How can I design a proper BJT current amplifier?

Discussion in 'General Electronics Chat' started by HiThanks, May 26, 2019.

  1. HiThanks

    Thread Starter New Member

    Jul 10, 2018
    2
    0
    In my project, I need to make current amplifier which have < 2mA input current and > 100 mA output current.
    Actually, I want to fully understand the BJT circuit, and then making a proper amplifier circuit.
    But project deadline is soon, so I just want to make the amplifier first.

    present circuit with two TIP122 (NPN BJT) is follow,
    upload_2019-5-26_19-45-54.png
    My goal is applying the linearly amplified AC current (100~200 mA) in R_load.

    The problem is ...
    - Signal source have current limitation ( maximum current : 2 mA, maximum voltage : -10 ~ +10 V ).
    - R3 have big DC current so, It is easily heated.
    - non-linearly amplified...

    So, can you suggest proper amplifier circuit for this situation?
    If you give some circuit scheme with detail parameter ( Ohm, Volts, Capacitance, etc. ), would be perfect!

    Thank you!

    // summary
    with 2 TIP 122,
    - Input AC signal : -10 V ~ +10 V sine wave (current limit ~ 2mA).
    - output AC signal : linearly amplified sine wave ( 100~ 200 mA )
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
    7,709
    1,553
    hi HiThanks.
    Is this a college assignment, if yes, I will move it to the Homework Forum, should improve responses.
    E
     
  3. ericgibbs

    Moderator

    Jan 29, 2010
    7,709
    1,553
    hi,
    If you are trying to test a Current amp, you should use a current input source.
    I will post a LTSpice sim of your circuit.
    E

    EDIT:
    This is what I see with your circuit.
     
    Last edited: May 26, 2019
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  4. crutschow

    Expert

    Mar 14, 2008
    22,119
    6,409
    You need to AC couple the input signal with a series capacitor if want to amplify AC.

    What is the purpose of R3 if you have a 5 ohm resistive load?
    Do you want the load to see only AC?
    If so, you need to AC couple the load also.
     
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  5. Zeeus

    Member

    Apr 17, 2019
    325
    10
    with output cap, what happens to negative swing as cap discharges through R3 compared to charging through Rload
     
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  6. crutschow

    Expert

    Mar 14, 2008
    22,119
    6,409
    It limits the magnitude of the negative swing. as compared to the positive swing.
     
    Last edited: Jun 3, 2019
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  7. HiThanks

    Thread Starter New Member

    Jul 10, 2018
    2
    0
    Thank you guys. I figured out.
     
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