I am hoping to build an oscillator with the same specs as the very first HP product: the HP 200A (can be found here:
http://www.hparchive.com/Manuals/HP-200A-Manual-1951.pdf). I expect it to take a while, but I figure I'll learn a lot in the process. I'd like to do everything analog, but don't really have any other design requirements. I've done some research and figured I'd use a Wien Bridge oscillator just like the 200A did, and found the very informative discussion here:
https://www.eevblog.com/forum/projects/wien-bridge-project/ .
This led me to Linear Technology's Application Note 43 (
https://www.analog.com/media/en/technical-documentation/application-notes/an43f.pdf), which begins its discussion of Wien Bridge oscillators on Page 29. From there, I found an LTSpice schematic of a similar circuit on the LT1037 page.
I figured my first step was to really understand how the most basic model of the circuit, such as the one found on Wikipedia or in Application Note 43 worked. I figured I could try to derive the Transfer Function myself. But that quickly led me to my first question. How is it possible that there is no input to the circuit? Looking at the LTSpice simulation, the only voltage sources present are both op amp supplies. My understanding from university classes is that op amps provide differential amplification of whatever is fed in to the +/- terminals, but it looks like in the oscillator, nothing is fed in. How is it possible at all that the output is non-zero?
I figure I'm missing something basic in my understanding of op amps, but haven't found much online.