I am studying Colpitts oscillators with one single JFET. These circuits are well known. However, I experienced that I sometimes have start-up problems. I know this is related to the loop gain being too low. I the book 'Practical Oscillator Handbook' by Gottlieb I found the Goral oscillator. I modified my circuit and found out that the Goral oscillator works much more stable and reliable than the ordinary Colpitts. However, besides the mentioned book I didn't find any other sources about those Goral oscillators, this is why I think these are not very well known or they must have some drawbacks I didn't find out so far. After whom are they named, what is the theory behind them and what are their pros and cons?


The goral oscillator used as a VCO: -

enter image description here

Taken from here.

Adding an emitter follower just boosts the drive current to the centre tap point of the capacitors in what would otherwise be a standard colpitts oscillator. I've used this design in a capacitance probe based on the probe feed line being a tuned half wave resonant transmission line. However, it wasn't something that I knew about beforehand but something that was developed to solve a problem indicated below.

Because the t-line can present a very low impedance (especially at high temperatures of +1000 degC) the extra drive capabilities of the emitter follower make this a good solution.

I have no idea why it is called this name but, after a quick google I think it is named after a guy called Pierre B. Goral.

The main drawback is that because they use two transistors in cascade, the highest frequency the pair can oscillate is limited to the low to mid hundreds of MHz (currently).

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  • \$\begingroup\$ Just out of curiosity, what the heck were you measuring with the capacitive probe? 1000°C is a bit low for steel. Ceramic processing? \$\endgroup\$ – Spehro Pefhany Nov 18 '16 at 16:23
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    \$\begingroup\$ @SpehroPefhany Spinning blades in an aircraft turbine - some probes (and don't ask me what they are made of cause I don't know) will do 1300 degC \$\endgroup\$ – Andy aka Nov 18 '16 at 16:26
  • \$\begingroup\$ You didn't by any chance know Tony Williams did you? I think they're Titanium in the compressor and nickel-iron superalloys in the combustion chamber, but that's a dim memory. \$\endgroup\$ – Spehro Pefhany Nov 18 '16 at 18:48
  • \$\begingroup\$ No, his name doesn't ring a bell but as I work with a few ex rolls Royce folk from the derby plant I might ask them on Monday. \$\endgroup\$ – Andy aka Nov 18 '16 at 18:59

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