ECEN474/704: (Analog) VLSI Circuit Design Spring spalermo/ecen474/lecture15_ee474...¢  Sam Palermo Analog

  • View
    1

  • Download
    3

Embed Size (px)

Text of ECEN474/704: (Analog) VLSI Circuit Design Spring spalermo/ecen474/lecture15_ee474...¢  Sam...

  • Sam Palermo Analog & Mixed-Signal Center

    Texas A&M University

    Lecture 15: Fully Differential Amplifiers & CMFB

    ECEN474/704: (Analog) VLSI Circuit Design Spring 2018

  • Announcements

    • Project Report Due May 1 • Email it to me by 5PM

    • Exam 3 is on May 3 • 3PM-5PM

    2

  • Agenda

    • Fully differential circuits

    • Common-mode feedback circuits

    • Multi-OTA stages CMFB

    • OTA-C filter w/ CMFB example

    3

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

    - 4 -

    Basic Operational Transconductance Amplifier Topologies

    SINGLE-ENDED

    FULLY-DIFFERENTIAL

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 5 -

    Fully-Differential Circuits

    - vo1vin1

    -

    vo2vin2 - +

    +

    In general:

     Hence

    oc odoooo

    o

    oc odoooo

    o

    vvvvvvv

    vvvvvvv

     

     

     

     

    222

    222

    2112 2

    2121 1

     

      

      

      

     

      

    ic

    id

    cccd

    dcdd

    oc

    od v v

    AA AA

    v v

    0Vidic

    od dc

    0Vicid

    od dd v

    vA v vA

    

    

    Differential-mode output Common-mode output

    0Vidic

    oc cc

    0Vicid

    oc cd v

    vA v vA

    

    

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 6 -

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

    - vo1vin1

    -

    vo2vin2 - +

    +

    A very important parameter: dc

    dd A ACMRR 

     

      

      

      

     

      

     

     

     

      

      

      

     

      

     

     

    ic m

    s id

    m

    s

    smm

    mm

    ic m

    s id

    m

    s

    smm

    mm

    v g Yv

    g Y

    Ygg Zggv

    v g Yv

    g Y

    Ygg Zggv

    1121

    221 02

    2221

    121 01

    2 1

    2 1

    Example:

    2IB

    IBIB

    v1 v2

    v01 v02

    Z2 Z1

    M1 M2

    Solving the circuit:

    Ys is the admittance associated with the current source 2IB

    2 12

    12 ii

    iciiid vvvvvv  and w/

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 7 -

    Fully-Differential Filters: Non-idealities

    2IB

    IBIB

    v1 v2

    v01 v02

    Z2Z1 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

    1 21

    21

    21

    012

    12

    2

    1

    1

    2

    21

    21

    012

    21

    1

    2

    2

    1 21

    21

    21

    012

    21

    2 1

    2 2

    22 12

    2

    2

    mm s

    smm

    mm

    vii

    oo cc

    mm

    s

    smm

    mm

    vii

    oo cd

    mm s

    smm

    mm

    vii

    oo dc

    mm

    s

    smm

    mm

    vii

    oo dd

    g Z

    g ZY

    Ygg gg

    vv vvA

    g Z

    g ZYZZ

    Ygg gg

    vv vvA

    g Z

    g ZY

    Ygg gg

    vv vvA

    g Z

    g ZYZZ

    Ygg gg

    vv vvA

    id

    ic

    id

    ic

     

      

     

     

      

     

    

    22

    11

    2

    1 1

    1

    1

    Zg ZgY

    Z Zg

    A ACMRR

    m

    m s

    m

    dc

    dd

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 8 -

    Fully-Differential Circuits

    - vo1Z1vin1

    ZF

    -

    vo2Z1vin2 ZF

    - +

    +

    112

    0201

    Z Z

    vv vvA f

    inin dd 

     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

    - vo1Z1vin1

    ZF

    -

    vo2Z1vin2 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

    - 9 -

    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 = RoutIB. Large common-mode offsets!

    How can this issue be fixed?

    2IB

    IB+IBIB+IB

    v1 v2

    v01 v02

    Z1 M1 M1 Z1

    M3M3 Vb

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 10 -

    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

    - 11 -

    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

    - 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.

    Adder

    CM-Feedback

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 13 -

    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

  • Agenda

    • Fully differential circuits

    • Common-mode feedback circuits

    • Multi-OTA stages CMFB

    • OTA-C filter w/ CMFB example

    14

  • TAMU-ECEN-474 Jose Silva-Martinez_08

    - 15 -

    A common mode feed-back circuit is a circuit sensing the common-mode voltage, comparing it wit

Recommended

View more >