Lernend B. wrote:
> I can adjust frequency but it effects amplitude too (amplitude is changing
> while adjusting frequency), how can i separate these two variables ?
Simple answer: you can't.

Or a little bit longer: you can, but you must add much more parts into 
your schematic to keep the oscillator sinusodial AND to stabilize the 
amplitude of the output signal.
So the first step is: generate a sine signal.
Second step is:  stabilize it to a stable amplitude.
And the last step is: amplify it to the desired output swing.

Thats why real world signal generators are more expensive compared to 
some C, R and a transistor.

> Any idea of it?
Whats the actual problem?

Lernend B. wrote:
> Hello folks,
> Recently I am working on RC oscillator circuit according to here
> (source:
> 
https://www.kynix.com/Blog/What-are-Oscillator-Types-Example-with-Diagrams.html),
> but I had problems with adjusting frequency.
> ..........................
> Any idea of it? Thank you in advance.

I think, for a real helpful answer, we need some more information:

* What is the required tuning range (frequency)?
* What is the required amplitude range?
* Is it allowed to vary two components in parallel?
* Are you required to use the shown circuitry (phase shift principle 
with a BJT)?

You should know that there are many different and versatile oscillator 
circuits (more than mentioned in the referenced paper), which - probably 
- are more suitable for your needs....