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Finding saturation range for an opamp?

  1. Mar 31, 2013 #1
    1. The problem statement, all variables and given/known data

    This isn't necessarily a direct homework problem that I have, but it is brought up a few times in homework problems for the chapter I'm reading through in my circuits book.. It will ask me if a certain voltage exceeds the saturation (or in other problems, at what value of the input voltage does the op amp reach saturation).

    I know in the cases where the opamp has a "V+" and a "V-", that those are the "bounds". If you have a V+ = 10V, and the input voltage times gain = 15V, then the output will still be 10V (in ideal circumstances, I believe that realistic applications that number is more like 85% of the V+\-)

    But what about circumstances in which a V+ and V- are not given? I read something about ideal opamps have an infinite open loop gain, but I don't really understand what that is supposed to mean. It seems to me that an opamp that doesn't have those V+ and V- bounds wouldn't be limited (possibly they would be by the fact if you have thousands of volts, despite minuscule currents, you could still fry the opamp).

    Any thoughts?
     
  2. jcsd
  3. Apr 1, 2013 #2
    The opamp V+ and V- are typically the power pins which power the device.

    Yes an ideal opamp does have infinite gain... a common op-amp style is a differential op-amp, where a finite gain of say 5 would amply a difference between the input terminals of the Op-Amp from 1Volt to 5Volts at the output.

    Real Op-amps do not have infinite gain, the infinite gain idea is introduced before the ideas of negative feedback opamps- where the output is threaded back to the input... which stabilizes this gain through resistor choices.

    At a simple level - think of an Op-Amp as a difference amplifier with a fixed gain.
    At a deeper level - Op-Amps are non-ideal devices with very high but not infinite gains which are stabilised through feedback & careful resistor selections.

    Real opamp circuits like instrumentation amplifiers often involve networks of Op-Amps connected together.

    The LM741 is the "standard" op-amp.

    I have spoken in a DC sense about op-amp operation.
    IF you have AC input to the two terminals of the op-amp.. then its response will vary according to the frequency of the op-amp. To properly understand this you need to know about circuit theory, laplace & fourier transforms.

    Its really only after some time that this has all started to "click" - I was very confused about it for a long time.
     
  4. Apr 1, 2013 #3

    CWatters

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    The output range of an opamp is normally limited. For example it might not be able to go within say 3V of the supply rails. So if the supply rails are ±15V then the output can swing ±12V. This is mentioned in the data sheets.

    If the gain of the circuit is 100 then the maximum input will be ±0.12V. If the input is more than that the output will be saturated.

    If the supply rails aren't mentioned in a homework problem then you can probably assume that they are large enough not to affect the answer.

    Most opamps have a very large gain (>100,000). When they are used in a circuit it is normal to add a feedback loop to reduce the gain to the required value (say 50 or 100). If the circuit is designed to produce a linear output with no distorsion you would want to ensure that the output doesn't go anywhere near the supply rails (or the data sheet value for saturation) so you reduce the gain to ensure it doesn't.

    Adding feedback also has other effects (such as increasing the bandwidth) but perhaps best not go there until you know a bit more about how they work.
     
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