Lecture04-Non-ideal Op Amps (Feedback circuit).pptx

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1 1 EE105 – Fall 2014 Microelectronic Devices and Circuits Prof. Ming C. Wu [email protected] 511 Sutardja Dai Hall (SDH) 2 Ideal vs Non-ideal Op Amps Ideal Op Amp A = R id = R o = 0 Non-ideal Op Amp A < R id < R o > 0 Other non-ideal characteristics: Finite common-mode rejection ratio (CMRR) Output voltage is not just proportional to the difference input voltage but also their absolute values Common-mode input resistance dc error sources (offset voltage, input bias current, offset current) Output voltage, slew rate, maximum current limits

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Transcript of Lecture04-Non-ideal Op Amps (Feedback circuit).pptx

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    EE105 Fall 2014 Microelectronic Devices and Circuits

    Prof. Ming C. Wu

    [email protected]

    511 Sutardja Dai Hall (SDH)

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    Ideal vs Non-ideal Op Amps

    Ideal Op Amp A = Rid = Ro = 0

    Non-ideal Op Amp A

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    Classic Feedback Systems

    A = transfer function of open-loop amplifier or open-loop gain.

    = transfer function of feedback network.

    T = loop gain = A For negative feedback: T(s) > 0 For positive feedback: T(s) < 0

    vd = vi v f

    vo = Avdv f = vo

    Av =vovi

    =A

    1+ A=1

    A1+ A#

    $ %

    &

    ' ( = Av

    Ideal T1+ T#

    $ %

    &

    ' (

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    Gain Error and Fractional Gain Error

    Gain Error: GE = (ideal gain) - (actual gain)

    For non-inverting amplifier,

    Fractional or percentage error:

    GE = 1 1

    T1+T

    $

    %

    & &

    '

    (

    ) ) = 1

    11+T

    $

    %

    & &

    '

    (

    ) )

    FGE =GE

    AvIdeal =

    1

    11+ T#

    $ %

    &

    ' (

    1

    =1

    1+ T1T

    for T >>1

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    Gain Error Example: Noninverting Amplifier

    Problem: Find actual gain and gain error for an amplifier Given data: Ideal closed-loop gain of 200 (46 dB), open-loop gain of

    op amp is 10,000 (80 dB).

    Approach: Amplifier is designed to give ideal gain and deviations from ideal case are determined.

    Note: R1 and R2 are not normally designed to compensate for finite open-loop gain of amplifier.

    Analysis:

    Av = AvIdeal T

    1+ T"

    # $

    %

    & ' with T = A =104

    1200"

    # $

    %

    & ' = 50

    AvIdeal =

    1

    = 200 Av = 200501+ 50"

    # $

    %

    & ' =196 FGE =

    200 196200

    = 0.02

    Note : FGE 1T

    =150

    = 0.02

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    Gain of Noninverting Amplifier

    For i = 0, v1 =R1

    R1 + R2vo = vo

    =R1

    R1 + R2

    vo = Avid = A vi v1( ) = A vi vo( )

    Av =vovi=

    A1+ A

    =1

    A1+ A"

    #$

    %

    &'=

    1

    T1+T"

    #$

    %

    &'

    T = A is called the loop gainFor T >>1

    Av AvIdeal =

    1=1+ R2

    R1 vid = vi v1 = vi vo

    vid = vi T

    1+Tvi =

    vi1+T

    Although no longer zero, vid is smallfor large loop gain T.Feedback factor

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    Output Resistance (Both Noninverting and Inverting Amplifiers)

    The output terminal is driven by test source vx and current ix is calculated to determine output resistance (all independent sources are turned off). Note that the equivalent circuit is same for both inverting and non-inverting amplifiers.

    Rout =vxix

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    Output Resistance (contd)

    Analysis : ix = io + i2

    io =vx Avid

    Ro and i2 =

    vxR2 + R1

    Also, vid = v1 and v1 =R1

    R2 + R1vx = vx

    1

    Rout=

    ixvx

    =1+ A

    Ro+

    1R2 + R1

    Rout =Ro1+ T

    R2 + R1( )

    Thus, shunt feedback reduces Rout .

    Typically, Ro 1+ T( )

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    Nonideal Op Amps Open-loop Gain: Design Example

    Problem: Design non-inverting amplifier and find the required open-loop gain

    Given Data: Av = 35 dB, Rout 0.2 , Ro = 250

    Analysis:

    AvIdeal =10

    35dB20dB = 56.2 | = 1

    AvIdeal =

    156.2

    Rout =Ro

    1+ A 0.2

    A 1

    RoRout

    1(

    ) *

    +

    , - | A 56.2

    2500.2

    1(

    ) *

    +

    , - = 7.02x104 or 96.9 dB

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    Input Resistance of Noninverting Amplifier

    Test voltage source vx is applied to input and current ix is calculated:

    ix =vx v1

    Rid | v1 = i1R1 i2R1

    Assuming i

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    Gain of Inverting Amplifier

    Let i1 be the current going from the input source through R1 and i2 be the current going through R2 into the amplifier output.

    i1 =vi vid( )

    R1 and i2 =

    vid( ) voR2

    i2 = i1 and vo = Avid

    After some algebra,

    Av =vovi=

    R2R1

    "

    #$

    %

    &'A

    1+ A= Av

    Ideal T1+T"

    #$

    %

    &'

    with = R1R1 + R2

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    Input Resistance of Inverting Amplifier

    R1 removed Rin =vxix=ixR1 + v

    ix= R1 +

    vix

    i 1= i + i2 =v1Rid

    +v1 voR2

    =v1Rid

    +v1 + Av1R2

    G1 =i1v1=

    1Rid

    +1+TR2

    Rin = R1 + Rid ||R2

    1+ A!

    "#

    $

    %& R1 +

    R21+ A

    For large A, Rin R1

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    Nonideal Amplifier Summary Inverting and Noninverting Amplifier

    =R1

    R1 + R2 and T = A

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    Feedback Amplifier Categories Voltage Amplifiers - Series-Shunt Feedback

    A voltage amplifier should have a high input resistance to measure the desired voltage and a low output resistance to drive the external load. To achieve the desired behavior, the input ports of the amplifier and feedback network are connected in series, and the output ports are connected in parallel (shunt).

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    Feedback Amplifier Categories Transresistance Amplifiers - Shunt-Shunt Feedback

    A transresistance amplifier converts an input current to an output voltage. Thus it should have a low input resistance to sink the desired current and a low output resistance to drive the external load. To achieve the desired behavior, the input ports of the amplifier and feedback network are connected in parallel (shunt), and the output ports are connected in parallel (shunt).

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    Feedback Amplifier Categories Current Amplifiers - Shunt-Series Feedback

    A current amplifier should provide a low resistance current sink at the input and a high resistance current source at its output. The input ports of the amplifier and feedback network are connected in parallel, and the output ports are connected in series.

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    Feedback Amplifier Categories Transconductance Amplifiers - Series-Series Feedback

    The transconductance amplifier converts an input voltage to an output current. This amplifier should have a high input resistance and a high output resistance. The input ports and the output ports of the amplifier and feedback networks are connected in series.

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    Feedback Amplifier Category Summary