LTspice simulation distortion function

[koolguy]

The formula of Phase shift oscillator is F=1/(2*pia*R*C(6+4Rc/R)) and the input impedance is R'=R3//R4 which is 2.4301 Kohm.

Your schematic has wrong values which makes it very difficult to calculate the frequency and amount of phase-shift for each RC network.

How to choose value of RC as my teacher told C the capacitor in Phase shift should be C=0.01uf or .005uf, why??

 

[audioguru]

The formula of Phase shift oscillator is F=1/(2*pia*R*C(6+4Rc/R)) and the input impedance is R'=R3//R4 which is 2.4301 Kohm.

But the fairly low input resistance of the transistor (do you know how to calculate it?) is in parallel with R3 and R4. The total resistance is supposed to be the same value as R5 or R6 which is 2.9k ohms ohms.

How to choose value of RC as my teacher told C the capacitor in Phase shift should be C=0.01uf or .005uf, why??

There are many phase-shift oscillator tutorials in Google and they all show the calculation for the frequency.
Maybe your teacher forgot that the transistor has a fairly low input resistance (do you know how to calculate it?).

The first link in Google shows a resistor in series with the base of the transistor to increase the total load resistance for the 3rd phase-shift capacitor.

Another link in google uses a darlington transistor that has a very high input resistance then the load for the 3rd phase-shift capacitor is calculated with the resistor values.

 

[koolguy]

But the fairly low input resistance of the transistor (do you know how to calculate it?) is in parallel with R3 and R4. The total resistance is supposed to be the same value as R5 or R6 which is 2.9k ohms ohms.

I/P impedance is R' = R1//R2//Beta*Re i think it it the formula.

 

[audioguru]

I/P impedance is R' = R1//R2//Beta*Re i think it it the formula.

Correct. But every transistor has a different amount of Beta so you don't know the exact input impedance unless you measure the Beta.

 

[koolguy]

the input impedance is R'=R3//R4 which is 2.4301 Kohm.

As the resistance is 2.9k and i/p impedance is 2.4 something should be exact??

 

[audioguru]

The input impedance of the transistor is about 1.3k ohms and is in parallel with the 2.4k of the resistors then the total resistance is only 843 ohms. It is supposed to be 2.9k ohms.

 

[koolguy]

The input impedance of the transistor is about 1.3k ohms

How, what is Beta??

 

[ericgibbs]

How, what is Beta??

Look at this link
YouTube - Transistor Beta X Transistor Beta = Darlington Beta

also consider the effect of using a Darlington pair in your oscillator circuit will have on the base input impedance of the circuit.

Recall you are requiring an input impedance/resistance of ~ 2.9K

 

[koolguy]

Look at this link
YouTube - Transistor Beta X Transistor Beta = Darlington Beta

also consider the effect of using a Darlington pair in your oscillator circuit will have on the base input impedance of the circuit.

Recall you are requiring an input impedance/resistance of ~ 2.9K

I am asking about in your schematic what value of Beta is put.

 

[audioguru]

I used a 2N3904 transistor that has a typical beta of about 150. In your phase-shift oscillator circuit its collector current is about 3mA and its Re is 0.026/3mA= 8.67 ohms. Its emitter resistor has a bypass capacitor so it does not enter the calculation for input impedance.
Then its input impedance is typically 150 x 8.67= 1.3k ohms.

The datasheet for the 2N3904 transistor has a graph showing its typical input impedance at different currents which shows a typical input impedance of 1.3k ohms at 3mA collector current.

 

[koolguy]

As for calculating single phase shift from one RC network, the formula is angle* =Tan-1(1/Omega*C*R).
Pls. tell in how to choose value correctly for phase shift.

Eg: if 90* is required then c or r should be = 0 or zero

 

[ericgibbs]

As for calculating single phase shift from one RC network, the formula is angle* =Tan-1(1/Omega*C*R).
Pls. tell in how to choose value correctly for phase shift.

Eg: if 90* is required then c or r should be = 0 or zero

hi Ritesh
You dont need 90deg per pole [ R/C] as you have 3 poles, so you need 60deg phase shift/pole to give 180deg total and the transistor gives another 180deg shift.

So the overall phase shift is 360deg and is what you need for oscillation to occur.

The transistor also provides gain in order to compensate for the attenuation in the 3 poles. OK

 

[koolguy]

You dont need 90deg per pole [ R/C] as you have 3 poles, so you need 60deg phase shift/pole to give 180deg total and the transistor gives another 180deg shift.

Yaa, I know but i was asking how to calculate it for 60degree shift.
I want to learn this in deep.

 

[ericgibbs]

Yaa, I know but i was asking how to calculate it for 60degree shift.
I want to learn this in deep.

hi,
Look at this useful tutorial.

**broken link removed**

 

[audioguru]

1) An oscillator needs a total of 360 degrees phase shift.
2) The transistor has a 180 degrees phase shift.
3) Therefore the total phase shift of the phase-shift parts must be 360 - 180= 180 degrees.
4) Simple arithmatic calculates the phase shift of each of the phase shift RC parts to be 180 degrees/3= 60 degrees each.

 

[koolguy]

1) An oscillator needs a total of 360 degrees phase shift.
2) The transistor has a 180 degrees phase shift.
3) Therefore the total phase shift of the phase-shift parts must be 360 - 180= 180 degrees.
4) Simple arithmatic calculates the phase shift of each of the phase shift RC parts to be 180 degrees/3= 60 degrees each.

I know this,
i am asking as angle* =Tan-1(1/Omega*C*R)., Omega=2*pia*F
how to get these value of F,C,R??

Is F is the required freq.??

 

[audioguru]

The frequency in the formula is f, not F.
The formula uses pi, not pia.

The formula for calculating the parts is in hundreds of articles about phase-shift oscillators that are found in Google.

 

[koolguy]

okk, i will see.!!

 

[koolguy]

Hi, see this I find in Google.

 

[audioguru]

Hi, see this I find in Google.

Use the program to design the transistor with a total input impedance of 2.9k ohms and a gain of 30 or almost 30dB. Then the transistor can be used in your phase-shift oscillator circuit and watch its output rise to clipping.