multisimoscillatorsimulation
I am trying to design a phase shift oscillator, using a CE Amplifier. I have a Vc of approximately 12V, gain of approximately 250 I aim for an oscillation frequency of 60 Hz.
But whenever I start the simulation, no oscillations take place. Can anyone tell me what is wrong with the circuit?
When I convert into hardware it works perfectly, but not in simulation. I am using NI Multisim circuit design suite 14.
I think you have a few issues that you need to address.
The primary issue is that the Q of your tank is very low. The Q of a parallel RLC circuit is:
$$Q=R\sqrt{\frac{C}{L}}$$
Considering that your resistive feedback is effectively in parallel with the tank, we can calculate Q:
$$ 1500\Omega \sqrt{\frac{1nF}{15\mu H}}=Q= 12 $$
The Q of 12 is VERY low, so this is probably your main issue. Increasing the magnitude of R1 and R2 should be sufficient for your circuit to oscillate properly. See my example simulation here. If your Q is too low, you will need more gain to compensate for the losses.
A second potential problem is that the op-amp you selected, the 741, has a bandwidth that is a bit low for the frequency you selected. The datasheet indicates a bandwidth of 1.5 MHz, and your oscillator frequency is 1.3 MHz. This may result in your op-amp not providing enough gain for the oscillator to function properly. There are MANY op-amps that would provide an improvement over a 741.
Another possible issue is that oscillators are a bit tricky to get working in simulators. While it sounds like this is not an issue for you, it is a potential pitfall. Often, the random noise that usually starts oscillators in reality does not occur in a simulator. Often a noise source or impulse is required to kick-start the oscillator.
Works for me, if you make a few tweaks to the circuit:
Adding R7 and C4 let you get more gain out of the circuit, while keeping the DC operating point approximately the same. Your circuit wasn't oscilating because the collector of Q1 is "loaded down" by the parallel impedance of the phase shift network, therefore the actual gain of the stage was significantly less than the standard \$\frac{R_c}{R_e}\$ formula would indicate. Most analyses of the phase shift oscillator assume that the output of the gain stage is low impedance and isn't affected by the phase shift network, and also that the phase shift stages themselves don't load each other.
Sometimes when simulating an oscillator circuit it's also necessary to start the supply voltage at 0 volts, to "kick start" the oscillation, as I've done here.
Best Answer
The third phase shift stage (that is closest to Q1's base) is "ruined" by the biasing resistor R2. R2 is in parallel with R8 and it greatly reduces the value of R8. Instead, remove R2 and make R5 much bigger so that the appropriate base DC voltage is produced (circa 1 volt).
A test you can do is to open the feedback loop and inject an AC signal into C1 (from the left) and do a bode plot to see what output voltage you get at the collector. At the point you have output and input in-phase there should be a gain greater than 1.