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LM386 Gain...

Discussion in 'Electronic Basics' started by [email protected], Dec 5, 2005.

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  1. Guest

    Hi,

    I hooked up an LM386 to a 9v supply (+ve to pin 6, -ve to pin 4).

    I left pins 2 and 3 (inputs) open, pin 7 open, and pins 1 and 8 open. I
    then measured the voltage at pin 5 and got the expected 4.5v.

    I then connected pin 1 to 8 using a wire (to increase the gain from 20
    to 200). The voltage at pin 5 dropped by about half a volt.

    Can someone explain why this happened???

    Thanks

    Gareth
     
  2. Even though both inputs are grounded through similar internal 50k
    resistors, the input transistors are not perfect duplicates. Any
    slight input offset voltage caused by the mismatch is amplifier more
    at the higher gain than it is at the lower gain.
     
  3. Guest

    Cool - that makes sense! Thanks.

    While I was trying to work out what was going on I was looking at the
    schematic (http://www.national.com/ds/LM/LM386.pdf) to see if could
    could see what baised the output at Vs/2. I couldn't!! Can someone
    explain it?

    Thanks

    Gareth
     
  4. The two inputs run at zero volts. The differential amplifier stage has
    two currents feeding into the common emitters, one from the supply and
    one from the output. But there is a resistor between these two
    inputs, so any difference between those two currents gets amplified by
    the differential darlington input stage. That is, there is a gain
    based on the inputs both having zero volts at their bases, but
    different voltages on their emitters. Since the current from the
    supply into the left emitter has two 15k resistors in series, but the
    current from the output to the right emitter has only one 15k
    resistor, the only way the two emitters see similar currents is if the
    output voltage is 1/2 of the supply voltage.

    Of course, the current to the left emitter varies as the supply ripple
    bounces around (both from the rectifier pulses and from load current
    pulses driving the speaker) so it is a really good idea to bypass pin
    3 to ground, so those short term variations are not forced to show up
    in the output signal via the bias scheme.

    If you want to trim the final output voltage, you can connect pin 3 to
    either the supply or to ground through a resistor much higher than 15k
    to tweak the left emitter current by trial and error. (or connect
    something like a 100k pot from supply to ground and connect the wiper
    to pin 3 through a 47k resistor to tweak the current to the left emitter).
     
  5. Pooh Bear

    Pooh Bear Guest

    It's a crap chip.

    Use something more modern. LM386s are notorious for parasitic oscillation
    for one thing and modern circuits rarely need the minimum 20x voltage gain.

    Graham
     
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