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"the FET gate goes into negative resistance under these conditions" . It was at this point that I got my coat.
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If you were to think about negative resistance from a physics point of view it could be a bit of a mind bender I guess...
But in the RF world, it's a much easier concept to understand
If you measure the reflection coefficient of a passive device the magnitude will be less than 1 because it will have resistive loss. This is because the reflected wave back from the device will be smaller than the incident (test) wave. So the mag of the reflection coefficient might be 0.9 for example. A passive device can't reflect more energy than is sent into it.
But if you have a device that has negative resistance it typically means that the reflected wave comes back bigger than the incident (test) wave. That behaviour is explained by the concept of 'negative resistance'. If the reflected wave is smaller than the incident wave it means there is loss in the circuit under test. If the reflected wave is exactly the same size as the incident wave then the circuit under test is lossless so the resistance is zero. But to cater for the case (with some types of active circuit) where the reflected wave comes back 'bigger' than the incident wave we can use the concept of 'negative' resistance to explain this phenomena.
If you have a series LC circuit and ground both ends and somehow induce energy into it (from a dip meter?) and then leave it alone it will oscillate forever at the resonance frequency if there is no resistance in these lossless components.
But we know that there will be resistive loss for real world components. Maybe 1 ohm for a typical LC combo at HF? So the oscillations fade to nothing very quickly. But if you fed one end of the LC circuit into a device (active device?) that had just over 1 ohm of 'negative' resistance then the oscillations would not just stay at the same level, they would grow and grow and grow until the limits of the active device were reached. This is because the active device is able to top up the LC circuit via the (bigger) reflected wave that comes back out of it. So the 1 ohm of real loss in the LC circuit is offset by the >1 ohm of negative resistance in the active circuit.