Variations on Final Build

My first challenge was to build an oscillator. The oscillator is an essential feature of the power source. I intended to duplicate the Sonicare system and then make modifications to scale up the power output. Of course, there are numerous options when choosing an oscillator. My first choice was a Colpitts oscillator.

I chose the Colpitts because it easy to assemble from discrete components, inexpensive and simple to troubleshoot should problems arise once the circuit became more complex. A final benefit of the Colpitts is that it can maintain oscillations into the megahertz if necessary.

I was unable to get this circuit to oscillate on the first build. I trimmed the leads on all components to reduce unwanted impedances. I finally was able to observe a 2.1MHz sinusoidal oscillation in the above configuration. I also observed a 8Mhz signal and as high as 13Mhz by adjusting the values of C2 & C3. It is important to note that this circuit is very temperamental.
At times it was difficult to initiate oscillation. Professor Joseph Clark pointed out that stray impedances on the breadboard are problematic at high frequencies. He says I would get better results on a soldered circuit board. When the Colpitts worked, the oscillations were stable and I measured 400mVPP on the output.

The next step was be to amplify this signal and drive a transmission coil. I attempted to use a 741 in an inverting configuration with a gain of 2. Unfortunately when the Colpitts was connected to the 741 it stopped working entirely. I attempted to use multiple amplification stages of different designs. Unfortunately, none of them were effective. It seemed that any loading whatsoever on the output of the Colpitts made it collapse. I decided to abandon the Colpitts.

For the sake of brevity I will not go into as much detail about the subsequent design alterations. My next permutation was to use a 555 timer IC. The out put was very stable but not sinusoidal. I considered adding a low pass filter to the output to correct this. For the sake of simplicity, I replaced the 555 with a Wein-Bridge oscillator

Now that I had a stable oscillator, I focused my attention on an amplification stage. There were many variations. Ideally there would be two stages: voltage gain and subsequent current gain. I attempted this with FETs, BJTs, a combination of both and finally with Op-Amps. I was never able to generate enough gain to drive a significant amount of current into my primary coil.