Download A Low-Voltage MOS Cascode Current Mirror for All Current Levels

A Low-Voltage MOS Cascode Current Mirror
for All Current Levels
Bradley A. Minch
Mixed Analog-Digital VLSI Circuits and Systems Lab
Cornell University
Ithaca, NY 14853–5401
[email protected]
August 6, 2002
CORNELL
1
Cascode Mirrors of Yore With Low Systematic Transfer Errors
• Each of these mirrors is self biasing, has
a high output impedance, and provides
a low systematic transfer error.
Iin
Vin
Iin
Iout
Vin
Iout
• Each requires an input voltage of two
diode drops.
• Each has an output compliance voltage
of a diode drop plus a saturation voltage.
• Neither is suitable for use with a low
power supply voltage.
Stacked
CORNELL
Super Wilson
2
Wide-Swing Cascode Mirrors With Low Systematic Transfer Error
• Each of these mirrors is self biasing, has
a high output impedance, and provides
a low systematic transfer error.
• Each has an output compliance voltage
of two saturation voltages.
Iin
Vin
m
Iin
Vin
• The Sooch mirror requires an input
voltage of two diode drops, which
makes it unsuitable for low-voltage
applications.
R
Iout
Iout
• The Brooks-Rybicki mirror requires an
input voltage of a diode drop plus
a saturation voltage, but requires a
different value of R for every Iin .
Sooch
CORNELL
Brooks & Rybicki
2
Wide-Swing Cascode Mirrors With Low Systematic Transfer Error
• To facilitate low-voltage operation, we can
remove the cascode bias-voltage generation
from the input branch.
• The output compliance voltage remains two
saturation voltages.
Iin
Vin
Iout
Vcn
• The input voltage becomes a diode drop,
comparable to that of a simple mirror.
• Iin is limited to Ic and the optimal value of Vcn
depends on Iin , which sometimes requires us to
generate Vcn adaptively.
CORNELL
Babanezhad & Gregorian
Ic
2
Wide-Swing Cascode Mirrors With Low Systematic Transfer Error
• To facilitate low-voltage operation, we can
remove the cascode bias-voltage generation
from the input branch.
• The output compliance voltage remains two
saturation voltages.
Iin
Vin
Iout
Vcn
• The input voltage becomes a diode drop,
comparable to that of a simple mirror.
• Iin is limited to Ic and the optimal value of Vcn
depends on Iin , which sometimes requires us to
generate Vcn adaptively.
CORNELL
Babanezhad & Gregorian
Ic
2
Wide-Swing Cascode Mirrors With Low Systematic Transfer Error
• Mulder et al. recently proposed a wide-swing
cascode mirror comprising a simple mirror
with source degeneration via ohmic MOS
transistors.
• The output compliance voltage remains two
saturation voltages.
• The input voltage is a diode drop plus a
saturation voltage.
• Iin is limited to Ib and the optimal value of Vbn
depends critically on Iin , which mandates that
we generate Vbn adaptively.
CORNELL
Iin
Vin
Iout
Ib
Vbn
Mulder et al.
2
Wide-Swing Cascode Mirrors With Low Systematic Transfer Error
• Mulder et al. recently proposed a wide-swing
cascode mirror comprising a simple mirror
with source degeneration via ohmic MOS
transistors.
• The output compliance voltage remains two
saturation voltages.
• The input voltage is a diode drop plus a
saturation voltage.
• Iin is limited to Ib and the optimal value of Vbn
depends critically on Iin , which mandates that
we generate Vbn adaptively.
CORNELL
Iin
Vin
Iout
Ib
Vbn
Mulder et al.
3
Development of a New Low-Voltage Cascode Current Mirror
• We recently presented a low-voltage cascode
bias circuit that generates a Vcn appropriate
for a unit-width transistor with a channel
current of about Ic.
• The degree to which the bottom transistor is
saturated depends on m and Ic/Ib —the larger
these values, the closer V is to VDSsat .
• Vcn is about a diode drop plus a saturation
voltage.
CORNELL
Ib
Ic
Vcn
m
V
Ic+Ib
3
Development of a New Low-Voltage Cascode Current Mirror
• Suppose we take this circuit and make Ic the
input current.
Ib
Iin
• Then, we produce an output current by adding
two transistors, as shown.
• In this mirror, V will be slightly lower than
V , giving rise to a systematic transfer errror.
In fact, it is easy to see that Iin < Iout <
Iin + Ib . If Iin Ib , this systematic error is
negligible.
CORNELL
Vin
m
V
Iin+Ib
3
Development of a New Low-Voltage Cascode Current Mirror
• Suppose we take this circuit and make Ic the
input current.
Ib
Iin
• Then, we produce an output current by adding
two transistors, as shown.
• In this mirror, V will be slightly lower than
V , giving rise to a systematic transfer errror.
In fact, it is easy to see that Iin < Iout <
Iin + Ib . If Iin Ib , this systematic error is
negligible.
CORNELL
Iout
Vin
m
V
Iin+Ib
V¢
Iout+
3
Development of a New Low-Voltage Cascode Current Mirror
• Suppose we take this circuit and make Ic the
input current.
Ib
Iin
• Then, we produce an output current by adding
two transistors, as shown.
• In this mirror, V will be slightly lower than
V , giving rise to a systematic transfer errror.
In fact, it is easy to see that Iin < Iout <
Iin + Ib . If Iin Ib , this systematic error is
negligible.
CORNELL
Iout
Vin
m
V
Iin+Ib
V¢
Iout+
3
Development of a New Low-Voltage Cascode Current Mirror
• If we make V equal to V , we could eliminate
the systematic gain error, but we would
effectively disable the cascode.
• Instead, we can inject another copy of Ib into
node V , as shown.
• If Ib is generated by a saturated pMOS
transistor, we can improve the circuit further
by adding a diode-connected transistor of
width m, as shown.
CORNELL
Ib
Iin
Vin
m
Iout = Iin
Ib
V
Iin+Ib
Iin+Ib
V
3
Development of a New Low-Voltage Cascode Current Mirror
• If we make V equal to V , we could eliminate
the systematic gain error, but we would
effectively disable the cascode.
• Instead, we can inject another copy of Ib into
node V , as shown.
• If Ib is generated by a saturated pMOS
transistor, we can improve the circuit further
by adding a diode-connected transistor of
width m, as shown.
CORNELL
Ib
Iin
Ib
Iout
Vin
m
V
Iin+Ib
V
Iin+Ib
3
Development of a New Low-Voltage Cascode Current Mirror
• If we make V equal to V , we could eliminate
the systematic gain error, but we would
effectively disable the cascode.
• Instead, we can inject another copy of Ib into
node V , as shown.
• If Ib is generated by a saturated pMOS
transistor, we can improve the circuit further
by adding a diode-connected transistor of
width m, as shown.
CORNELL
Ib
Iin
Ib
Iout
Vin
m
V
Iin+Ib
m
V
Iin+Ib
3
Development of a New Low-Voltage Cascode Current Mirror
• The resulting mirror has a low systematic
transfer error and a high output impedance.
Ib
Iin
• The output compliance voltage of this mirror
is two saturation voltages.
• The input voltage of this mirror is a diode
drop plus a saturation voltage.
• The bias current, Ib, does not represent an
upper limit on Iin and does not need to track
Iin adaptively.
CORNELL
Ib
Iout
Vin
m
V
Iin+Ib
m
V
Iin+Ib
4
Experimental Results: Input Characteristics
4.5
4
3.5
Vin (V)
3
simple mirror
stacked mirror
Sooch mirror
new mirror, m = 4, Ib = Iin
new mirror, m = 4, Ib = Iin/10
new mirror, m = 4, Ib = Iin/100
2.5
2
1.5
1
0.5
0 -9
10
CORNELL
10
-8
-7
10
-6
10
Iin (A)
-5
10
-4
10
-3
10
4
Experimental Results: Output Characteristics Iin = 1.00 nA
1.12
simple mirror
stacked mirror
Sooch mirror
new mirror, m = 4, Ib = Iin
new mirror, m = 4, Ib = Iin/10
new mirror, m = 4, Ib = Iin/100
1.11
1.1
Iout (nA)
1.09
1.08
1.07
1.06
1.05
1.04
1.03
1.02
0
CORNELL
0.5
1
1.5
2
2.5
3
Vout (V)
3.5
4
4.5
5
4
Experimental Results: Output Characteristics Iin = 10.0 nA
11.1
simple mirror
stacked mirror
Sooch mirror
new mirror, m = 4, Ib = Iin
new mirror, m = 4, Ib = Iin/10
new mirror, m = 4, Ib = Iin/100
11
10.9
Iout (nA)
10.8
10.7
10.6
10.5
10.4
10.3
10.2
10.1
0
CORNELL
0.5
1
1.5
2
2.5
3
Vout (V)
3.5
4
4.5
5
4
Experimental Results: Output Characteristics Iin = 100. nA
111
simple mirror
stacked mirror
Sooch mirror
new mirror, m = 4, Ib = Iin
new mirror, m = 4, Ib = Iin/10
new mirror, m = 4, Ib = Iin/100
110
109
Iout (nA)
108
107
106
105
104
103
102
101
0
CORNELL
0.5
1
1.5
2
2.5
3
Vout (V)
3.5
4
4.5
5
4
Experimental Results: Output Characteristics Iin = 1.00 µA
1.1
simple mirror
stacked mirror
Sooch mirror
new mirror, m = 4, Ib = Iin
new mirror, m = 4, Ib = Iin/10
new mirror, m = 4, Ib = Iin/100
1.09
1.08
Iout (mA)
1.07
1.06
1.05
1.04
1.03
1.02
1.01
1
0
CORNELL
0.5
1
1.5
2
2.5
3
Vout (V)
3.5
4
4.5
5
4
Experimental Results: Output Characteristics Iin = 10.0 µA
10.7
10.6
10.5
Iout (mA)
10.4
10.3
10.2
10.1
simple mirror
stacked mirror
Sooch mirror
new mirror, m = 4, Ib = Iin
new mirror, m = 4, Ib = Iin/10
new mirror, m = 4, Ib = Iin/100
10
9.9
9.8
9.7
0
CORNELL
0.5
1
1.5
2
2.5
3
Vout (V)
3.5
4
4.5
5
4
Experimental Results: Output Characteristics Iin = 100. µA
105
104
103
Iout (mA)
102
101
simple mirror
stacked mirror
Sooch mirror
new mirror, m = 4, Ib = Iin
new mirror, m = 4, Ib = Iin/10
new mirror, m = 4, Ib = Iin/100
100
99
98
97
96
95
0
CORNELL
0.5
1
1.5
2
2.5
3
Vout (V)
3.5
4
4.5
5
4
Experimental Results: Output Characteristics Iin = 1.00 mA
1.01
1
0.99
Iout (mA)
0.98
0.97
simple mirror
stacked mirror
Sooch mirror
new mirror, m = 4, Ib = Iin
new mirror, m = 4, Ib = Iin/10
new mirror, m = 4, Ib = Iin/100
0.96
0.95
0.94
0.93
0.92
0.91
0
CORNELL
0.5
1
1.5
2
2.5
3
Vout (V)
3.5
4
4.5
5