NPN Circuit with LM386

Discussion in 'General Electronics Chat' started by BackyardBrains, Jul 1, 2012.

  1. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
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    Attached is the circuit for the wonderful Mini-Audio Amplifier product from Radio Shack: RS product 062620.

    I am trying to put a similar amplifier on my circuit board, but am running into terrible feedback at stability issues. I would like to understand this circuit better, but am confused about a few things:

    1) Pins 1 and 8 are supposed to set the gain on the LM386. Why does this amplifier have a capacitor between 1 and 5? I have used the LM386 before, but only in it's default state (20x, nothing b/w pins 1 and 8).

    2) What is the purpose of the NPN before the input of the LM386? I am trying to get better at analyzing these circuits, but would love some pointers on where to start. Is it a pre-amp? What is the gain? Why is this transistor even needed?

    Thanks so much.

    Greg
     
  2. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
    33
    0
    Update. I found a more current schematic. (attached below). It looks more recognizable, pins 1 and 8 handle the gain. But it still has Q1 (NPN) before the NJM386 IC. Are R1 and R2 setting the gain?

    Greg
     
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    1,393
    I have built an amplifier with this IC. I followed the schematic in the LM386 datasheet, and it works fairly well. However, when I first assembled it on the solderless breadboard, there was some instability.

    Here's the thread, which has a photo and a schematic.

    http://forum.allaboutcircuits.com/showthread.php?t=62896
     
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  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    R1 supplies + bias & neg feed back, R2 raises input Z & more neg feed back, R5 adjusts gain between 20 & 200.
     
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  5. upand_at_them

    Active Member

    May 15, 2010
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    Uhh, where is the collector current coming from in that circuit?

    And, yes, R1 and R2 are setting the gain for the transistor (acting as a pre-amp).
     
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  6. BackyardBrains

    Thread Starter Member

    Aug 26, 2010
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    Thanks for this information on R1/R2. I have been trying to understand the circuit a bit better by modeling it in SPICE (attached). I did see that there was a pre-amp gain, but it wasn't very high (7-10x).

    Would the main reason for adding the NPN be to add more gain to the LM386? Or does the NPN provide another function that makes this circuit so stable?

    I wish schematics were more descriptive as to why things are there, not just what. it helps when trying to learn how they work. I am trying to develop my own intuition on circuits, but am not quite there yet.
     
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