LM380 help

Discussion in 'The Projects Forum' started by kahafeez, Dec 24, 2008.

  1. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
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    hi people, i tried to use an LM380 to get a gain of 50. i applied a sine wave of 0.4Vpp bt at the output i got a 6.4Vpp signal instead of a 20Vpp signal...... though my Vcc was 15V so i couldnt get 20Vpp bt still 6.4Vpp is way less than it should have given me..... i used the schematic attached..... i performed the ckt in a lab and grounded all of its pin that were meant for grounding.....


    i also observed this that whenever i took my hand near to the circuit the noise in the output suddenly increased very high..... i dont know what mistake was there....
     
  2. DrNick

    Active Member

    Dec 13, 2006
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    That circuit will not produce a gain of 50. you are running it open loop. Look in to op-amp circuits using feedback.

    As an experiment, What frequency is the signal that you are getting out? Compare it to the one you are putting in.
     
  3. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
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    ok then wat do i do to make it a close loop.... bcz every schematic i found on the web was an open loop one....

    further more, i'd also like to know that how will i find out that the output has a power of 2.5Watt???? plz tell me how to measure the power of a signal.....
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    What exactly do you want to build, a simple amplifier or an active filter?
     
  5. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
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    amplifier.... i need to send a wave into air.... that wave i dont know what its power is .... so i've been told that to send a wave u've to increase the power... i thought that 2.5W power would be enough for abt 10feet or even lesser i dont care abt the range.... so i chose LM380 to make the power of the wave of freq 200KHz to 2.5W....
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    The LM380 is an audio amp. The degradation of signal as you hand approaches means that some wiring is poor. Check your grounds. You might see about adding a load to the output to see if performance improves.

    The spec sheet should give the frequency range for the amp. If your signal is above or below that range of frequencies, the output will be reduced.
     
  7. mik3

    Senior Member

    Feb 4, 2008
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  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    As others have noted, LM380 is not an op amp, it's an audio amp with a fixed gain of 50.
     
  9. DrNick

    Active Member

    Dec 13, 2006
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    Oops, that will learn me to not check datasheets...heh
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    To the OP - the LM380 is an audio amplifier. It is not possible for it to send radio frequency signals. All it can do is drive a loudspeaker with, say, music.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Ironically, I suspect it is oscillating because it is improperly loaded and/or has inadequate power supply decoupling. That would explain why it is sensitive to hand proximity.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    The LM380 has an output power of 2.5W into an 8 ohm speaker. Its power into a 75 ohm antenna is much less, about 0.27W.
    An antenna for 200kHz must be many miles long.
     
  13. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
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    ok then what do i do to deliver a better power to the antenna.....

    cz LM12 is very costly....

    what else can i do??? actually the problem with the freq is that i can detect higher freq than 500KHz at the reciever.... so i thought i should use 200KHz bcz i've to make an oscillator too of the said freq.... i can make a 200KHz sine using wein bridge and TL071 bt how to make sine of 450KHz?
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    An LM12 has even lower bandwidth than the LM380 that you are trying to use.

    There is a rather important matter of the legality of transmitting using radio frequencies in your country.

    Here in the USA, if you are caught transmitting without a license for a specific frequency or band or at excessive power levels, your equipment will be confiscated, you will be put in jail, and large fines must be paid.

    I don't know what the authorities might do to you in Pakistan, but they may be much more strict than here in the States.

    Radio frequencies are closely regulated in most countries. Before you attempt to build a transmitter, you should find out what your country's regulations are.
     
  15. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
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    thats an important matter piece of advice SgtWookie..... i'm thankful to u abt this....
     
  16. DrNick

    Active Member

    Dec 13, 2006
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    Again,

    What is the frequency of the signal you are getting out? As Ron H suspects, it may be oscillating. If the frequency of the sinusoid that you are getting out is not that of which you are putting in, you have a stability problem...
     
  17. kahafeez

    Thread Starter Active Member

    Dec 2, 2008
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    i distroyed the circuit that i made on the breadboard..... i'll try again and then tell that whether the freq is changing or staying the same....
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    A breadboad causes many audio circuits to oscillate because its capacitance between tracks and wires is too high. Use a small pcb or stripboard and solder the parts and wires.
     
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