Electronic – Op Amp based Hartley oscillator

operational-amplifieroscillatorsimulation

I would like to build an oscillator that will satisfy a few requirements. The main requirement is to have exactly one capacitor. Another one is to have high Q and sine wave output.

I am considering Hartley's oscillator design. Since it has only one cap. I don't what to built the biasing network and select a transistor that will suit the oscillator's needs, so I figured that there must be a way to build Hartley's oscillator around an operational amplifier as shown on the figure below.

enter image description here

I have made the simulation as shown above, and I believe it should oscillate, since the gain of the Op Amp negative feedback loop is greater than \$\frac{L1}{L2}\$ ratio. But the oscillations start and die. I can see that the frequency of the decaying oscillations is correct, but they would not sustain. I event tried to crack the gain up to some unreasonable number, but this doesn't help at all.

Can anyone advise me as to what I'm overlooking and how to make this circuit oscillate?

PS. The simulation uses 741. If is is not the best op-amp for the job, then what would be a good replacement?

Best Answer

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.