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Posts posted by teragiga

  1. Hi all!
    This is my first post so, sorry if I would not be proficient in discussing electronics :)

    I am stuck in a practical experiment, I studied how amplifier for obtaining gain work. And I created a small circuit with an astable multivibrator, and operational, 2 resistors, two batteries of 9v:
    Something like this:
    Where the settings are :
    non inverting input.
    Rf = 300 K, Rg=150K
    And the expected gain should be : Av= 1 + Rf/Rg = 3.
    Doing a simulation I obtain something like this
    Where the output in volts of the operational is effectively augmented.
    But when I build the real circuit it doesn't happen:
    The oscillator generates a square wave that I can detect with
    oscilloscope and even with a tester (between -4.80v +4.80v ), the output of the amplifier is instead stuck to 7.8v.
    I have done this with a LM741, and also using a LM386 (one with
    single supply) and nothing.
    The crazy thing is that I obtain constant 7.8 v independently from  the value of the feedback resistors (like they don't matter) and even if I completely disconnect the input.
    So, it is like if whatever I do with this configuration
    the amplifier saturates to 7.8v and end of the story.
    I also tried the inverting input configuration and nothing.
    Where is the mistake ? Or I completely miss something in what "gain" means ?

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