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What is meant by the input impedance of a voltage source?

  1. Oct 15, 2013 #1
    I am familiar with the concept of the internal resistance of a voltage source, but what is meant by the input impedance?
  2. jcsd
  3. Oct 15, 2013 #2

    Simon Bridge

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  4. Oct 15, 2013 #3
    OK, maybe I get it now.. The input impedance of a voltage source is the impedance 'seen' by that source? So it's not a property of the source itself?

    In that case I am still confused about this problem I'm supposed to solve (this should maybe go in the homework section, though):

    "A circuit has input voltage V1 and V2. Design the circuit so that the output voltage is V0 = 10∫V1dt -5V2.
    The input impedance of both voltage sources should be ≥ 100kΩ. Assume ideal op amp."

    So if my understanding is correct, then I'm not sure what is being asked for here?
  5. Oct 15, 2013 #4

    Simon Bridge

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    A voltage source does not have an "input impedance", since it does not have "inputs".
    I think I've seen the term used in the context of a voltage source before but am havig trouble finding an example besides yours.

    ... you appear to be asked to design a circuit using an op-amp - the circuit in question has two voltage sources in it - which should have some information about frequency and phase as well and amplitude (voltage) if only implied (i.e. perhaps they are variable DC sources?)

    When you draw your circuit diagram, the voltage sources should probably be drawn as ideal sources in series with an ideal impedance. Consider: why do the impedances need to be ≥ 100kΩ? What is special about that? How does it impact on the design of the circuit?

    Have you had some lessons about op-amp circuits?
    Notice that the op-amp has to integrate the first source - do you know how to use the op-amp as an integrator?
    Last edited: Oct 15, 2013
  6. Oct 17, 2013 #5


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    I agree for the most part. I think the problem is probably referring to the input impedances as seen by the voltage sources. Think of the voltage sources as signal sources that would be loaded down by anything less that 100K ohms. They should probably still be modeled simply as ideal voltage sources.
  7. Oct 18, 2013 #6
    OK, thanks, I get it now. What threw me off whas the fact that I can't decide what the input impedances are, since these will depend on the load; but of course I can make sure that they are greater than 100 kΩ regardless of load.
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