I found these oscillator configurations(all are similar) at various sites on the web, but here and here the author says its a negative resistance oscillator. It's a popular oscillator among amateur radio enthusiasts due to its ability to oscillate at a wide range of frequencies, its often called emitter coupled oscillator/differential pair oscillator etc. Such one is even briefly touched on the book Experimental Methods in RF Design
Circuit 3
Circuit 4
Circuit 6
My question is that, is these circuits negative resistance oscillators, if so how is negative resistance achieved in these circuits? If not, how does such circuits oscillate?
EDIT: I found another different circuit that is said to be negative oscillator too. I would like to add it to the question too. The page also contains several other circuits similar to the ones shown above.