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op amp troubleshooting

Discussion in 'Electronic Basics' started by jim, Dec 15, 2003.

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  1. jim

    jim Guest

    Can anyone help here? As a newbie,I'm experimenting with the 741 op
    amp just to familiarize myself with its operation and it's acting
    rather peculiarly. The supply pins read at +/- ( +/- 9V) appropriatly
    and I'm running it as an inverting amp with feedback resistor of 10K
    and my input resistor of 1K and my signal into this. My + input is
    grounded
    as is pin8. Both offsets are open. So my gain should be 10. The first
    problem is that I'm getting a triangular wave when my input is
    anything higher than around 50mV rms. I'm showing gain,not anywhere
    near 10. If I feed it 10mV rms I get a nice gain back with a well
    formed sine wave,but as I progressively go up ,it changes to a
    triangular wave,and the gain dumps.,no clipping noted. This is all at
    100khz,the lowest my generator will go. I'm working on those
    breadboards from radio shack. Any ideas on whats going on? Thanks.I
    just went through several bad 741"s,nothing on the output pin and I
    thought I was going crazy until I got one of the older model 741's and
    at least I'm getting signal with gain but the waveform has me
    flumoxed. Any help is appreciated. jim
     
  2. John Larkin

    John Larkin Guest

    The 741 is slew rate limiting; its output can't slam fast enough to
    keep up with the ideal output waveform at 100 KHz. Check the 'slew
    rate' spec. Also keep in mind that its gain-bandwidth product is only
    about 1 MHz, so it can't deliver a correct gain of 10 at 100 KHz. You
    need faster opamps or a slower signal generator!

    John
     
  3. jim

    jim Guest

    Yep,Thanks,I kinda of was thinking along those lines but I guess I was
    expecting clipping rather than the triangular wave. I was also worried
    about the effect of stray capacitance the breadboards are noted to
    have. jm
     
  4. Paul Blitz

    Paul Blitz Guest

    Yup, it's the gain-bandwidth limit that's upsetting things!

    I gave up using 741's ages ago, and now use the TL071 / TL072 / TL074
    (single / dual / quad) range. They are just as cheap (probably cheaper),
    very well behaved, and perform a LOT better than the 741 does.

    Paul
     
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