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

## Main Question or Discussion Point

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
Homework Helper
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?

Simon Bridge
Homework Helper
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?