question about feed back gain of oscillator.

[goldsmith]

Hi guys
I know that if the B* av become 1 , the circuit can tendency to the oscillation . but when the feed back network is applied to the amplifier , How can i find the new AV and B( feed back gain) ?
for example for a simple Hartley oscillator?
Best Regards
Goldsmith

 

[LvW]

Goldsmith,

the answer to your question would me much easier if you would not use symbols and abbreviations (B, AV) but instead WORDS.

 

[goldsmith]

Dear LvW
Hi
Thanks for your reply. here is my circuit:


I want to find the B , and av. i think the av without feed back is like simple ce amp? is it right?
and i think that the feed back loop will change all things!
Is it possible that you guide me , please?
Best Regards
Goldsmith

 

[LvW]

Goldsmith,
why don't you follow my recommendation to use WORDS instead of symbols? It is really important to know the meaning behind such abbreviations/symbols - otherwise you cannot understand the operation principle of such circuits. For example: For oscillators the term "loop gain" is a keyword. Therefore, how are your symbols B and av related to the loop gain?

 

[goldsmith]

Dear LvW
What do you mean by :"WORDS instead of symbols"?
My mean by B is the feed back loop gain . and my mean by A is the amplifier gain . and i think if A*B become 1 or grater than 1 the circuit can work as an oscillator. is it right? and how can i find them in that circuit?
Thank you for your patience and guide.
Goldsmith

 

[LvW]

Dear LvW
What do you mean by :"WORDS instead of symbols"?
My mean by B is the feed back loop gain . and my mean by A is the amplifier gain . and i think if A*B become 1 or grater than 1 the circuit can work as an oscillator. is it right? and how can i find them in that circuit?
Thank you for your patience and guide.
Goldsmith

If A is the amplifier gain and A*B is the loop gain (unity for oscillation) then B is simply the "feedback factor" (always <1).
There is nothing like "feedback loop gain". Perhaps now you can see how important it is to use words/descriptions rather than simply symbols or letters.
Otherwise, misunderstandings are possible.

---------- Post added at 08:53 ---------- Previous post was at 08:50 ----------

"Loop gain" is the gain of the complete feedback loop which for the purpose of calculating ot simulating has to be opened at an appropriate node.
Therefore, firstly identify the feedback loop!

 

[Qaisar Azeemi]

I think such gains can be easily found by using a multimeter(Volt meter) and a few mathematical calculations.

since the amplifier gain depends upon input and output voltages i.e; Av=Vout/Vin
so you can find out the amplifier gian of your oscillator circuit usin a multimeter.

after finding Av now use it in the following formula to get the New feed back gain:
Av*Vin - Av*B*Vout = Vout
or
B= (Av*vin - Vout)/(Av*Vout)

since you have measured Vin, Vout and Av so it is very easy to find new Feed Back gain....

 

[goldsmith]

Dear Qaisar Azeemi
With voltmeter we can not measure high frequencies , but with oscilloscope , we can do it. but i want to design this oscillators , and i have to do this calculations at theory (at first step , and after designing , i want to build these circuits).
What is your idea about finding Av and B , of this circuit?
Best Regards
Goldsmith

---------- Post added at 12:18 ---------- Previous post was at 12:15 ----------

Dear LvW
Is it possible that you say , how can i find the closed loop gain of that circuit? i can calculate the gain of amplifier , without feed back network , simply . but with this type of feed back i can not find it.
Sincerely
Goldsmith

 

[LvW]

Is it possible that you say , how can i find the closed loop gain of that circuit? i can calculate the gain of amplifier , without feed back network , simply . but with this type of feed back i can not find it.
Sincerely
Goldsmith

Dear Goldsmith,

in another thread I gave you a general recommendation how to make some progress in understanding electronics.
Regarding this thread: An oscillator has no "closed loop gain" because it is not an amplifier with an output-to-input ratio.
You shouild know that all parts connected to the transistor (including the feedback network) contribute to the gain value.
Don't try to search for an answer - it's rather complicated in this case. Start your studies with circuits that are easier to understand.
For example: Do you know why most opamps are "universal-compensated"?

 

[goldsmith]

Dear LvW
Thank you for your guide and good advice , really. and about your suggest here:"For example: Do you know why most opamps are "universal-compensated"? " no i don't know . but i want to know that what is your mean by universal compensated and why they are universal compensated?
Yours sincerely
Goldsmith

 

[LvW]

Dear LvW
Thank you for your guide and good advice , really. and about your suggest here:"For example: Do you know why most opamps are "universal-compensated"? " no i don't know . but i want to know that what is your mean by universal compensated and why they are universal compensated?
Yours sincerely
Goldsmith

Intentionally, I will give you an answer that is incomplete: Because of stability reasons.
Now, you should try to find out by yourself the principle behind this answer. An internet search using the keyword "frequency compensation" or "universal compensation" should give you some information.

 

[fareen]

i m also designing a similar oscillator but doesnt know a thing i need help with the calcuation
can you ppl guide me
here is my circuit

goldsmith what cosideration you take when baising transistor
how much maxium amplitude do u want??

 

[goldsmith]

Dear Fareen
Hi
What do you need exactly? is your problem , designing the biasing network of your oscillator?
Best Wishes
Goldsmith

---------- Post added at 00:05 ---------- Previous post was at 00:00 ----------

it will be better , that you prevent from designing your oscillators with CC amplifiers. the AV of them will be unity ( in fact will be lower than unity ) . so , cc amplifiers are not good selections for , oscillator design aim .because the av *bv should become greater than 1 . and the av*bv can not exceed from 0.9 with cc amplifier. thus these are not good things for oscillators.