# Opamp

Discussion in 'Homework Help' started by tommydyhr, Sep 25, 2013.

1. ### tommydyhr Thread Starter Active Member

Feb 3, 2009
39
4
Today I was tasked with determining the output resistance as seen by the load resistor, RL, in the circuit below.

As far as I can tell, the circuit is a current generator, whose output current solely depends on the size of Rf (and the current in Rf should equal the current in RL since no current goes to the non-inverting input of U1 due to the summing point constraint), and is thus completely independent on the load resistor, which in turn would mean that the output resistance (given ideal components) is infinite, but I am by no means sure, as I've no way of proving it.

Am I correct in my assumptions? How would I approach an actual analysis of this circuit?

And apologies for the lackluster title. Forum wouldn't let me write anything more than a single word.

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Last edited: Sep 25, 2013
2. ### WBahn Moderator

Mar 31, 2012
17,455
4,701
The forum has a limit on the total length of a title, but doesn't limit it to just a single word.

The most general way to find output resistance is to use a voltage source as your load and then determine the relationship between the voltage of that source and the current through it. Take the derivative of that relationship and find how the voltage changes in response to a change in the current and you will have the output resistance.

There are several other ways you can go about it, too.