Understanding the Wien Bridge Oscillator Circuit

[Omri]

Hi all,

I am doing a project on electronics about the Wien Bridge Oscillator. I learned about impedances and operational amplifiers and now I'm trying to understand the Wien Bridge circuit. I'm having some problems understanding it and I would be very happy to get some help. I still have not understood why oscillations occur in this circuit, what are the conditions for oscillations to occur, and how it works mathematically. I would be extremely thankful if anyone could instruct me or relate me to some good material about it.

EDIT: to be more specific, I've been trying to understand the analysis here:
http://en.wikipedia.org/wiki/Wien_bridge_oscillator"
If it gives some more specific reference... but I am totally willing to try other materials as well.

Thanks a lot,
Omri

 

[Omri]

Can anyone please help...?

 

[negitron]

Check out this informative Powerpoint presentation: http://jeremya.com/engr/engr332/WienBridge-Oscillator.ppt

 

[Omri]

Correct me if I'm wrong, but it looks like the ratio should be:
$$\frac{V_O}{V_A} = \frac{Z_S + Z_P}{Z_P}$$
and the presentation says it's inverse. In addition, I don't really understand how the resistors R1 and R2 affect the system - am I supposed to multiply the right hand side of the equation above by $$1 + \frac{R_2}{R_1}$$ to get it right?
The most significant question I have is, why does the circuit create sinusoidal voltage? I can get why oscillations will occur but I don't get why they should be sinusoidal.

Thanks for you help :-)