Hi!
First: I'm brazilian, so, sorry for my english. it sucks!
Second: what really matter, my question: how can I equate the output voltage of a wien oscillator? all the circuits with op amp that I have seen have an input, and the output is related to this input. the wien oscillator has no input. the circuit is below:

that's all folks! thanks!

 

Here is a pretty good article that describes the wien oscillator.

http://www.ti.com/sc/docs/apps/msp/journal/aug2000/aug_07.pdf#search="opamp wien oscillator"

As for your question on the seeming lack of an input, the answer is that the circuit does have an input. The source of the input signal is the output. A portion of the output is feed back to the positive input of the opamp. This fraction of the input is used to cause the opamp to oscillate at a frequency governed by the network that is connected between the output and the opamp's positive input.

hgmjr

 

Thanks!