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

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Discussion Overview

The discussion revolves around the concept of input impedance in the context of voltage sources, exploring its definition and implications in circuit design, particularly involving operational amplifiers. Participants also address a specific homework problem related to designing a circuit with certain input impedance requirements.

Discussion Character

  • Conceptual clarification
  • Homework-related
  • Debate/contested

Main Points Raised

  • One participant expresses familiarity with internal resistance but seeks clarification on input impedance, questioning whether it is a property of the source itself.
  • Another participant suggests that input impedance refers to the impedance 'seen' by the voltage source, indicating it may not be an inherent property of the source.
  • A participant challenges the notion of input impedance for a voltage source, stating that a voltage source does not have inputs and raises questions about the context in which the term is used.
  • Concerns are raised about the specific homework problem, with participants discussing the design of a circuit involving two voltage sources and the requirement for their input impedance to be ≥ 100kΩ.
  • One participant proposes that the problem likely refers to the input impedances as seen by the voltage sources and emphasizes the importance of avoiding loading effects on the sources.
  • A later reply acknowledges the confusion regarding the determination of input impedances, noting that they depend on the load but can be designed to exceed 100 kΩ regardless of the load.

Areas of Agreement / Disagreement

Participants express differing views on the definition and applicability of input impedance for voltage sources, with no consensus reached on whether it is a relevant concept in this context. The discussion regarding the homework problem remains unresolved, with various interpretations presented.

Contextual Notes

Participants note that the input impedance may depend on the load and that the specific requirements of the homework problem introduce additional complexity regarding circuit design and the behavior of operational amplifiers.

Zarquon
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I am familiar with the concept of the internal resistance of a voltage source, but what is meant by the input impedance?
 
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Simon Bridge said:
Have you tried looking up the term?
Can you describe where the regular definitions online lose you?

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

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?
 
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.

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.
... 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:
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.
 
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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