darlington pushpull newbie

Discussion in 'General Electronics Chat' started by sofiaD, Feb 15, 2014.

  1. sofiaD

    Thread Starter New Member

    Feb 15, 2014
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    Hi everybody,

    I'm trying to design a darlington pushpull circuit,

    here is my beginning :
    darlington base.png
    View attachment darlingtonPshPll base.zip

    Can somebody tell me if all these resistances will be usefull for me to design an amplifier which
    => has a voltage gain of 1 ( VS = VE)
    => has in input 40 mA, and I want 1,6 A at output of this amplifier, that means I want 800 mA at XMM12 and XMM13.
    => it should work at the frequency of 13,56 Mhz.

    I know how it works for one 2N2222, but for darlington pushpull I have no idea, I looked in the web but there was none for my application...

    I don't know how design it, because it's very difficult, maybe somebody can teach me the "beginning" ?

    Thanks a lot, your help is welcome :p
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Is it 1356 MHz or 13.56 MHz ?
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You have the emittor and collector of Q3 and Q4 switched.
    See the picture how they should be:

    [​IMG]

    Bertus
     
  4. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    What software is that? ??
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    That's NI MultiSim
     
  6. bertus

    Administrator

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    Hello,

    I only changed the transistors using a kind of paint program KolourPaint on my linux machine to show how they should be.
    I can not change the parameters or meter values.

    Bertus
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    And V2 is wired wrong.
    It says -15V but instead you have wired it as +15V
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    why is Re2 & Re3 values different.
     
  9. sofiaD

    Thread Starter New Member

    Feb 15, 2014
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    Thanks a lot for your answers, what a wonderful community... i'm very happy..;)
    I didn't find the section where I could introduce myself..

    It's 13,56 Mhz (RFID^^)

    @Dr.killjoy : Yeah i'ts multisim 13.00 pro edition, pro edition because there are so much components that other versions. the compiler is SPICE (same as in Altium and LTSpice)

    @ bertus : thanks , I will look into your modification and will report how it changes the results.

    @ R!f@@ : Thanks ! I just saw the -15V failures, I tought it was possible to plug like that but I will change it and reports too.

    I will repost in one/2 hours, when I will back home to use my laptop for multisim.
    xoxo
     
  10. sofiaD

    Thread Starter New Member

    Feb 15, 2014
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    Hey forumers !

    I did a new simulation with your modifications, here is the schematics and my results :
    View attachment Darlington push pull.zip

    screen darlington push pull.jpg

    Can you help me for these questions please ?

    - My norton circuit (at the bottom) should simulate a card which supply 0,2 Watts, and a 0 to 10 V voltage. The voltage is correct but why the current is only 0,0069 A (6,9 mA) ? it should give 0,040 A (40 mA)... (ammeter XMM5 at the input of the darlington circuit)

    - What can I do, please, in order to get Voltage input of darlington circuit = Voltage output of darlington ? ie : a voltage gain of 1, because in this simulation he is a little below 1.

    - Is my circuit safe ? There are not a lot of resistances or capacities... I think it's very strange and unsafe, can you tell me a way in order to make it safer please ?

    - Why the ammeter XMM12 doesn't supply the same intensity as XMM13 ? according to the symetry it should be the same, Am i wrong?

    - can somebody tell me the formula in order to get my current gain in this schematic ?

    - What do you think, please, about values of R1 and R2 & RE1 and RC2 ?


    Of course all others advices are welcome !!

    Sof,
    xoxo
     
  11. #12

    Expert

    Nov 30, 2010
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    Off Topic Forum. First thread on the page.
     
  12. sofiaD

    Thread Starter New Member

    Feb 15, 2014
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    @#12 Thanks
     
  13. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Measure the voltage between D1 and D2 and ground, with no input signal.
    Idealy the DC voltage should be 0 Volts.
    With no input signal, the output voltage to ground should also be 0 Volts.

    To avoid a DC offset from the input signal, you could put a capacitor between the D1, D2 connection and the - connection of XMM5.

    Bertus
     
  14. sofiaD

    Thread Starter New Member

    Feb 15, 2014
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    @ Berthus : Thanks for your answer, How do you choose your capacitor ? I think it's related with the frequency ..

    I did what you say, but maybe my "Vgnd" is wrong, I let you see it :

    without capacitor :
    darlington DC without C.png

    with capacitor :
    darlington DC with C.png
    darlington PP with C.png


    It's strange because the signal is worse with capacitor

    Thanks a lot
     
  15. bertus

    Administrator

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  16. sofiaD

    Thread Starter New Member

    Feb 15, 2014
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    @Berthus : Thanks Bertus for your help,

    In multisim I can't mesure a DC variable if it is plugged directly to the ground,
    the one thing I can do is on this screen :
    dc offset with capa.png

    Here is the result with capa which is the same so I think my measure are wrong.. : dc offset with capa.png

    What is strange is that with capacitor my signal is worse :
    simu with capa.png

    Sorry if I ask stupid questions :(

    BTW : Can I know why you chose this value of capa ? Or maybe was it just an example ?

    Thanks, you are very helpful for me and I hope i could help you too one day
    xoxo
     
  17. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I see that you have changed the values of R1 and R3 from 1K5 to 3K3.
    Is there a reason for this?

    I have chosen the 10 nF as given in table III of the following PDF:
    rf_proto-1.pdf

    In there 10 nF is used for DC block and decoupling in HF circuits.

    Bertus
     
  18. sofiaD

    Thread Starter New Member

    Feb 15, 2014
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    @Bertus : Thanks for your explanations..

    You know usually in a circuit with one transistor, R1 and R3 are usefull to place the bias point. But here I have 4 transistorss and no ideas how bias their point... and because of the symetry of my schematic, it was very strange that R1 was different of R3..

    I just know that the sum of R1 + R3 should be high, near K Ohms, because if R1 + R2 is low, there will be high power loss in my circuit.

    Do you know the right way to choose them ? ^^

    If I understood, the capacitor was only here in order to calculate DC values. Now that I calculated these values, can I remove this capacitor ?

    You know in multisim you have a function "DC Bias point analysis" which allows you to calculate it without modify it ;)

    Your pdf is absolutely fantastic for me :p !!! thanks so much !!!

    Do you think my circuit can work in "real" ? The goal is to create it :D

    I Input/ I Output is near 58 when I input is 5 - 20 mA !

    If I understood the gain is regulated by RE1 and RC2 in my schematic ? Thanks

    xoxo
     
  19. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Have a look at the attached PDF on calculating the resistors for the amplifier.
    It gives an example for an audio amplifier.

    Bertus
     
  20. sofiaD

    Thread Starter New Member

    Feb 15, 2014
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    @Bertus : Thanks Bertus you are sooo helpful !!
    I did all their procedure and I found 1,4 KOhms for the sum of RB1 + RB2.

    So I took RB1 = RB2 = 750 Ohms .

    Do you think I should let the capacitor just after the input, please ?

    Can somebody tell me if my circuit is safe / realistic or not, please ? :D
    I will make the PCB and test it in real :)

    have a good day !
     
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