Vout
Vdd
Transistor-basedresistive divider
S = 1Vdd
Vref
If KN = KP and VTN = |VTP|
S = 1Vdd
Vref
Bias Circuits involving Diodes
GND
Vout
R
Vdd
GND
Iout
Current Source
MOSFET Source
Bias Circuits involving Diodes
GND
Vout
R
Vdd
GND
Iout
Current Source
MOSFET Source
GND
Vout
R
Vdd
BJT Source
Bias Circuits involving Diodes
Significant Power Supply Rejection (higher PSRR)
GND
Vout
R
Vdd
GND
Iout
Current Source
MOSFET Source
GND
Vout
R
Vdd
BJT Source
Output Voltage Sensitivity
GND
Vref
R
Vdd
BJT Source Small-Signal model:
R
Vref
1/gm
GND
Vdd
Output Voltage Sensitivity
GND
Vref
R
Vdd
BJT Source Small-Signal model:
R
Vref
1/gm
GND
Vdd Vref = Vdd / (1 + gm R)
gm = (Vdd0 – Vref0)/ R UT
Output Voltage Sensitivity
GND
Vref
R
Vdd
BJT Source Small-Signal model:
R
Vref
1/gm
GND
Vdd Vref = Vdd / (1 + gm R)
gm = (Vdd0 – Vref0)/ R UT
Vref (1 + (Vdd0 – Vref0)/ UT ) = Vdd
S = (Vref / Vdd) (Vdd0 / Vref0)
= (Vdd0/Vref0)/(1 + (Vdd0–Vref0)/UT )
Vdd
Vref
Output Voltage Sensitivity
GND
Vref
R
Vdd
BJT SourceS = (Vref / Vdd) (Vdd0 / Vref0)
= (Vdd0/Vref0)/(1 + (Vdd0–Vref0)/UT )
Vdd
Vref
Output Voltage Sensitivity
GND
Vref
R
Vdd
BJT SourceS = (Vref / Vdd) (Vdd0 / Vref0)
= (Vdd0/Vref0)/(1 + (Vdd0–Vref0)/UT )
Vdd
Vref
Vdd0–Vref0 >> UT
S = (Vdd0/ (Vdd0–Vref0)) (UT / Vref0) Vdd
Vref
Output Voltage Sensitivity
GND
Vref
R
Vdd
BJT SourceS = (Vref / Vdd) (Vdd0 / Vref0)
= (Vdd0/Vref0)/(1 + (Vdd0–Vref0)/UT )
Vdd
Vref
Vdd0–Vref0 >> UT
S = (Vdd0/ (Vdd0–Vref0)) (UT / Vref0) Vdd
Vref
~1
Output Voltage Sensitivity
GND
Vref
R
Vdd
BJT SourceS = (Vref / Vdd) (Vdd0 / Vref0)
= (Vdd0/Vref0)/(1 + (Vdd0–Vref0)/UT )
Vdd
Vref
Vdd0–Vref0 >> UT
S = (Vdd0/ (Vdd0–Vref0)) (UT / Vref0) Vdd
Vref
~1
With Vref0 ~ 0.65, S = 0.04Vdd
Vref
Bias Circuits involving Diodes
Significant Power Supply Rejection (high PSRR)
GND
Vout
R
Vdd
GND
R1
R2
Allows wider variation of output voltage
Breakdown Diode Reference
Breakdown Diode Reference
Breakdown Diode Reference
Big Issue: Very noisy output voltage
SVREFVDD
VREF
VDD
VDD
VREF
vref
vdd
VDD
VBV
rZ
rZ R
VDD
VBV
Bootstrapped Reference
GND
Vout1
Vdd Vdd
M3
M4
GND
Vout2
M2
M1
Iref
AI = (W/L)1 / (W/L)2
R
Bootstrapped Reference
GND
Vout1
Vdd Vdd
M3
M4
GND
Vout2
M2
M1
Iref
AI = (W/L)1 / (W/L)2
VR
Bootstrapped Reference
GND
Vout1
Vdd Vdd
M3
M4
GND
Vout2
M2
M1
Iref
AI = (W/L)1 / (W/L)2
In subthreshold (or BJTs): V = UT ln(AI) = Iref R
Iref = UT ln(AI) / R V
R
Bootstrapped Reference
GND
Vout1
Vdd Vdd
M3
M4
GND
Vout2
M2
M1
Vdd
Iref
Iref
GND
Iref
AI = (W/L)1 / (W/L)2
In subthreshold (or BJTs): V = UT ln(AI) = Iref R
Iref = UT ln(AI) / R V
R
Bootstrapped Reference
Bootstrapped Reference
Bootstrapped Reference
Startup Circuit
Bootstrapped Reference
Bootstrapped Reference
Bootstrapped Reference
Startup Circuit