Operational amplifier(OP AMP) questions plz help

Discussion in 'Homework Help' started by shank09, Jan 8, 2012.

  1. shank09

    Thread Starter New Member

    Jan 8, 2012
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    1. Define positive and negative feedback.
    and
    2. Will an op amp with positive feedback operate in the linear region, or saturation? why?
     
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Hello shank, welcome to All About Circuits

    I take it you are studying op amps - what is your course?

    The rules here require some discussion of your thoughts from you first so do you have any thoughts?
     
  3. shank09

    Thread Starter New Member

    Jan 8, 2012
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    Yes i am studying Op amps.. my course is Computer Engineering (in high school :eek:)

    My thoughts about...
    for negative feedback i got "When we connect the output of an op-amp to its inverting input it’s called a negative feedback"
    but for positive feedback i dont get it much... the only thing that i know is that the output connects to the (+) positive input, and its a non-inverting input or something right?
    and
    for the last question i have no clue...
    plz help me expand more on these questions... :confused:
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    OK

    First feedback.

    Feedback is where we take some or all of the output and connect it to the input along with the original input.

    Now we assume that the delay between input and output is so small that when we do this the feedback is still the same as the input.

    But

    Just like with an ordinary battery which can be connected either way round, the feedback can be connected in the same direction or the opposite direction to the input.

    When the feedback is connected in the same direction it is called positive feedback and it adds to the driving signal (makes it bigger).

    When the feedback is connected in the opposite direction it is called negative feedback and it opposes or subtracts from to the driving signal (makes it smaller).

    So the result is that with negative feedback the final input and therefore the final output is smaller and with positive feedback the final output is larger.

    As a student programmer have you heard of recursion?

    This analysis assumes that the amplifier is still amplifying ie is operating in its linear region.

    Of course all good things come to an end so eventually if we keep increasing the positive feedback, thereby increasing the final output we can reach a situation where any given amplifier gives up and enters what we call saturation.

    In saturation the output no can longer increase, and in general the output no longer responds to the input.

    Electronic amplifiers often have two terminals or inputs one for positive and one for negative feedback. This makes circuit connection easier, but it is not necessary. For instance a microphone fed into an amp and placed in front of the driven speaker exhibits howl - which is positive feedback.

    does this help?
     
    Last edited: Jan 8, 2012
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  5. Adjuster

    Well-Known Member

    Dec 26, 2010
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    You really need to revise your course material. These are basic descriptive questions, about things you need to understand.

    You have the beginning of an idea about how an op-amp might be configured for negative or positive feedback, but that is a long way from knowing what feedback is, or what it results in.

    These concepts are key to many systems in engineering, but also beyond it into many other subjects like biology and even economics.

    Here are a some links to descriptions of feedback.

    http://www.allaboutcircuits.com/vol_3/chpt_8/4.html

    http://www.allaboutcircuits.com/vol_3/chpt_8/12.html

    http://en.wikipedia.org/wiki/Negative_feedback_amplifier

    http://en.wikipedia.org/wiki/Positive_feedback
     
  6. shank09

    Thread Starter New Member

    Jan 8, 2012
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    Thanks, Studiot and Adjuster
    This was very helpful...

    btw i am in grade 10, this is my first time taking this course, so i dont know much about it...

    Oh yea i need one more help (quick question).


    [​IMG]

    As you can see the pic above is a basic Negative Feedback... you can see that the Positive (+) input is going to ground right? My question is that what will happen to the Negative (-) input when it is a Positive Feedback... will it be also connected to ground?

    Thanks...
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Yes, for positive feedback just swop the + and - signs.

    The amplifier triangle you show is actually a differential amplifier. That means that it amplifies the difference between its + and - inputs.
    So if one is connected to ground it amplifies the difference between ground and the input. This can happen even without Rf (ie no feedback).

    Now let me ask you a question.

    What do you think would happen if Rf was not a plain resistor but a frequency sensitive component?
     
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  8. shank09

    Thread Starter New Member

    Jan 8, 2012
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    Hey, isnt this the differential amplifier...
    [​IMG]
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Yes, an Opamp differential amplifier has TWO inputs and FOUR resistors like this:
     
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  10. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Actually you need to reread what I said carefully.

    The triangle is the differential amplifier. It does amplify the difference between the two inputs, when correctly configured. Audioguru has shown one such configuration more fully. Not all (operational) amplifiers have both inputs, particularly the early ones. Equally not all amplifiers with both inputs are configured as differential amplifiers to the outside world.

    We are trying to help you along here and you did not answer my question about frequency so here is a different one.

    Every signal input is (connected) between two terminals. Audioguru's circuit shows V1 and V2. What do you think the second terminal is for each of these signals?
     
    Last edited: Jan 9, 2012
  11. crutschow

    Expert

    Mar 14, 2008
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    To add to studiot's post: The op amp is indeed a differential amplifier block with a very high (theoretically infinite) DC gain. Ag's post shows a differential amplifier circuit constructed with an op amp that has a fixed gain defined by the external components.
     
  12. Jony130

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    Feb 17, 2009
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