# Phase shift oscillator

1. Feb 13, 2016

### suv79

1. The problem statement, all variables and given/known data

Gain & phase response of TL072 op-amp

2. Relevant equations
'Frequency of oscillation was observed to be about 15 % lower'

Why ???
3. The attempt at a solution

so at this frequency the op-amp has 45 phase shift, but why will this impact the oscillator and lower the frequency.

PSpice simulation

analysis

why is my frequency 22 Hz

2. Feb 13, 2016

### suv79

how can i find an 'ideal op-amp' in PSpice ?

3. Feb 13, 2016

### LvW

An ideal opamp model in PSpice is simply called up with "OPAMP".
Regarding your result - I suppose the critical parameter is the limited slew rate which drastically lowers the frequency (and causes a triangle form).

4. Feb 13, 2016

### suv79

but would the slew rate effect the frequency or only distort the waveform,
the phase shift oscillator should producesa sine wave output.

5. Feb 13, 2016

### LvW

Yes - the slew rate SR lowers the frequency because SR causes in addition a negative phase shift (delay corresponds with phase shift).

6. Feb 13, 2016

### suv79

65 kHz is my worked out frequency but PSpice is 22 Hz

7. Feb 14, 2016

### LvW

I have simulated your circuit - and the result is at 55 kHz.
What are the time steps you have chosen?
My choice: End of simulation 1ms and resolution 1µs.
Why such a large simulation time for 55 kHz(1s)?

Comment: A much better waveform can be achieved if you split the feedback resistor in two (33k=23k +10k) and connect two diodes (antiparallel) across the 10k resistor.

Last edited: Feb 14, 2016
8. Feb 14, 2016

### suv79

you got 55 kHz that is what it should be about 15% lower.

9. Feb 14, 2016

### suv79

i think something is wrong with this iam still getting 22 Hz

10. Feb 14, 2016

### suv79

i have to write a report about why the circuit is 15% lower from designed frequency, when the TL072 op-amp was used.

have simulated your circuit - and the result is at 55 kHz.
What are the time steps you have chosen?
My choice: End of simulation 1ms and resolution 1µs.
Why such a large simulation time for 55 kHz(1s)?

Comment: A much better waveform can be achieved if you split the feedback resistor in two (33k=23k +10k) and connect two diodes (antiparallel) across the 10k resistor.

did you use PSpice to simulated the circuit? can you see any differences between your circuit and my one ?

11. Feb 14, 2016

### suv79

could you email me your simulated circuit ?

12. Feb 14, 2016

### LvW

I have simulated exactly your circuit using a realistic model for TL072.
Why didn`t you answer my question regarding simulation time amd time resolution??

Last edited: Feb 14, 2016
13. Feb 14, 2016

### suv79

the time i used was 1 Second

but time resolution i have no idea.

14. Feb 14, 2016

### suv79

the run time should not change the frequency

15. Feb 14, 2016

### LvW

OK - if you know better I have no problems.
It seems you are very familiar with simulation programs.

I have again removed my explanation for the frequency variation in my answer#12.
I see no necessity to give you hints if you know better.

16. Feb 14, 2016

### suv79

but i dont understand what is wrong with my circuit, why is it not getting 55 kHz

17. Feb 14, 2016

### suv79

i have chanced the run time to 1m the freq is now 21 kHz

but where can i change time resolution ?

18. Feb 14, 2016

### Staff: Mentor

What is the DC value of your circuit's output, and what value were you expecting it to be?

What pk-pk amplitude are the oscillations you are seeing?

19. Feb 15, 2016

### suv79

i have no idea how to work out the amplitude of the waveform

20. Feb 15, 2016

### suv79

why when the stop time is change dose it effect the frequency.