Download Analog Integrated Circuits Fundamental Building Blocks

Analog Integrated Circuits
Fundamental Building Blocks
Integrated current sources and sinks
Faculty of Electronics
Telecommunications and
Information Technology
Gabor Csipkes
Bases of Electronics Department
Outline
 ideal and real current sources – parameters
 a simple transistor as current source
 resistive degeneration – negative feedback for increased output resistance
 cascoding
 enhanced output resistance cascode current source
Analog Integrated Circuits – Fundamental building blocks – Integrated current sources and sinks
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Ideal and real current sources
 ideal current source → the current is independent on the voltage across terminals
 real current source → finite output resistance → the output current depends on the
voltage across the terminals
 current sinks → same as the current source but they absorb the current
ideal
real
I out  I 
Vout
 f (Vout )
Rout
 key parameters to consider:
 output resistance → must be as large as possible → Iloss close to zero
 minimum required output voltage for device biasing in integrated sources
Analog Integrated Circuits – Fundamental building blocks – Integrated current sources and sinks
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A single transistor as current source
 a single transistor – voltage controlled current source
 bipolar transistors biased in FAR and MOS transistors in saturation
Rout
Vout

I out
Bipolar
MOS
Rout  rCE
Rout  rDS
(hundreds of kΩ)
(hundreds of kΩ)
Vo min  VBE
Vo min  VDSat
MOS transistors are typically used as
current sources when available in the
target technology.
Analog Integrated Circuits – Fundamental building blocks – Integrated current sources and sinks
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A single transistor current source
 a tens or hundreds of kΩ output resistance may not be sufficient in some applications
→ use series-series negative feedback to enhance Rout
VG  VDSat  VTh  I out R
VTh  f VBS 
Vout  I out R  VDS

VDS   I out  I1  I 2  rDS

 I1  g mVGS
I  g V
mb BS
 2
resistive
degeneration
Rout  rDS  R   g m  g mb  rDS R
Vo min  VDSat  I out R
A large output resistance causes Vomin to rise → method to
increase Rout not efficient
Analog Integrated Circuits – Fundamental building blocks – Integrated current sources and sinks
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The cascode current source
 goal → eliminate the linear dependence of Vomin on R and keep it low while Rout grows
very large
 solution → replace R with a non-linear resistor that features very large resistance for
low biasing voltage → a second transistor
Vout  VDS 1  VDS 2

VDS 1   I out  g m1VGS 1  rDS 1
V  I  g V  g V
m 2 GS 2
mb 2 BS 2  rDS 2
 DS 2  out
Rout  rDS 1  rDS 2   g m 2  g mb 2  rDS1rDS 2
Vo min  VDSat1  VDSat 2
Analog Integrated Circuits – Fundamental building blocks – Integrated current sources and sinks
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The cascode current source - biasing
 transistor biasing:
VGcas  VDSat 2  VTh 2  VDS 1

VGbias  VDSat1  VTh1
 how is VDS1 chosen ? → consider transistor operating regions as Vout decreases →
becomes important later in amplifier design for maximizing signal swing
 M1 is maintained in saturation by the constant VGS of M2 → if the voltage budget
permits, VDS1 is typically chosen (1.5-2)VDSat1
Analog Integrated Circuits – Fundamental building blocks – Integrated current sources and sinks
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The enhanced cascode current source
 very large Rout while keeping Vomin low → increse the gain of the negative feedback loop
 VDS1 is set by the constant voltage Vct at the positive amplifier input
Vout  VDS 1  VDS 2

Rout  rDS 1  rDS 2   a  1 g m 2  g mb 2  rDS 1rDS 2
V

I

g
V
r
m1 GS 1  DS 1
 DS 1  out

Vo min  VDSat1  VDSat 2
VDS 2   I out  g m 2VGS 2  g mb 2VBS 2  rDS 2
V   aV
DS 1
 G2
Analog Integrated Circuits – Fundamental building blocks – Integrated current sources and sinks
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Bibliography
 P.E. Allen, D.R. Holberg, CMOS Analog Circuit Design, Oxford University Press, 2002
 B. Razavi, Design of Analog CMOS Integrated Circuits, McGraw-Hill, 2002
 D. Johns, K. Martin, Analog Integrated Circuit Design, Wiley, 1996
 P.R.Gray, P.J.Hurst, S.H.Lewis, R.G, Meyer, Analysis and Design of Analog Integrated
Circuits, Wiley,2009
 R.J. Baker, CMOS Circuit Design, Layout and Simulation, 3rd edition, IEEE Press, 2010
Analog Integrated Circuits – Fundamental building blocks – Integrated current sources and sinks
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