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