Hello!
I've got a project to design a RC phase shift oscillator, but without an OpAmp.
Basically, my job is to design an amplifier with phase inversion (because a group of 3 RC circuits will invert the phase by 180 which represent the positive feedback).
I've done some research and found the amplifier in the picture suited for the purpose (at least it seems to) .
I've tried to simulate in Multisim software and as you can see in the figure the output signal is weird and I do not understand why it has this shape. I need to design it in order to get at least a gain of 29.

The frequency of the oscillation should be around 5kHz and that is set by choosing the proper values for R and C.

Any suggestions ?

Thank you !

 

You are overdriving the input.
Obviously if the circuit has a gain of >1 the output will be saturated with an input of 5V since your supply is 6V.
Reduce the source to 1 mVac or so

 

I've reduced the source to 10mVac and the output looks like this.
As I can see from the oscilloscope the gain is about 40.

 

Your simulation did you so much good. In the old days we did it with one transistor.

 

See

 

To be clear, I mean this in the nicest possible way: 431 people are weird.

ak

 

How do I transform this in an opAMP, I mean I need like 'infinite' gain. What stage should I add and how ?

 

To be clear, I mean this in the nicest possible way: 431 people are weird.

ak

What are you talking about ?

 

How do I transform this in an opAMP, I mean I need like 'infinite' gain. What stage should I add and how ?

First you said you don't want to use an opamp, now you do.
Do you mean you want to add parts to the circuit you have to create an opamp circuit?

And you initially say you need a gain of 29, now it's "infinite".

 

To be fair Ts said that gain of at least 29 was needed. However there is a large gap between 29 and infinite.

 

First you said you don't want to use an opamp, now you do.
Do you mean you want to add parts to the circuit you have to create an opamp circuit?

And you initially say you need a gain of 29, now it's "infinite".

I am not allowed to use an opamp (meaning the integrated circuit as a symbol - the triangle), but I can use transistors to create the circuit that is 'inside' the triangle. As we all know, an inverting opamp with 2 resistors as a negative feedback has a gain of -R2/R1 only if the open loop gain is very high.
So, I thought that I need a circuit with very high gain, then I connect a negative feedback with 2 resistors to it and the closed loop gain should be higher than 29.

Am I clear enough ?

Thank you!

 

Opamps just make setting gain easy. You can do it without high-gain-plus-negative-feedback. Google "transistor phase shift oscillator ".