12-20-2019 02:29 AM
Hey there, from what I know, we can convert DC to AC by using Phase Shift Oscillators. I'm able to pull off the manual calculations, but as for simulation on Multisim 14.2, I didn't get the desired graph(phase shifted). Maybe my understanding is wrong, so please let me know how can I make the following circuit to oscillate.
I set my output at node after C2.
05-23-2020 04:17 PM
Hello coldcoldwater,
If my understandings are right you want to build generator.
In this case you should have postie feedback to do so but form the schematic I can see that your op-amps positive terminal is connected to ground..
05-23-2020 11:00 PM - edited 05-23-2020 11:09 PM
Hello, this is an oscillator, not a DC to AC converter. A converter has an input and an output while an oscillator essentially amplifies its own noise and passes it through a very narrow resonant spectrum to provide a sine wave at the output. The oscillator does not need an input to provide output signal, unless a temporary trigger is needed.
The feedback is fine at the opamp negative input because your global feedback must be positive. The opamp provides 180deg phase shift while the RC network the other 180deg in the loop gain so your signal gets back in phase to the opamp input.
Take a closer look at VCC and VEE. The supplies seem inverted in your schematic. Look at the opamp datasheet to identify + and - supply terminals.
Now, you have connected 188.5k in parallel with 6.5k which gives a total feedback resistance of 6.28k. From hand calculations of the loop gain it results that for starting oscillations the feedback resistor should be 12x larger than the 6.5k used in the RC network. With larger feedback resistor the oscillations will start faster, but the output signal will be more distorted.
On a further note, this oscillator does not provide in itself an amplitude control. This means that the output amplitude grows until it is limited by the supply rails and you practically get something that resembles a square wave rather than a sine. For a sine wave you should consider adding an amplitude control circuit. See here https://electronicsarea.com/nonlinear-amplitude-control-of-a-sinusoidal-oscillator/ for a simple example.