I just built this phase shift oscillator as I need a sinewave to experiment and learn a little about transistor circuits and although it works just fine, I am noticing the rise time is a little faster than the fall time, which component controls this please so I can make the adjustment for equal rise and fall times?

Neil.

 

p.s the 4k7 pot in this circuit is a fixed 4k7 resistor in mine since I don't have a pot of that value.

 

There will always be distortion in this kind of oscillator as the amplitude increases until output waveform is limited by the supply voltage. to avoid this you can use a circuit which reduces the gain when the amplitude reaches some acceptable limit.

 

Hello,

You might want to have a look at this application note.

Bertus

 

Thankyou for your rapid replies...I was thinking there was something wrong with my circuit, now I know not to expect a perfect waveform and wont worry about it, since I don't need it to be perfect for my application.

thanks again

Neil.

 

Oh one more thing....can someone breakdown this equation for me and tell me how to calculate it, my
maths is shitty at best and I am a little lost here about what I should be adding and or multiplying.

Neil.

 

There are two ways to arrange the phase shift network--the one you have where the capacitors are in series and the resistors in shunt, or the other way with resistors in series and capacitors in shunt. The capacitors in series style is equivalent to a high pass filter and harmonics in the sine wave (distortion in other words) pass through. The capacitors in shunt style attenuates the harmonics and yields a lower distortion sine wave.

See here for an example of the capacitors in shunt style, and the full calculations for the frequency of oscillation: http://www.crbond.com/papers/ent23.pdf

 

Thanks Electrician but im looking for more of a step by step, line by line approach......i hate math but in this case it is necessary....

Neil.

 

What i need is one of them phone apps where you punch in the equation numbers in the relevent boxes and it gives the answer

 

Have a look here: http://www.aikenamps.com/index.php/designing-phase-shift-oscillators-for-tremolo-circuits

Math in appendix A at the bottom of the page.

 

Look at the voltage on the right-hand side of each of the "C" capacitors with a high impedance scope probe. The amplitude will decrease as you move to the right, but you should see the distortion change as you move along the loop.

ak

 

Oh one more thing....can someone breakdown this equation for me and tell me how to calculate it, my
maths is shitty at best and I am a little lost here about what I should be adding and or multiplying.

Neil.

I think I misunderstood what you are asking here. I now think you want to evaluate this formula for the frequency, rather than being shown how it is derived. So with that in mind, did you use the values for the capacitors and resistors as shown in the schematic of post #1?

 

As long as you have 3 stages, (ie resistor /capacitor) , then your Frequency in Hz is =

F=1/ ( 15.39 x R x C)

C is in Farads
R is in Ohms

With your values that is 1.38Khz

 

F=1/ 15.39 x R x C

Ambiguous.
F=1/ (15.39 x R x C)

 

I just built this phase shift oscillator as I need a sinewave to experiment and learn a little about transistor circuits and although it works just fine, I am noticing the rise time is a little faster than the fall time, which component controls this please so I can make the adjustment for equal rise and fall times?

You can reduce gain by adding capacitor 51...330pF between collector and base of transistor.
In my case it was 180pF.
See pictures below.


Another way to reduce gain is replacing resistor 680 Ohm by pot 680 Ohm, then capacitor 22uF should be connected to wiper of pot instead emitter of transistor.