How is this circuit a negative resistance oscillator

Question

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 1 Circuit 2

Circuit 3

Circuit 4

Circuit 5

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.

Circuit 7

Answer 1

Here's just the negative resistance part.

With zero voltage on the base of Q1, there's a current that flows into the base of Q1 and the collector of Q2 (through R2). If the voltage at the base of Q1 is raised, the voltage at the emitter of Q2 is raised. This decreases the amount of current being pulled into the parallel combination of Q1 base and Q2 collector.

So more voltage gets you less current -- that's effectively a negative resistance.

You can find this out more formally by simulating the circuit, or by drawing it out and analyzing it on paper.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

Each oscillator with a frequency-selective feedback loop can also be seen as a negative-resistance oscillator.

• In some cases, the oscilator function can best be explained using the "negative-resistance" view, but

• In most cases, it is easier to explain its working principle with the "loop gain" method.

Example: The well-known WIEN-Oscillator is mostly (if not always) described using the bandpass like feedback (damping 1/3 at fo) and a fixed opamp gain of "3" (unity loop gain). However, looking into the inverting input (all resistors remain connected), the input resistance at this point is the parallel combination of the grounded resistor (Ro) and a negative (frequency dependent) resistance (-Rin) of the same value. At the oscillating frequency fo we have Ro||(-Rin) >>> infinite.