PSPICE amplifier circuit problem

Discussion in 'General Electronics Chat' started by Rated007, Dec 21, 2014.

  1. Rated007

    Thread Starter New Member

    Dec 21, 2014
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    As shown in the attached files, I created an amplifer circuit using an inverting amplifier as the pre amp and a class AB circuit for the power amp. I used a voltage divider to supply 4.5V. My pre amp works fine. But the amplitude of the output of the power amp is very much lower than i want it to be. I can fix this upto a certain extent using low resistor values for the voltage divider but then those resistors get immensely heated up and also, the phase shift alters. So can someone please provide me a solution as to how i can modify my circuit to get a well amplified and less noise distorted output. The IC used is LM324 and the resistance of the output speaker is 8 ohms. The pspice schematic and simulation are attached below. Thank you very much
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi 7,
    The virtual ground created by the 741 OPA is not able to sink the current required for a 8R power output stage.
    E
     
  3. Rated007

    Thread Starter New Member

    Dec 21, 2014
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    Hi eric,
    What's the best alternate solution? Is it possible to change the resistor values to resolve this issue? Thank you.
    7.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Welcome to the forum!
    R8 and R9 are not needed.
    Check the datasheets for the 2N3904 and 2N3906! They have a maximum collector current rating of 200mA and are unsuited to driving an 8 Ohm load, as is a 9V PP3 battery.
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You will have to remove R8 and R9 (both 330 Ohms at the output of the virtual ground circuit).

    If you want to have a more stable virtual powersupply, have a look on this project:
    CMV2A virtual ground circuit

    As said a 9 Volt PP3 battery wont last long.

    Bertus
     
  6. Rated007

    Thread Starter New Member

    Dec 21, 2014
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    Hi Alec, thank you very much. But please do check the following attachments. Also, could you please give me the code for the transistors that are best suited for this problem if 2N3904 and 2N3906 are not.
    Thank you
     
  7. Rated007

    Thread Starter New Member

    Dec 21, 2014
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    Hi bertus,
    thank you for the info but its compulsory for us to use the configuration of my schematic. So if its possible could you please let me know how i can improve my circuit without making heavy alterations to it.
    Thank you
    7.
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,648
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    Hello,

    Did you have a look at the schematic I pointed you?
    The input resistors there are 10 K in stead of the 100 K you are using.
    Also there are capacitors accross the resistors, any value between 10 and 100 nF will do.

    Bertus
     
  9. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    380
    hi 7,
    By using AC coupling throughout the amplifier and to the 8R speaker, via say a 1000uF, you could eliminate the need for a virtual ground source.
    Use BD135 and BD136 transistors to make a medium power audio amp

    The 9V, PP9 battery will have a short life!

    E
     
  10. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You could also use BD135/136 or similar transistors as a push-pull output stage to boost the current available from your virtual ground circuit.
     
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