ELE6308

Transcription

ELE6308

S
G
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ELE6308
Microélectronique analogique et mixte
--- Sources de courant ----- Voltage and current references --Mohamad Sawan, Professor
Kamal El-Sankary, Ph.D.
Laboratoire de neurotechnologies Polystim
http://www.cours.polymtl.ca/ele6308/
[email protected]
M5418
Page 1
Current Mirrors & V/I references
Plan
I.
II.
Current Mirrors
I.
Simple
II.
Cascode/ Improved cascode (high swing)
III.
Wilson
Voltage and current references
I.
Sensitivity
II.
Power supply independent references
III.
Bandgap circuits
Page 2
ELE6308 - Microélectronique analogique et mixte
CIRCUITS MOS
Source de courants

Miroir de courant

Structure simple
iref
M1
iout
+
M2 Vout
-
Page 3
ELE6308- Microélectronique analogique et mixte
CIRCUITS MOS
Source de courants
•
Miroir de courant simple (suite)
 Analyse DC
iref
+
M2 Vout
M1
VGS = Vth +
I OUT
iout
-
2 I ref
mCox (W L )1
2 I ref
(W L)2
1
= mCox (W L )2
=
I ref
2
mC ox (W L )1 (W L)1
 Impédance de sortie
DV = VGS - Vth =
rout = rds2 = 1/(λ IOUT)
Page 4
2 I ref
mCox (W L )1
CIRCUITS MOS
Source de courants

Miroir de courant «cascode»
 Analyse DC
i ref
i out
+
M3
M2
2(Vth +Veff)
M4
Vth +Veff
VA
Vout
M1
-
∆V =Veff= VGS - Vth à ID =Iref
VOUT ! Vth + 2"V
Vout = 2 Vth+ 2 Veff - Veff -Vth + Veff
Page 5
CIRCUITS MOS
Source de courants

Miroir de courant cascode (suite)
M2
+
ix
Vx
i ref
i out
-
M1
+
M3
2(Vth+Veff)
M2
VA
M4
Vth+ Veff
Vout
Vx
M1
-
rds2
g m2 Vgs2
+
Vsb2
-
L'impédance de sortie est augmentée par (1 + av2):
Page 6
g mb2 Vsb2
rds1
vx
= Rout @(1 + gm 2 rds1 )rds 2
ix
CIRCUITS MOS
Source de courants

Miroir de courant Wilson
M3
M1
M2
 Analyse DC
I OUT =
(W L )2
(W L )1
I REF
rO ! gm 3 rds 3 rds 1
Page 7
CIRCUITS MOS
Source de courants

Miroir de courant cascode amélioré
Avantage: nécessite seulement 2 Veff
Inconvénients: énergie et complexité.
Page 8
CIRCUITS MOS
Source de courants

Miroir de courant cascode amélioré (High swing)
Page 9
Current Mirrors & V/I references
Plan
I.
II.
Current Mirrors
I.
Simple
II.
Cascode/ Improved cascode (high swing)
III.
Wilson
Voltage and current references
I.
Sensitivity
II.
III.
Bandgap circuits
Page 10
Power Supply Independence
How do you characterize power supply independence?
Use the concept of:
Application of sensitivity to determining power supply dependence:
Thus, the fractional change in the reference current is equal to the
sensitivity times the fractional change in the power supply voltage.
For example, if the sensitivity is 1, then a 10% change in VDD will cause a
10% change in IREF.
Page 11
Simple Bias/Reference Circuits
Page 12
MOSFET-Resistance Voltage References
Assume that VDD = 5V, W/L = 2 and R = 100kΩ,
Thus, VREF ≈ 1.281V and
S = 0.283
Page 13
MOSFET-Resistance Voltage References
This circuit allows VREF to be larger.
If the current in R1 (and R2) is small
compared to the current flowing
through the transistor, then
Page 14
Simple Current Reference
Page 15
MOS Widlar Current Reference
=>
=>
Where
Page 16
Bootstrapped Current Source/Sink
Start up
Principle:
If T4 = T5, then I1 ≈ I2. However, the T1-R loop gives:
Solving these two equations gives
The output current, Iout = I1 = I2 can be solved as:
Power supply independent !!
Page 17
Temperature Dependence
The objective is to minimize the fractional temperature coefficient defined as,
parts per million per °C or ppm/°C
Note that VREF, VT, β, and R are all functions of
temperature.
Page 18
Temperature Dependence
Bootstrapped Current Source/Sink
The output current was given as:
Assume that:
If the resistor is polysilicon, then
The temperature performance of this circuit is not that good.
Page 19
Temperature-Independent
References
• Reference voltages and/or currents with little dependence to temperature
prove useful in many analog circuits.
• If two quantities with opposite temperature coefficient are added with
proper weighting, the resultant quantity theoretically exhibits zero
temperature coefficient.
• For V1 and V2 with opposite temperature dependence, the
coefficients c1 and c2 can be chosen in such a way that
Thus, the reference voltage
coefficient.
exhibits zero temperature
Page 20
Bandgap Voltage Reference
• The most popular technique for both Bipolar and CMOS technologies.
• Generates a fixed dc reference voltage that does not change with
temperature.
• Cancels the negative temperature dependence of a PN junction with a
positive temperature dependence from a PTAT (proportional-to-absolutetemperature) circuit.
• The term with negative temperature dependence is the forward-biased
voltage of a diode (usually base-emitter junction): VBE
• The PTAT term is realized by amplifying the voltage difference of two
forward-biased diodes ( i.e., base-emitter junctions): VBE1 - VBE2
Page 21
Bandgap Voltage Reference
Page 22
Bipolar Circuit
Page 23
CMOS circuits
• In CMOS technologies, where the
independent bipolar transistors are not
available, parasitic lateral bipolar transistors are
used.
Page 24
CMOS circuits
Page 25
CMOS circuits
Page 26
CMOS circuits
• Usually make n=integer2-1, e.g. n=8
• Layout:
Page 27
CMOS circuits
• If |VBE| of a unit device is 700mV at room temperature and I=50µA
• Decided to use n=8
Page 28
CMOS circuits
Page 29
CMOS circuits
• For low voltage reference: Add currents proportional to VBE and kT/q,
instead of stacking voltages
Page 30

ELE6308

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