which oscillator has good amplitude stability?

The best amplitude stability comes from a triangle wave generator feeding a wave shaping network to form a sine wave. Several manufacturers make single chip oscillators of this type. The amplitude is as stable as the power supply voltage.

For super precision, you could use an FET switching the output of a voltage reference. This would generate a square wave with an amplitude stability of 1ppm or better.

If only a sine wave will do, use a DAC fed by a digital synthesizer. Incredibly stable DACs are available, and you could use a PIC chip as the synthesizer. No feed back is involved here.

In this age of Numerically Controlled Oscillator (NCO) with fantastic stability (both amplitude as well as frequency and even phase), why do you go for obsolete (half a century old) technologies!

Direct Digital Synthesis is the technology. Qualcomm produces some good chips.

Wow you got good answers. Historically I think that the first successful product from Hewlett Packard was an audio generator that had a very stable output. The circuit if I remember correctly was a Wein bridge type with a lamp filament as the stabilizing element. This is just a historic note you already have good answers.

How it made depends on the application

Audio, RF, micro-wave oscillators all require some feedback mechanism.

It is this feedback mechanism that maintains the output level.

Then the overall accuracy (drift over time…) becomes the sole function of the detector

Hope this properly answers your question

look here

http://www.google.ca/search?hl=en&q=level+detector...

maybe this is better

http://www.google.ca/search?hl=en&q=oscilator+leve...