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?
Electronic – Goral Oscillators
oscillator
Related Solutions
It's not the same circuit on the emitter. You've got a 1k pot in series with a 1k resistor and the pot wiper connects to a grounding capacitor. You've also got a 10pF cap where it is 100nF in LTSpice.
I'm not saying any of these matter but you can't possibly make comparisons with these glaring disparities.
EDIT
Following comments from the OP, the problem I think arises from the new "intended" operating frequency of the circuit (he uses a 10pF cap on the multisim circuit to set the resonant frequency whilst the LTSpice circuit has 100nF). Added to this is the lack of emitter capacitor - the multisim circuit shows it connected via the wiper of a pot set midway and this will drastically reduce the gain and prevent or delay oscillation. Remember that the collector has to generate enough signal to drive the 10nF capacitor (C3) and at a much higher frequency this 10nF (via C5, 1uF) will probably look like a short circuit - the gain of the transistor has to be greater than 1 for it to begin oscillation and with no directly connected emitter capacitor and a 10nF effectively connected to the collector I think the gain will be less than unity.
I'm not totally familiar with this type of Colpitts oscillator but it seems to me that the LTSpice circuit should oscillate at about 41kHz whereas the MS circuit should operate at about 4.1MHz - this isn't going to happen with a 10pF - its reactance at 4.1MHz will be nearly 4 kohms and this will get "battered" by R1 and R2 in the imedance versus resistance race. Do the sensible thing and start with the same circuit. On the LT circuit, the 100nF will have an impedance of about 40 ohms at 41kHz and therefore not be hardly affected by R1 and R2.
Is there any reason MOSFETs in Colpitts oscillators seem so rare?
The most basic problem with a MOSFET in a colpitts oscillator is that the gate-source junction is much more capacitive than either a JFET or a BJT (like a hundred to a thousand times more). So, that gate-source capacitance throws a big wrench in the works.
I mean, they can work (sub 1 MHz for instance) but due to the unknown value of the gate-source capacitance, the frequency they produce is a little unpredictable and, with varying gate bias conditions, the capacitance can also vary and add further woe to the performance and stability. After all, you want an oscillator to be stable and predictable.
Best Answer
The goral oscillator used as a VCO: -
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).