A RC Wien Bridge oscillator outputs a pure sine wave. The frequency depends on the resistor-capacitor combination in the oscillator. Embedded Eric built one using instructions from “Analog Circuit Design“:
I have been slowly reading through Jim Williams “Analog Circuit Design” book. Not slowly because it is boring, it is actually very interesting and a good read; I just get easily distracted and it has been a few weeks since I picked it up. I am about 200 pages or so into it and my favorite chapter so far has been the one where Jim Williams describes the steps he took while recreating William Hewlett’s Model 200A Wien Bridge Oscillator. It is essentially the same project described in Linear Tech’s AN43 Appnote, but with a lot more background and Jim describes the inner workings very conversationally in the book.
Jim’s first iteration of the circuit consisted of a RC Wien Bridge, an LT1037 Opamp and a #327 Lamp. The Lamp is used as a variable resistor that starts out at a very low 98 Ohms DC resistance that helps get the Opamp oscillating (because of the increased gain) and ends up stabilizing at around 235 Ohms once it heats up (thus servo-ing the gain down to an equilibrium point that keeps the circuit oscillating).
RC Wien Bridge oscillator – pure sine wave - [Link]