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Most of the op-amps use differential amplifier

  1. Sep 30, 2008 #1
    1.My text says 'most of the op-amps use differential amplifier as the input stage.The two transistors must be biased correctly.But it is not possible to get exact matching of two transistors.Thus the input terminals which are the base terminals of the transistors do conduct small dc current'.
    I cant understand this.If the characteristics of the two transistors are not similar would it result in the op-amp drawing small dc current?.
    2.There's one line which says the op-amp characteristics do not depend on temperature and there's another statement which states the input-offset voltage depends on temperature.which is true?
     
    Last edited: Sep 30, 2008
  2. jcsd
  3. Oct 1, 2008 #2

    berkeman

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    Staff: Mentor

    Re: op-amps

    For BJT inputs, there will certainly be an input bias current, independent of the quality of the matching of the differential pair. Even for FET inputs, there is a very tiny input bias current. The mismatching of the input differential pair gives rise to the Input Offset current/voltage specifications, not the Input Bias Current specification.

    And all of those depend on temperature. Your text must have been referring to something else when it mentioned temperature independence.
     
  4. Oct 2, 2008 #3
    Re: op-amps

    thank you for replying.so the op-amp will draw some bias current(due to indvidual transistors ) and the offset current will add to this current when the transistors are mismatched.am i right?
     
  5. Oct 2, 2008 #4

    berkeman

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    Staff: Mentor

    Re: op-amps

    Sort of right. The bias currents are just that -- they are what help to bias up the input differential pair in their linear region. The "offset" current is the extent to which these input bias currents are different. This is important in some applications, because the input offset current will generate an input offset voltage via the input bias resistors that you put outside the opamp, and this offset voltage will get multiplied by the gain of the opamp and cause an output offset voltage error. In some applications, this doesn't matter; in others, it can be a problem.
     
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