Current-Mode : an Overview
J-F Perotto, CSEM & EICN
jean-felix.perotto@csem.ch
Technology Leadership Day 2000
Current Mode: an Overview
© J-F Perotto, EICN
Definitions
Voltage-Mode
Usually, information is represented by voltage at the nodes of the circuit.
Current-Mode
An other way is to represent information by current flowing in the branches of
the circuit.
Technology Leadership Day 2000
Current Mode: an Overview
© J-F Perotto, EICN
In integrated circuits world,
Current-Mode offers some advantages over Voltage-Mode :
• performances improvement
• structural advantages
• specific features
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high speed
low power consumption at high frequency
high signal dynamic range
low cross-talk & switching noise
controlled gain without feedback components
current summing without components
schematic simplicity
well suited for low voltage applications
pseudoconductance networks
current switching technique
Current Mode: an Overview
© J-F Perotto, EICN
Current-Mode in analog continous-time circuits
Two examples :
• Finite gain amplifier
• Pseudoconductance network
Technology Leadership Day 2000
Current Mode: an Overview
© J-F Perotto, EICN
Voltage-Mode Amplifier : needs a low output resistance
to drive the parasitic capacitance.
Idd
+
Fc =
Rout
Uin
2π Rout Cout
Cout
-
High FC ⇒ low Rout ⇒
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1
Uout
high Idd or complex B-class output stage
needs relatively high supply voltage
Current Mode: an Overview
© J-F Perotto, EICN
Voltage-Mode Amplifier : needs high value feedback and
ponderation resistors.
Uin1
Uin2
Uin3
R4
R1
R2
+
R3
-
R1 .. R4 >> Rout ⇒
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Cout
Uout
large silicon area to implement the resistors
high parasitic capacitance ⇒ low FC , instabilities
Current Mode: an Overview
© J-F Perotto, EICN
Current-Mode Amplifier : - quasi-infinite output resistance,
- no feedback or summing components,
- simplicity
Vdd
Iin1
I0
GI0
Iout = G (Iin1 + Iin2 + Iin3)
Iin2
Iin3
S1
S2
Cout
• gain G is given only by the transistors aspect ratio S=W/L : G = S2/S1
• high Fc due to ∆us = 0
• amplifier is inconditionally stable
Technology Leadership Day 2000
Current Mode: an Overview
© J-F Perotto, EICN
Pseudoconductance
MOS current can be expressed as :
Which can be rewrited as:
i = I S [ f (Vg ,Vd ) − f (Vg ,Vs )]
i = G * (VA* − VB* )
G* =
IS
V0
V0 is an arbitrary voltage
giving G* and V* conductance
and voltage dimension
V * = V0 f (Vg ,V )
i = G (VA − VB )
VA
2
with I S = 2nβU T
G
VB
i
*
*
A
*
B
i = G (V − V )
G* V
B
VA
i
From the current point-of-view, MOS transistor behaves as a resistor 1/G
which only depends of β and Vg
Technology Leadership Day 2000
Current Mode: an Overview
© J-F Perotto, EICN
Pseudoconductance (cont.)
In strong inversion :
V − VD 2
V − VS 2
2nβU T2
i >> I S : i =
[V0 ( sat
) − V0 ( sat
) ]
V0
2U T
2U T
G* = cte
VA*
G*
VB*
In weak inversion :
V
−V
−V
D
S
2nβU T2 UsatT
i << I S : i =
e [V0e UT − V0e UT ]
V0
G*(Vg)
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VA*
G*(Vg)
VB*
Current Mode: an Overview
© J-F Perotto, EICN
Linear network with pseudoconductances
Example : R-2R lattice AD converter
I
I
2G
G
I/2
2G
G
I/4
2G
G
I/8
G
Vpol
G
β
I/16
I/2
Iout
β
I/4
2β
β
β
β
I/8
I/16
Iout
Version « resistor based »
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2β
2β
Version « transistor based »
Current Mode: an Overview
© J-F Perotto, EICN
Current-Mode in analog discrete-time circuits
(Switched-Current technique)
Three examples :
• current integrator
• current biquadratic filter
• switched-current ∆ modulator
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Current Mode: an Overview
© J-F Perotto, EICN
Classic Voltage-Mode Track-and-Hold
Φ
follower
+
UIN
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C
-
Current Mode: an Overview
UOUT
© J-F Perotto, EICN
Current-Mode Track-and-Hold
Vdd
Φ
IBIAS
IBIAS
IIN
IOUT
Φ
IIN
T&H
IOUT
symbol
Phase 1 : Φ = ON
Track (copy IIN into IOUT)
Phase 2 : Φ = OFF
Hold (IOUT latched)
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Current Mode: an Overview
© J-F Perotto, EICN
Classic Voltage-Mode Integrator
C
Resistor version
R
-
UIN
Switch-cap version
+
UOUT
C
Φ
Φ
-
UIN
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αC
Current Mode: an Overview
+
UOUT
© J-F Perotto, EICN
Current-Mode Integrator
Φ
IIN
T&H
Φ
FS
current duplicator
IOUT
T&H
IIN
IOUT
1
FS
s
IOUT
symbol
MOS implementation : 12 transistors
I OUT ( n) = I OUT (n − 1) + I IN (n)
1
H ( z) =
z −1
z-domain
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OSR>>1
H ( s ) ≈ FS
1
s
Laplace
Current Mode: an Overview
© J-F Perotto, EICN
Application: Biquadratic Filter
Vdd
1
a0
FS
IIN
FS
1
s
b0
a2 s 2 + a1 FS s + a0 FS2
H (s) = 2
s + b1 FS s + b0 FS2
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a1
a2
FS
FS
1
s
b1
IOUT
1
1
Such a filter is fully programmable by simply adjusting
the aspect ratio ai and bi of the coefficient transistors
Current Mode: an Overview
© J-F Perotto, EICN
Application : 1st Order 1 Bit ∆ Modulator
Vdd
IREF
FS
IIN
act as current
sign detector
1
FS
s
D Q
Output bit stream
F ’S
-IREF
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Current Mode: an Overview
© J-F Perotto, EICN
Conclusion
• Current-mode is well adapted to low power – low voltage integrated circuits,
• It often conducts to very simpler solutions than voltage-mode approaches,
• It is the way to realize analog functions in submicron technologies, which are
largely oriented toward digital performances.
Technology Leadership Day 2000
Current Mode: an Overview
© J-F Perotto, EICN