alow-voltage mos cascode current mirror for all current...

A Low-Voltage MOS Cascode Current Mirrorfor All Current Levels

Bradley A. Minch

Mixed Analog-Digital VLSI Circuits and Systems LabCornell University

Ithaca, NY 14853–5401

[email protected]

August 6, 2002

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Cascode Mirrors of Yore With Low Systematic Transfer Errors

• Each of these mirrors is self biasing, hasa high output impedance, and providesa low systematic transfer error.

• Each requires an input voltage of twodiode drops.

• Each has an output compliance voltageof a diode drop plus a saturation voltage.

• Neither is suitable for use with a lowpower supply voltage.

Iin

IoutVin

Iin

IoutVin

Stacked Super Wilson

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Wide-Swing Cascode Mirrors With Low Systematic Transfer Error

• Each of these mirrors is self biasing, hasa high output impedance, and providesa low systematic transfer error.

• Each has an output compliance voltageof two saturation voltages.

• The Sooch mirror requires an inputvoltage of two diode drops, whichmakes it unsuitable for low-voltageapplications.

• The Brooks-Rybicki mirror requires aninput voltage of a diode drop plusa saturation voltage, but requires adifferent value of R for every Iin.

Iout

Iin

Vin

m

Iout

Iin

Vin

R

Sooch Brooks & Rybicki

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• To facilitate low-voltage operation, we canremove the cascode bias-voltage generationfrom the input branch.

• The output compliance voltage remains twosaturation voltages.

• 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 togenerate Vcn adaptively.

Iout

Iin

Vin

Vcn

Ic

Babanezhad & Gregorian

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• Mulder et al. recently proposed a wide-swingcascode mirror comprising a simple mirrorwith source degeneration via ohmic MOStransistors.

• The output compliance voltage remains twosaturation voltages.

• The input voltage is a diode drop plus asaturation voltage.

• Iin is limited to Ib and the optimal value of Vbn

depends critically on Iin, which mandates thatwe generate Vbn adaptively.

Iout

Iin

Vin

Vbn

Ib

Mulder et al.

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Development of a New Low-Voltage Cascode Current Mirror

• We recently presented a low-voltage cascodebias circuit that generates a Vcn appropriatefor a unit-width transistor with a channelcurrent of about Ic.

• The degree to which the bottom transistor issaturated depends on m and Ic/Ib—the largerthese values, the closer V is to VDSsat.

• Vcn is about a diode drop plus a saturationvoltage.

m

Ib Ic

Ic+Ib

V

Vcn

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• Suppose we take this circuit and make Ic theinput current.

• Then, we produce an output current by addingtwo transistors, as shown.

• In this mirror, V ′ will be slightly lower thanV , 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 isnegligible.

m

Ib Iin

Iin+Ib

V

Vin

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• Suppose we take this circuit and make Ic theinput current.

• Then, we produce an output current by addingtwo transistors, as shown.

• In this mirror, V ′ will be slightly lower thanV , 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 isnegligible.

m

Ib Iin

Iin+Ib

V

IoutVin

Iout+V¢

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• If we make V ′ equal to V , we could eliminatethe systematic gain error, but we wouldeffectively disable the cascode.

• Instead, we can inject another copy of Ib intonode V ′, as shown.

• If Ib is generated by a saturated pMOStransistor, we can improve the circuit furtherby adding a diode-connected transistor ofwidth m, as shown.

m

Ib Iin

Iin+Ib

V

Iout = IinVin

Iin+Ib

V

Ib

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Ib

m

Ib Iin

Iin+Ib

V

IoutVin

Iin+Ib

V

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Ib

m

Ib Iin

Iin+Ib

V

IoutVin

m

Iin+Ib

V

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• The resulting mirror has a low systematictransfer error and a high output impedance.

• The output compliance voltage of this mirroris two saturation voltages.

• The input voltage of this mirror is a diodedrop plus a saturation voltage.

• The bias current, Ib, does not represent anupper limit on Iin and does not need to trackIin adaptively.

Ib

m

Ib Iin

Iin+Ib

V

IoutVin

m

Iin+Ib

V

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Experimental Results: Input Characteristics

simple mirrorstacked mirrorSooch mirrornew mirror, m = 4, Ib = Iin

new mirror, m = 4, Ib = Iin/10new mirror, m = 4, Ib = Iin/100

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Iin (A)

Vin

(V

)

10-9 10-8 10-7 10-6 10-5 10-4 10-3

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Experimental Results: Output Characteristics Iin = 1.00 nA

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 51.02

1.03

1.04

1.05

1.06

1.07

1.08

1.09

1.1

1.11

1.12

Vout (V)

I out

(nA

)



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Experimental Results: Output Characteristics Iin = 10.0 nA

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 510.1

10.2

10.3

10.4

10.5

10.6

10.7

10.8

10.9

11

11.1

Vout (V)

I out

(nA

)



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Experimental Results: Output Characteristics Iin = 100.nA

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5101

102

103

104

105

106

107

108

109

110

111

Vout (V)

I out

(nA

)



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Experimental Results: Output Characteristics Iin = 1.00µA

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 51

1.01

1.02

1.03

1.04

1.05

1.06

1.07

1.08

1.09

1.1

Vout (V)

I out

(m

A)



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Experimental Results: Output Characteristics Iin = 10.0µA

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 59.7

9.8

9.9

10

10.1

10.2

10.3

10.4

10.5

10.6

10.7

Vout (V)

I out

(m

A)



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Experimental Results: Output Characteristics Iin = 100.µA

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 595

96

97

98

99

100

101

102

103

104

105

Vout (V)

I out

(m

A)



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Experimental Results: Output Characteristics Iin = 1.00 mA

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50.91

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1

1.01

Vout (V)

I out

(m

A)



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alow-voltage mos cascode current mirror for all current...

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