amplifier - Analysis of an Armstrong oscillator circuit

I am trying to analyse the following Armstrong oscillator circuit from HF Tesla coil experiments. I am particularly interested in this topology because of the class E operation and the high efficiency mentioned in the original project.

As the first step, I simulate the circuit and my LTSpice circuit is as follows. MOSFET R6020PNJ is used instead of 2SK2698 (The MOSFET model (STW14NM50) that is available in the same author's post was not giving me the oscillation). Inductance of primary, secondary and tickler coils are calculated from the geometry parameters provided in the project description. A series capacitor is added to the secondary to make the secondary self resonating at 4.76MHz to be same as the original project. A load resistance is added to make the secondary loop.

I understand the objective of original project is to generate an arc inside a Faraday cage, however, here, I am trying to use the secondary as the load.

First, I want to understand the operation of this topology. Particularly, the role of C3 and C4 is not clear to me.

Next, I would like to derive equations for oscillation frequency and the oscillation voltages across L1 and L2. I understand From the simulations that the frequency of the oscillation (~7.8MHz in this simulation) is highly dependent on the secondary resonance, but not equal to the self-resonance of secondary coil, or, L1-C4 resonance. (apparently, I also noticed that the input and output capacitance of the MOSFET are substantially larger than C4 : C_iss=2040p, C_oss=1660pF, C_rss=70pF) How Can I calculate the oscillation frequency?

A supplementary question to this analysis is asked here:

Answer

I want to understand the operation of this topology. Particularly, the role of C3 and C4 is not clear to me.

Lc and C3 form a supply voltage filter - in other words they try to prevent interference getting back onto the 60 volt line. This might be better explained if you gave a link to where the circuit came from. For instance, those components will also prevent noise from the 60 volts getting on to the oscillator but, the bigger picture isn't available so it's guesswork.

C4 (and the parasitic MOSFET drain source capacitance) dictate the resonant frequency of the oscillator along with L1. Loading effects due to RL on the secondary may shift this frequency slightly due to the coupling factor between the coils.

Next, I would like to derive equations for oscillation frequency

Any derivation needs to take into account plenty of minor as well as major factors and this is best served using a simulation tool but, the main factors involve: -

• L1


• C4 in parallel with MOSFET capacitance


• The above form a resonant circuit


• C5 and the secondary coil form a series tuned circuit that feeds the load


• That series tuned circuit and the coupling (0.8) will modify the oscillation frequency.

If I were to analyse it I'd probably use a sim and do an AC analysis that involved stimulating L1 and C4 (plus MOSFET capacitance) with a current source and looking where the resonant peaks might occur in the frequency spectrum for different loading values.

From that I'd consider going down the route of making a formula based on what I'd seen in the simulation.