Spice-Simulation Using LTspice Part 2 - IEC & Associates

Note:Resistor R4 / 680Ω gives the positive feedback which causes the oscillation. A part of the emitter AC current isflowing into the resonant circuit through this resistor and „compensates the losses“.So if you increase the resistor’s value, the positive feedback will get weaker and weaker. This gives less distortionbut the oscillation will suddenly stop.Task:Try to find out this stop point by increasing the value of R4 and repeating the simulation.Watch the improvement of the waveform and check the better attenuation of the harmonics by the FFT-simulationof the frequency spectrum..Also be aware of the „oscillation start delay“ when reducing the feedback. When approaching the criticalpoint you get signficantly longer delays before the oscillation starts.(Note: with a resistor value of 6.8k you are not far from this point…please check it!)This is the result for 6.8k. The oscillation start delay is increasing….…but now you geta nearly perfect curve!126