BJT Biasing Cont. - seas.upenn. ese319/Lecture_Notes/Lec_5_BJTBias2_12.pdfBJT Biasing Cont. Biasing...

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Transcript of BJT Biasing Cont. - seas.upenn. ese319/Lecture_Notes/Lec_5_BJTBias2_12.pdfBJT Biasing Cont. Biasing...

  • ESE319 Introduction to Microelectronics

    12009 Kenneth R. Laker, update 21Sep12

    BJT Biasing Cont. Biasing for DC Operating Point Stability BJT Bias Using Emitter Negative Feedback Single Supply BJT Bias Scheme Constant Current BJT Bias Scheme Rule of Thumb BJT Bias Design

  • ESE319 Introduction to Microelectronics

    22009 Kenneth R. Laker, update 21Sep12

    Simple Base Biasing Schemes

    What's wrong with these schemes?

  • ESE319 Introduction to Microelectronics

    32009 Kenneth R. Laker, update 21Sep12

    Another Simple Biasing Scheme

    RB

    RC

    I E

    V CC

    Is this scheme any better? If it is, why?

  • ESE319 Introduction to Microelectronics

    42009 Kenneth R. Laker, update 21Sep12

    Biasing for Operating Point StabilityA practical biasing scheme must be insensitive to changes in transistor and operating temperature! Negative feedback is one solution.

    IC

    IB

    IE R

    E

    RC

    VCC

    VB

  • ESE319 Introduction to Microelectronics

    52009 Kenneth R. Laker, update 21Sep12

    I C I S eV BEV T

    V BE=V BRE I E=V BREI C

    Basic relationships:I E= I BI C=1 I B

    I E=1

    I C=

    1

    I C

    Given the (DC bias) equations:

    Assume: V BE=0.7 V

    Let: VB, VCC and IC be specified;

    Compute: RE to obtain IC:

    RE=V BV BE

    I C=

    V B0.7I C

  • ESE319 Introduction to Microelectronics

    62009 Kenneth R. Laker, update 21Sep12

    Negative feedback makes the collector current insensitive to VBE, IS, and . If IC increases due to an increase in IS then VBE will decrease; thus, limit-

    ing the magnitude of the change in IC.

    The equations that must be satisfied simultaneously are:

    Negative Feedback via RE

    I C=V BV BE1RE

    =V BV BE

    RE

    V BE=V BREI C=V B

    1

    RE I CI C=I S eV BEV T and

    if IS => IC when IC => VBE IC > VBE VBE insensitivityI C

    V BRE

    =>

  • ESE319 Introduction to Microelectronics

    72009 Kenneth R. Laker, update 21Sep12

    Scilab Analysis of IC Insensitivity to ISSimultaneous equations:

    I C= I S eV BEV T

    I C=V BV BE

    RE

    or: V BV BE

    RE=I S e

    V BEV T

    If we plot the exponential function and the straight line function,the solution values of IC and VBE for the circuit occur at theirintersection.

    Let RE=4k V B=4.7Vand (want VB >> VBE)

  • ESE319 Introduction to Microelectronics

    82009 Kenneth R. Laker, update 21Sep12

    Scilab Program//Calculate and plot npn BJT bias characteristicbeta=100;alpha=beta/(beta+1);VsubT=0.025;VTinv=1/VsubT;VBB=4.7;Re=4;vBE=0.0:0.01:1;iCline=alpha*(VBB-vBE)/Re;//mA.plot(vBE,iCline);iC=0.01:0.01:2; //mA.!IsubS =1E-16; //mA.for k= 1:1:8 IsubS=10*IsubS; vBE2=VsubT*log(iC/IsubS); plot(vBE2,iC); //Current in mA.end

    I C=V BV BE

    RE

    V BE=V T lnI CI S

  • ESE319 Introduction to Microelectronics

    92009 Kenneth R. Laker, update 21Sep12

    IC vs IS Results PlotI S=10

    11 I S=1018

    I C0.1mA Insensitive to I

    S!

    I C=V BV BE

    RE

    I C= I S eV BEV T

  • ESE319 Introduction to Microelectronics

    102009 Kenneth R. Laker, update 21Sep12

    RE=V B0.7

    I C

    Example: =100V B=4.7VI C=1 mA

    RE=0.994.70.7

    103

    =100101

    0.99

    I C=V B0.7

    RE=

    1

    103 A

    Writing IC as a function of :

    Assume: 100 200100101

    103=0.990 mA I C200201

    103=0.995 mA

    CONCLUSION: IC is insensitive to changes in !

    RE4000

    Insensitivity to Beta

  • ESE319 Introduction to Microelectronics

    112009 Kenneth R. Laker, update 21Sep12

    Non-zero Base Resistance

    I B

    ICV B=I B RBV BE1 I B RE

    I B=V BV BE

    RB1RE

    I E=1 I B

    I C= I B=

    RB1REV BV BE

    I C= I B

    1V BV BE

    RE=

    V BV BE RE

    If RB1REI E

    RC

    RE

    RB

    VB

    VCC

    If RB1RE

    I C= I BRB

    V BV BE no feedback!

    IE

  • ESE319 Introduction to Microelectronics

    122009 Kenneth R. Laker, update 21Sep12

    Biasing for Operating Point Stability Quick Review

    What is the purpose RE?

    What does RB represent?

    What is the condition on the value of V

    B?

    What is the condition on the value of R

    E?

    What is our rule of thumb for achieving x >> y?

  • ESE319 Introduction to Microelectronics

    132009 Kenneth R. Laker, update 21Sep12

    Observations Emitter feedback stabilizes base voltage source bias. To reduce the sensitivity of IC to VBE, choose . RB 0, emitter feedback stabilization works well if RB is

    not too large, i.e.RB1RE

    Ideal rule of thumb (if possible):

    RB1

    101RE

    V BV BE

    RB=10010

    RE=10 RELet 1=100

    orRB1

    101RE RB=

    110

    1RE

  • ESE319 Introduction to Microelectronics

    142009 Kenneth R. Laker, update 21Sep12

    Emitter-Feedback Bias Design

    Voltage bias circuit Single power supply version

    R1

    R2

    VCC

    RC

    RE

    IC

    IE

    IB

    VB

    +

    -

  • ESE319 Introduction to Microelectronics

    152009 Kenneth R. Laker, update 21Sep12

    RTh=RB=R1R2

    RTh

    VTh

    V Th=V B=R2

    R1R2V CC

    Thevenin Equivalent

    I 1

    I B I 1 I C

  • ESE319 Introduction to Microelectronics

    162009 Kenneth R. Laker, update 21Sep12

    Using Thevenin EquivalentUse Thevenin's theorem to simplify base circuit:

    If VCC, VB and RB are inde-pendently specified then R1 and R2 are found by solving Eqs. (1) and (2):

    Design Eqs.

    I B0

    V Th=V B=R2

    R1R2V CC

    I1

    RTh=RB=R1R2=R1 R2

    R1R2R1R2=

    R1 R2RB

    V B=R2 RBR1 R2

    V CCR1=RBV CCV B

    V B=R2

    R1R2V CC R1 V BR2 V B=R2 V CC

    R2=R1V B

    V CCV B

    R2=R1V B

    V CCV B

    R1=RBV CCV B

    (1)

    (2)

    Can VCC, VB and RB be inde-pendently specified?

  • ESE319 Introduction to Microelectronics

    172009 Kenneth R. Laker, update 21Sep12

    Using Thevenin Equivalent

    If VCC, VB and RB are independently specified then R1 and R2 are found by solving Eqs. (1) and (2):

    Design Eqs.

    R2=R1V B

    V CCV B

    R1=RBV CCV B

    Can VCC, VB and RB be independently specified?

    RECALL: VB >> VBE and RB1RE

  • ESE319 Introduction to Microelectronics

    182009 Kenneth R. Laker, update 21Sep12

    1st Rule of Thumb for Single Supply Biasing1. Choose RT = R1 + R2 so that I1

  • ESE319 Introduction to Microelectronics

    192009 Kenneth R. Laker, update 21Sep12

    IC

    R1

    R2

    RC

    RE

    V CCI

    E

    V CE+-

    V RE+

    -

    +

    -

    Three design goals so farRB1REV BV BE

    Constraint: V CC=V RCV CEV RETRADEOFF Increase VRC and/or VRE => Reduce VCE VRC too large => possible saturation & reduced op-erating range (i.e. V

    CE -> 0.2 V or smaller).

    VRC too small => possible cutoff & reduced operating range (i.e. I

    C -> small or zero).

    NEED A COMPROMISE!

    An Unavoidable Design Tradeoff

    I 1 I C also I 1 I B

    (VCC

    fixed)

    V RC

    V RE=V BV BE

    B

  • ESE319 Introduction to Microelectronics

    202009 Kenneth R. Laker, update 21Sep12

    2nd Useful Rule of Thumb 1/3, 1/3, 1/3 Rule

    V RE=V CC

    3V RC= I C RC=

    V CC3

    V CE=V CC

    3

    V CC=V RCV REV CE

    RC=V RCI C

    =V CC3 I C

    RE=V REI E

    =V CC3 I E

    =V CC3 I C

    V RE=V CCR2RT

    V BE=V CC

    3

    Design Equations

    where

    IC

    R1

    R2

    RC

    RE

    V CCI

    E

    V CE+-

    +

    -

    +

    -

    V B=V BEV RE I 1 R2V BE=0.7V

    VB >> VBE

    V RE= I E REV B

    I 1=I C10

    V CCRT

    R2=RTV BV CC

    13

    RT

    R1=RT 1V BV CC

    23

    RT

    RT = R1 + R2

    I1 V RC

    V RE

  • ESE319 Introduction to Microelectronics

    212009 Kenneth R. Laker, update 21Sep12

    Biasing for Operating Point Stability Quick Review

    What is the condition on the value of VB?

    What is the condition on the value of RE?

    What is the purpose for R1 & R

    2?

    What happened to RB?

    What is the constraint on the value of I1?

    What is the 1/3, 1/3, 1/3 Rule?

    IC

    R1

    R2

    RC

    RE

    V CCI

    E

    V CE

    I1

    IB

    V RC

    V CC=V RCV CEV RE

    V RE

    V CC=V RCV CEV RE

    V B

    V CCV RCV CEV B

  • ESE319 Introduction to Microelectronics

    222009 Kenneth R. Laker, update 21Sep12

    Constant Emitter Current BiasThe current mirror is used to create a current source:

    1. A BJT collector is the cur-rent source:

    I C= I S eV BEV T

    I REF

    I

    2. A diode-connected transistor sets the current.

    3. Choose Rref for the desiredcurrent:

    Rref =V CC0.7

    I REF

    V CC0.7I C1

    V BE2=V BE1

    I O

    I REF=I C1I B1I B2 I C1 I E1

    I REF=V CCV BE1

    RrefRref

    V CC IC1

    I B1 I B2

    V CE1=V BE1

    4. If Q1 = Q2 then IC2 = IC1 => I O I refmatched

    I O=I C2

    and VCE2 = VCE1

  • ESE319 Introduction to Microelectronics

    232009 Kenneth R. Laker, update 21Sep12

    Constant Emitter Current

    If Q1 and Q

    2 have the same saturation current:

    Now: VBE1 = VBE2I S1= I S2

    And the transistors are at the same temperature: T 1=T 2

    The two collector currents set primarily by Rref are equal, as long as Q2 is not saturated.