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  • Sam Palermo Analog & Mixed-Signal Center

    Texas A&M University

    ECEN474: (Analog) VLSI Circuit Design Fall 2012

    Lecture 17: Fully Differential Amplifiers & CMFB

  • Announcements & Agenda

    Preliminary report due 11/19

    Fully differential circuits Common-mode feedback circuits

    2

  • TAMU-Elen-474 Jose Silva-Martinez-08

    - 3 -

    v I + v I - M2 M1

    I TAIL

    v O - v I + v I - v O - M2 M1

    I TAIL

    v O + v O +

    M2 v I + v I -

    i o v I - v I +

    i o M1 M2 M1

    I TAIL I TAIL

    v O

    (a) (b)

    (c) (d)

    v O

    I B I B

    Basic Operational Transconductance Amplifier Topologies

    SINGLE-ENDED

    FULLY-DIFFERENTIAL

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 4 -

    Fully-Differential Circuits

    - vo1 vin1

    -

    vo2 vin2 - +

    +

    In general: Hence

    ocodoooo

    o

    ocodoooo

    o

    vvvvvvv

    vvvvvvv

    +=+

    +

    =

    +=+

    +

    =

    222

    222

    21122

    21211

    =

    ic

    id

    cccd

    dcdd

    oc

    odvv

    AAAA

    vv

    0Vidic

    oddc

    0Vicid

    oddd v

    vAvvA

    ==

    ==

    Differential-mode output Common-mode output

    0Vidic

    occc

    0Vicid

    occd v

    vAvvA

    ==

    ==

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 5 -

    Fully-Differential Filters: Effects of current source inpedance and mismatches

    - vo1 vin1

    -

    vo2 vin2 - +

    +

    A very important parameter: dc

    ddAACMRR =

    +

    ++=

    +

    ++=

    icm

    sid

    m

    s

    smm

    mm

    icm

    sid

    m

    s

    smm

    mm

    vgYv

    gY

    YggZggv

    vgYv

    gY

    YggZggv

    1121

    22102

    2221

    12101

    21

    21

    Example:

    2IB

    IB IB

    v1 v2

    v01 v02

    Z2 Z1

    M1 M2

    Solving the circuit:

    Ys is the admittance associated with the current source 2IB

    212

    12ii

    iciiidvvvvvv +== and w/

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 6 -

    Fully-Differential Filters: Non-idealities

    2IB

    IB IB

    v1 v2

    v01 v02

    Z2 Z1 M1 M2

    Voltage gain: Note the effects of the mismatches, especially in Adc and Acd

    ( )

    ( )

    ( )( )

    +

    ++=

    ++

    =

    +

    ++=

    +

    =

    ++=

    +

    =

    +++

    ++=

    =

    =

    =

    =

    =

    2

    1

    1

    2

    21

    21

    012

    12

    1

    2

    2

    121

    21

    21

    012

    12

    2

    1

    1

    2

    21

    21

    012

    21

    1

    2

    2

    121

    21

    21

    012

    21

    21

    22

    2212

    2

    2

    mms

    smm

    mm

    vii

    oocc

    mm

    s

    smm

    mm

    vii

    oocd

    mms

    smm

    mm

    vii

    oodc

    mm

    s

    smm

    mm

    vii

    oodd

    gZ

    gZY

    Ygggg

    vvvvA

    gZ

    gZYZZ

    Ygggg

    vvvvA

    gZ

    gZY

    Ygggg

    vvvvA

    gZ

    gZYZZ

    Ygggg

    vvvvA

    id

    ic

    id

    ic

    +

    =

    22

    11

    2

    11

    1

    1

    ZgZgY

    ZZg

    AACMRR

    m

    ms

    m

    dc

    dd

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 7 -

    Fully-Differential Circuits

    - vo1 Z1 vin1

    ZF

    -

    vo2 Z1 vin2 ZF - +

    +

    112

    0201

    ZZ

    vvvvA f

    inindd =

    =Ideal voltage gain

    Ideally even-order distortions are cancelled

    Ideally common-mode signals are rejected

    What sets the output common-mode of these circuits?

    Function of the amplifier output resistance

    - vo1 Z1 vin1

    ZF

    -

    vo2 Z1 vin2 ZF - +

    +

    IB

    Common-mode offsets can impact the

    performance of the following stages

    Can exceed the common-mode input

    range of preceeding stages

    With finite Acc can accumulate in a

    multi-stage amplifier circuit

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 8 -

    Fully-Differential Amplifiers: COMMON-MODE DC offset

    If IB is positive transistors M3 eventually will be biased in triode region (small resistance)

    dc gain reduces drastically

    Linear range is further minimized

    THD increases

    The common-mode output impedance is the parallel of the equivalent output resistance (M1 and M3) and the parasitic capacitors.

    For large dc gain, the output impedance at nodes v01 and v02 are further increased and IB produces a dc offset = RoutIB. Large common-mode offsets!

    How can this issue be fixed?

    2IB

    IB+IB IB+IB

    v1 v2

    v01 v02

    Z1 M1 M1 Z1

    M3 M3 Vb

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 9 -

    Fully-Differential Amplifiers: Characterization

    Common-mode current offset of 0.01 mA per side is added on purpose

    Tail current is 0.5 mA while the current sources on top are 0.26 mA!

    Differential input voltage is set at 0

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 10 -

    Fully-Differential Amplifiers: Characterization

    Offset current is integrated, and output voltage moves upstairs and reach steady state when the current sources on top become equal to 0.25 mA!

    Differential input voltage is set at 0

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 11 -

    Fully-Differential Amplifiers: Common-mode Feedback

    Gcmf=100 A/V. If the current offset is 10 A, then the offset voltage needed for compensation is 100 mV.

    Expected DC outputs after settling are 100 mV.

    Adder

    CM-Feedback

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 12 -

    Fully-Differential Amplifiers: Common-mode Feedback

    Gcmf=100 A/V. If the current offset is 10 A, then the offset voltage needed for compensation is 100 mV.

    Expected DC outputs after settling are 100 mV.

    If necessary to reduce this 100mV offset further, then use a larger Gcmf

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 13 -

    A common mode feed-back circuit is a circuit sensing the common-mode voltage, comparing it with a proper reference, and feeding back the correcting common-mode signal (both nodes of the fully-differential circuit) with the purpose to cancel the output common-mode current component, and to fix the dc outputs to the desired level.

    What is a common-mode feed-back correction circuit ?

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 14 -

    Fully-Differential Filters: CMFB Principle

    - vo1 Z1 vin1

    ZF

    -

    vo2 Z1 vin2 ZF - +

    +

    A common-mode feedback loop must be used: Circuit must operate on the common-mode signals only!

    BASIC IDEA: CMFB is a circuit with very small impedance for the common-mode signals but transparent for the differential signals.

    Use a common-mode detector (eliminates the effect of differential signals and detect common-mode signals)

    Analyze the common-mode feedback loop: Large transconductance gain and enough phase margin

    Minimum power consumption

    2vvv 0201cm

    +=

    vo1 vo2 vcm

    Z Z

    Simplest common-mode detector

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 15 -

    CMFB Principles: Analysis of the loop for common-mode signals only

    Analysis for common-mode noise; for instance noise due to power supplies: io1=io2=icm_noise

    The two outputs can be connected together for the analysis of the CMFB loop!

    mg

    1Zcm=

    -

    BASIC CONCEPTS: The c

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