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.
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.
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.
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.
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
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
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.
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.
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.
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.