Re: Random thoughts on sinewave oscillators

On 19/10/2024 7:58 pm, Liz Tuddenham wrote:

Edward Rawde <invalid@invalid.invalid> wrote:
 [...]

A Wien bridge has an overall voltage gain of 1/3 ...

 If all the components are perfect and correctly matched.
 In a practical oscillator where the resistors are switched or the
capacitors ganged, there will be slight mis-matches due to tolerances
and the loss will be greater (and unpredictable).

The whole point about Wein bridges is that they are perfectly linear.
You have to introduce a controllable non-linearity that lets you tweak the gain to be exactly three when they are oscillating at the right amplitude, and dial it back if the amplitude is too big, or dial it up if it is too low.
In practice tolerance on the two crucial capacitors and resistors means that the gain for stable output is never going to be exactly three.
Just for kicks I simulated one with a thirty degree phase shift in the RC pairs (rather than the classical forty five degree phase shift) and it worked fine too.
--
Bill Sloman, Sydney