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Amplifier chaining

  1. May 2, 2010 #1
    I am trying to chain two op amp amplifiers. I thought that I could couple them through a capacitor so the output offset of the first amplifier doesn't get amplified. Now the first one is oscillating because of the capacitive load. Is there a way to do ac coupling without using a capacitive load for the first amplifier, or another way to prevent the oscillations?
  2. jcsd
  3. May 2, 2010 #2
    1. What is the frequency of oscillation?
    2. What is the gain of the op amp circuits?
    3. What is the frequency range you are amplifying?
    4. As always, a schematic will help.
  4. May 3, 2010 #3
    1.) 8MHz
    2.) 10000
    3.) 20kHz - 100kHz
    4.) G=100 Instrumentation amp followed by a G=100 inverting amp followed by a sallen key 100kHz low pass.
    I haven't really found the source of the oscillations, but driving capacitive loads is discouraged, so I wondered if there is any way around it.
  5. May 3, 2010 #4


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    A coupling capacitor is not generally considered a capacitive load.
  6. May 3, 2010 #5
    I know this is obvious, but have you tried using a smaller ceramic/polyfilm capacitor?
  7. May 3, 2010 #6
    I found the bug: Ironically the low pass was doing the MHz oscillations. The cable on the output was enough of a capacity! 20 Ohms in series and the problem is gone...
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