Yes, ION you are absolutely right. Without knowing what is considered "important" one really can't choose between the two transistors.
Myself, I like to see the high voltages on the output without seeing excessive transistor heating, so for me, the TIP35C works "Better" than the TIP122. But in my construction, changing from one transistor to the other is simply a matter of loosening and tightening three screws in a terminal block. Don't even have to fire up the soldering iron. So it is easy for me to do comparisons, and if someone will just tell me what is important, what are the desired waveforms, etc. I can easily change to the transistor that will give me those characteristics.
But the TIP35C still even gives me those long-duration oscillations after disconnecting the main power supply, so I'm not convinced that the high gain of the TIP122 is actually important for even that effect either.
Yes the TIP35C will go out to 3 MHz GBW. https://en.wikipedia.org/wiki/Gain%E2%80%93bandwidth_productRegarding the long run times, if the circuit is not providing any output power, it can "freewheel" for a long time, until the caps run down. How long does it run without the two power supply bypass caps? What is important to remember that Blocking Oscillators are different in operational principles than typical feedback oscillators, but the various parasitic modes are produced when the circuit is "changing modes" or when the blocking oscillator is "between modes" With the higher gain of the TIP122 you are more likely to have the required gain to support some of the intermediate oscillation modes i.e modes available when the transistor is not fully off or fully on but at some point in between. These are the normal feedback oscillator modes and gain available must support oscillation criteria if it is to oscillate in these "between state modes" On the other hand due to the 3MHz GBW of the TIP35C you may have just enough gain to be able to hit some really high frequency (but less than 3MHZ) modes of oscillation. Then there is the other factor: stiffness of the power source. A saggy or current limited supply or supply chain can cause a low frequency "burst mode" of oscillation due to the recharge time constants of the rail capacitors. We used to call this "motorboating". There are many causes for "motorboating" besides a "soft" supply. It could also be caused by drive being quenched due to long time constants in the base drive scheme. A highly recommended read for all in this investigation: https://en.wikipedia.org/wiki/Motorboating_%28electronics%29we see there is the odd heavy handed base drive that has it's own long and short time constants due to sheer capacitor size and esr, and also capacitance of the diode in that circuit. We cannot at this time know what the builder had in mind in this choice of circuit configuration. The rest is straight JT with multiple output paths. Lots of stuff to juggle here. A good designer would present the rationale for his choice of each of the components in a complete circuit description, that is,if he truly wanted to teach an art. If he wanted to play cat and mouse games, that is another story.
« Last Edit: 2017-04-29, 23:50:09 by ION »
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