current sources
Post on 13-Jan-2016
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Current Sources
Ever wonder howwe make one of these?
Current Sources
Ever wonder howwe make one of these?
GND
Vb M5
Vout
Iout
CurrentSink
Current Sources
Ever wonder howwe make one of these?
GND
Vb M5
Vout
Iout
CurrentSink
V1
Vdd
M6
Iout
CurrentSource
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
BJT Current Source (Sink)
Current Source Concepts
Cascode SourceGate Overdrive
Von
Current Mirrors
GNDGND
Iin
Vb M5Mb
Vout
Iout
nFET Current Mirror
Current Mirrors
GNDGND
Iin
Vb M5Mb
Vout
Iout
Iout = ( (W/L)5 / (W/L)b ) Iin
nFET Current Mirror
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
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
Current Mirror
GNDGND
Iin
Vb
M5
Mb
Vout1
Iout1
Iout = ( (W/L)5 / (W/L)b ) Iin
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
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 )
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 )
BJT Current Mirror
GND
Iin
GNDGND
Iout
Vi
C
No base current effects, same emitter areas: I
out = I
in
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/ ) )
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/ ) )
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
Basic MOS Cascode Source(Sink)
An Improved Source (Sink)
One Further Improvement
Adding Gain Increase Ro
Low Saturation Voltage Source
Current Source Design
SPICE Results
Current Sources
• Basic concepts
• Cascode Circuits
• Advanced Cascode Circuits
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