Current Sources
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Transcript of Current Sources
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Current Sources
Ever wonder howwe make one of these?
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Current Sources
Ever wonder howwe make one of these?
GND
Vb M5
Vout
Iout
CurrentSink
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Current Sources
Ever wonder howwe make one of these?
GND
Vb M5
Vout
Iout
CurrentSink
V1
Vdd
M6
Iout
CurrentSource
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Current Sources
Ever wonder howwe make one of these?
GND
Vb M5
Vout
Iout
CurrentSink
V1
Vdd
M6
Iout
CurrentSource
How “good” a current source? Early Effect
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BJT Current Source (Sink)
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Current Source Concepts
Cascode SourceGate Overdrive
Von
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Current Mirrors
GNDGND
Iin
Vb M5Mb
Vout
Iout
nFET Current Mirror
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Current Mirrors
GNDGND
Iin
Vb M5Mb
Vout
Iout
Iout = ( (W/L)5 / (W/L)b ) Iin
nFET Current Mirror
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Current Mirrors
GNDGND
Iin
Vb M5Mb
Vout
Iout
Iout = ( (W/L)5 / (W/L)b ) Iin
nFET Current Mirror pFET Current Mirror
Vdd Vdd
Vb
Iin
Iout
M7M4
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Current Mirrors
GNDGND
Iin
Vb M5Mb
Vout
Iout
Iout = ( (W/L)5 / (W/L)b ) Iin
nFET Current Mirror
A good way to generate a bias current
pFET Current Mirror
Iout = ( (W/L)7 / (W/L)4 ) Iin
Vdd Vdd
Vb
Iin
Iout
M7M4
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Current Mirror
GNDGND
Iin
Vb
M5
Mb
Vout1
Iout1
Iout = ( (W/L)5 / (W/L)b ) Iin
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Current Mirror
GNDGND
Iin
Vb
M5
Mb
Vout1
Iout1
GND
M6
Vout2
Iout2
GND
M7
Vout3
Iout3
Iout = ( (W/L)5 / (W/L)b ) Iin
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Current Mirror
GNDGND
Iin
Vb
M5
Mb
Vout1
Iout1
GND
M6
Vout2
Iout2
GND
M7
Vout3
Iout3
Iout = ( (W/L)5 / (W/L)b ) Iin Iout / Iin =
( (W/L)6 / (W/L)b )
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Current Mirror
GNDGND
Iin
Vb
M5
Mb
Vout1
Iout1
GND
M6
Vout2
Iout2
GND
M7
Vout3
Iout3
Iout = ( (W/L)5 / (W/L)b ) Iin Iout / Iin =
( (W/L)6 / (W/L)b )
Iout / Iin =
( (W/L)7 / (W/L)b )
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BJT Current Mirror
GND
Iin
GNDGND
Iout
Vi
C
No base current effects, same emitter areas: I
out = I
in
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BJT Current Mirror
GND
Iin
GNDGND
Iout
Vi
C
No base current effects, same emitter areas: I
out = I
in
Base current, identical , same emitter areas: I
out = I
in / (1 + (1/ ) )
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BJT Current Mirror
GND
Iin
GNDGND
Iout
Vi
C
No base current effects, same emitter areas: I
out = I
in
Base current, identical , same emitter areas: I
out = I
in / (1 + (1/ ) )
Base current, n outputs, identical , same emitter areas: I
out = I
in / (1 + (1/ ) )
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Diode-Capacitor Dynamics
C (dVi/dt) = I
in - Iso exp(V
i/U
T)
Iout
= Ico
exp(Vi/U
T)
(C / Iout
) (d Iout
/dt) = Iin - I
out
C (d Iout
/dt) = Iout
( Iin - I
out )
GND
Iin
GNDGND
Iout
Vi
C
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Basic MOS Cascode Source(Sink)
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An Improved Source (Sink)
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One Further Improvement
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Adding Gain Increase Ro
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Low Saturation Voltage Source
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Current Source Design
SPICE Results
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Current Sources
• Basic concepts
• Cascode Circuits
• Advanced Cascode Circuits