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Current Source Analysis

  1. Mar 16, 2014 #1
    Thank you for any help that you can offer.

    Did I answer all of the questions correctly and thoroughly? Can you please find any errors, and point them out to me so that I can fix them?

    1. The problem statement, all variables and given/known data

    Show that :

    VB = 1.6V
    VE = 1.0V
    IE = 1.0 mA
    IL = 1.0 mA
    What is the voltage compliance range of the current source?
    For what range of load resistors RL will the circuit function as a current source?

    https://scontent-a.xx.fbcdn.net/hphotos-prn1/t1.0-9/1012267_10151937081750919_819156677_n.jpg

    2. Relevant equations

    RB1/RB2 = {10V - VB}/VB
    ...................................................................................................
    IL = IE = VB - 0.6/RE
    ............................................................................................
    VLimit < Vcomp < {VRE/RE}RL = 1.03mA[/B]
    ....................................................................................................................
    iout = Vin/RE
    ..................................................................

    3. The attempt at a solution

    2(a)1

    RB1/RB2 = {10V - VB}/VB
    ..................................................................................................
    8.2k/1.6k = {10V - VB}/VB
    ....................................................................
    5.125VB = {10V - VB}
    ..............................................................
    5.125VB + VB = 10V
    ...........................................................
    6.125VB = 10V
    .....................................
    VB = 10V/6.125
    ....................................
    VB = 1.63V
    ................................

    2(a)2
    .................................
    IL = IE = VB - 0.6/RE
    ............................................................................................
    IL = IE = 1.63 - 0.6/1000k
    ....................................................................
    IL = IE = 1.03/1000k
    ............................................................
    IL = IE = 1.03mA
    .......................................................
    VB - VBE = VE
    ...................................................................
    VE = 1.03V
    ...............................
    VE/RE = 1.03V/1000k
    .............................................................
    IE = VE/RE = 1.03mA
    .............................................................................


    What is the voltage compliance range of the current source?

    The device is in comlpliance when:

    VLimit < Vcomp < {VRE/RE}RL = 1.03mA[/B]
    .............................................................................
    VCC - ILRE < Vcomp < {VRE/RE}{VCC/IE}
    .........................................................................................
    10V - (0.001A)(1000[ohms]) < Vcomp < {1V/1000[ohms]}{10V/0.001A}
    .................................................................................................................
    10V - 1V < Vcomp < 10V
    ..................................................
    9V < Vcomp < 10V
    ..................................................


    For what range of load resistors RL will the circuit function as a current source?

    The range of load resistors can be found by:

    iout = Vin/RE
    .................................................................
    9V/1000[ohms] < Vcomp/RL[ohms] < 10V/1000[ohms]
    ..........................................................................................................
    Your assistance will be greatly appreciated
     
  2. jcsd
  3. Mar 16, 2014 #2

    rude man

    User Avatar
    Homework Helper
    Gold Member

    "Compliance range" needs to be better defined.

    The circuit is a current source for R_L only if the 8.2K resistor stays at +10V.
    Then you can determine how low Vcc to R_L can be before the transistor saturates.

    There is in addition a high voltage limit due to the breakdown voltages of the transistor, but that can't be determined with the data you're given.

    The last question's answer depends on the voltage to R_L as well as the value of R_L, so I guess assume Vcc = 10V for R_L.
     
  4. Mar 16, 2014 #3

    @rude man


    Thank you. I will make corrections.
     
  5. Mar 17, 2014 #4
    @rude man,


    If the compliance voltage is the range of output voltage in a constant current (dc) power supply over which the load regulation is within certain limits, how can I use this knowledge to verify that my last answer is correct. Can the variable load resistor range from 0 to 8k4 ohms. If it is higher than that, what will happen?

    Thank you
     
  6. Mar 17, 2014 #5

    rude man

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    Homework Helper
    Gold Member

    If your load is 8000 ohms, what is the collector voltage going to be?
    If it's 9000 ohms, what then?

    Hint: if the emitter to collector voltage is < about 0.2V the transistor is in saturation.

    BTW this is not voltage compliance. It assumes the voltage to RL is a constant +10V.

    Voltage compliance is a function of R_L. Try to figure out how low the voltage at the top of R_L can be for a given R_L.
     
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