Download Basic Current Mirrors •Cascode Current Mirrors

Current Mirrors
•Basic Current Mirrors
•Cascode Current Mirrors
Hassan Aboushady
University of Paris VI
References
• B. Razavi, “Design of Analog CMOS Integrated Circuits”,
McGraw-Hill, 2001.
H. Aboushady
University of Paris VI
1
Copying Currents
• Definition of current by a resistive divider
I out ≈
µ nCox W ⎛
2
⎞
R2
⎜⎜
VDD − VTH ⎟⎟
L ⎝ R1 + R2
⎠
2
• Iout depends upon supply, process and temperature.
• A relatively complex circuit is used to
generate a stable reference current IREF
• IREF is then copied to the different
current sources of the system.
H. Aboushady
University of Paris VI
Basic Current Mirror
• How to guarantee that Iout = IREF ?
• 2 identical MOS devices that have equal gate-source voltages
and operating in saturation carry equal currents (assuming λ=0).
I REF =
I out =
H. Aboushady
µ nCox ⎛ W ⎞
2
⎜ ⎟ (VGS − VTH )
⎝ L ⎠1
µ nCox ⎛ W ⎞
2
2
⎜ ⎟ (VGS − VTH )
⎝ L ⎠2
I out =
(W / L )2 I
(W / L )1 REF
2
University of Paris VI
2
Taking channel Length modulation into account
I D1 =
I D2 =
µ nCox ⎛ W ⎞
2
⎜ ⎟ (VGS − VTH ) (1 + λVDS 1 )
⎝ L ⎠1
2
µ nCox ⎛ W ⎞
2
⎜ ⎟ (VGS − VTH ) (1 + λVDS 2 )
⎝ L ⎠2
2
VDS1 ≠ VDS 2
If
I D1 (W / L )2 (1 + λVDS 2 )
=
I D 2 (W / L )1 (1 + λVDS 1 )
⇒ I D1 ≠ I D 2
H. Aboushady
University of Paris VI
Cascode Current Mirror
VGS 0 + VX = VGS 3 + VY
if
(W / L )3 = (W / L )2
(W / L )0 (W / L )1
Then VGS 3 = VGS 0
and VY = V X
H. Aboushady
University of Paris VI
3
Limited Voltage Swing
VDS 3 = VGS 3 − VTH
VP min − VY = VN − VY − VTH
VP min = VN − VTH
VP min = VGS 1 + VGS 0 − VTH
VP min = (VGS1 − VTH ) + (VGS 0 − VTH ) + VTH
• Low Voltage Cascode Current Mirror:
VP min = (VGS 3 − VTH ) + (VGS 4 − VTH )
H. Aboushady
University of Paris VI
4