# help need for frequency response

#### jjohn

Joined Feb 18, 2009
13
I am studying RF circuit. I read the book "Elsevier - Telecommunication Circuits & Technology - 2000".The page image is attached. I know f= 1/2*pi*(Ro//RL) C where Ro is output resistance of transistor and RL is load resistor .
How I try is shown below
It look like 3C for R3//R5//R6//R7 and f' = 1/2pi*3C*(R3//R5//R6//R7),
there is another C for Re and f" =1/2*pi*RC ,
f= 1/(2pi* RC *(1/{(RC)^2 + (R3//R5//R6//R7)}))
I try to solve for equation(1.3) , I still cannot get that answer. Can anyone help me or hint me to do .thanks.

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#### KL7AJ

Joined Nov 4, 2008
2,229
I am studying RF circuit. I read the book "Elsevier - Telecommunication Circuits & Technology - 2000".The page image is attached. I know f= 1/2*pi*(Ro//RL) C where Ro is output resistance of transistor and RL is load resistor .
How I try is shown below
It look like 3C for R3//R5//R6//R7 and f' = 1/2pi*3C*(R3//R5//R6//R7),
there is another C for Re and f" =1/2*pi*RC ,
f= 1/(2pi* RC *(1/{(RC)^2 + (R3//R5//R6//R7)}))
I try to solve for equation(1.3) , I still cannot get that answer. Can anyone help me or hint me to do .thanks.

Howdy:

You can't just add the capacitors or the resistors in this configuration. Each R/C pair has to be calculated independently. The effect is to have a SHARPER cutoff frequency with three pairs than you would have with just one, but the cutof frequency will be the same. (Also the phase shifts accumulate for each R/C pair.

Eric

#### jjohn

Joined Feb 18, 2009
13
Dear KL7AJ
Thank for reply .But I still cannot figure out . Can you tell more detail like which resistor is need to parallel and how many f needed to calculate and how to combine f(total)?
Can you kindly see my new drawing and tell me what wrong with my calculation. thanks.Can you recommend any book for that . I find for electronic circuit analysis but cannot get answer.
thanks.

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#### jjohn

Joined Feb 18, 2009
13
Sorry about calculation is wrong . Correct approximated calculation is attached.But I still want detail calculation . for R3/R
If I was wrong , correct me.
thanks.

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#### The Electrician

Joined Oct 9, 2007
2,970
Your first post showed an oscillator circuit with a transistor as the active device; now you are trying to analyze a circuit with an opamp as the active device.

The reason the 4 R3/R term appears in the book solution is that the transistor output impedance is not zero, but approximately R3. The opamp circuit assumes that the opamp output impedance is zero.

See here for some explanation:
http://www.crbond.com/papers/ent23.pdf

and here:
http://en.wikipedia.org/wiki/Phase_shift_oscillator

Which circuit would you like to analyze? Your first post with the transistor, or your later post with the opamp?