Download Low Voltage Power Supply Current Source

ECE 607(Edgar Sanchez-Sinencio)
C
S
Low Voltage Power Supply
Current Source
M
A
• Simple implementation of a current source in many
applications including a tail current yields a low output
impedance.
• Cascode
C
d iimplementations
l
t ti
off currentt sources yields
i ld a larger
l
output impedance, however the trade-off is the reduction
on the headroom.
• One alternative implementation of a current source with
higher output impedance without sacrificing significant
overhead is presented next.
next
TAMU-AMSC
• The current source discussed here has an output resistance about 25 times
larger than that of a single transistor current source.
• This current source improves the common-mode input range and the CMRR
of differential pairs.
• Another use of this LV CS is for the voltage follower topologies.
Low Voltage CURRENT-SOURCE
Thi currentt source is
This
i applied
li d att the
th PMOS diff
differential
ti l pair
i ( M2i and
d M1i) att node
d Y
Vdd
M2
Z
M1
A
− +
X
I
Vi+
M1i
R0S
I
Y
IB
R0S
Vx
R0B
M2i
Vi-
(b)
(a)
Analog and Mixed-Signal Center (AMSC) TAMU
A conceptual Schematic of the low voltage current source. (a) Current source representation
( )A
(b)
Architecture
i
You, F., Embabi, H.K., Duque-Carrillo, J.F., Sanchez-Sinencio, E., “An improved tail current source for low voltage applications ,”
IEEE Journal of Solid-State Circuits , Volume: 32, Issue: 8 , Aug 1997, Page(s): 1173 –1180
R os =
where
1 + g m1A o /(g o1 + g oB)
g o 2(1+ Aog m1 /(g o1 + g
g m1 (g m 2 ), g o1 (g o 2 )
respectively.
Ao
)− Aog m 2 / g o 2 )
are the transconductance and output conductance of M1
is the DC gain of the error amplifier “A” and
conductance
d t
((resistance)
it
) off the
th reference
f
currentt source
g o1 = g o 2
oB
(1)
I B.
g oB (R oB )
A
Assuming
i that
th t
(M 2 ) ,
is the output
g m1 = g m 2
and
d
, equation (1) can be simplified as:
R os ≈ − R oB
B
(2)
Note that the resistance is negative and is equal to the resistance of the reference source
R os ≈ −
g m4
g o3g o 4
I B.
(3)
Analog and Mixed-Signal Center (AMSC) TAMU
Vdd
Z
M2
Cc
X
Io
M1
Y
R0S
M4
Vb
IB
ID
M3
V
Current Ref.
Vss
Error Amplifier A
Full implementation of the LV current source.
Analog and Mixed-Signal Center (AMSC) TAMU
FREQUENCY RESPONSE OF THE TAIL CURRENT OUTPUT IMPEDANCE
(a) WITHOUT DIFFERENTIAL PAIR AND (b) WITH DIFFERENTIAL PAIR
• This LV Current Sources has one negative feedback loop and one positive feedback loop.
• The
Th negative
ti feedback
f db k ( consisting
i ti off the
th error amplifier
lifi A andd transistor
t
i t M1)must
M1)
t be
b
greater than the positive( which consists of A and transistor M2) to have stability.
• The analysis is based on the small signal equivalent circuit shown in the next slide.
• Cx
C andd Cy
C are th
the parasitic
iti capacitances
it
att nodes
d X andd Y.
Y
• goB is the conductance of the current source IB
• The error amplifier is characterized by one dominant pole at CZ/goa
Open loop transfer function with Cc
•
•
•
The frequency response is simulated for two cases; i) With a simple transistor
tail current and ii) using the proposed LV current source.
Th effect
The
ff t off Rc
R is
i shown
h
in
i curve ( c )
Fig 5 (a) does not have Cc, thus its poor phase margin. Adding Rc to CC in series yields Fig 5( c )
Vdd
I
Vb
Vi1
M6
Vi2
Mi1
Mi 2
Vout
Cc
M3
M4
M5
Vss
U off the
Use
h proposed
d current source iin simple
i l two stage amplifier.
lifi
Cc has a resistor in series Rc to provide better stability.
Analog and Mixed-Signal Center (AMSC) TAMU
Frequency response of two stage amplifiers using LV current source (dashed)
and simple current source (solid).
Analog and Mixed-Signal Center (AMSC) TAMU
Measured output current of the simple (curve A) and LV (Curve B) current source.
Analog and Mixed-Signal Center (AMSC) TAMU
Measured I-V characteristics of the LV current source.
Analog and Mixed-Signal Center (AMSC) TAMU
LOW VOLTAGE CURRENT SOURCES
VZ
M2
M1
A
B2
B1
X
Vi+
Y
Ros
M1i
M2i
rBref
IBref
Vi-
• Amplifier “A” forces the VDS of M1 and
M2 to be equal. Thus IB1 = IB2 .
• R os = − R Bref
MAX
MAX
• VREF = Vy = VDD − VSG ,AUX − VSD,AUX
• Large variations of VZ are caused when
M1 and M2 enter in triode mode
M1 and M2 enter in triode mode.
Using an auxiliary differential amplifier LV Current Source
MA4
MA3
VZ
Vy
A
z
Vx
Ao
A(s ) =
1 + s CZ g Oa
1+
VX
MA1
MA2
IB
14
An Alternative Approach for a LV Current Source*
• The auxiliary amplifier is implemented by NMOS cascoding transistor MCP
connected between node Z and node y.
Thus
A=
VZ
= g m MCP ro MCP
VY
Vy = VCP + VSG
Then
VyMAX = VDD − VDSSAT
Vicm can be extended to the upper rail V MAX
icm = VDD
* J. Ramirez-Angulo, R. G. Carvajal and A. J. Lopez-Martin, “Single Transistor High Impedance Tail Current Source with Extended
Common-Mode Input Range and Reduced Supply Requirements”, IEEE Trans. on Circuits and Systems II, Vol. 54, No. 7,
pp. 581-585, July 2007.
MCB
VBC
• To reduce mismatch betweeen
VDS1 and VDS2 a diode connected
NMOS MDP is inserted.
IBC
VZ
M2
MDP
M1
MCP
Vy
-
v in
M1i MBP
IBREF + IBC
VBP
M2i
+
v in
• Implementation of an enhanced LV current source where amplifier is implemented
byy a simple
p cascodingg (two
(
transistor MCB and MBP))
• (W L )1i , 2i = (W L )
• (W L )1, 2 = 2(W L )
• (W L )MPB = 4(W L )