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the inductive impedance connected to drain can cause negative resistance at gate?

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Alex_Zhan

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Hello everyone! I read a paper but I am puzzled when I read this sentence. 4.JPG

In my mind, I think the resistance at gate is extremely large, how can it became negative after an inductive impedance is connected to the drain?
 

Hi,

you just post a snippet of the text. We don´t know the context. So we need to guess.

I guess it should be "impedance" instead of "resistance". And this may be caused by the C_GD. Also do a search for "miller capacitance".


Klaus
 

Question: Is the circuit which belongs to the quoted text an oscillator?
Note that each oscillator with a feedback loop can also be seen as a negative-resistance oscillator.
In this case, a negative resistance cancels out the positive damping resistances and, thus, allows oscillation.

Example: The oscillator circuit called "Clapp oscillator" shows a negative input resistance at he gate node.

- - - Updated - - -

Second example: WIEN oscillator with fixed gain A=1+Rf/Ro.
Without the grounded resistor R0 the input resistance at the inverting node is Rin=- Rf/2 (at f=fo=1/RC) .
With Ro=+Rf/2 this leads to the well-known oscillation condition with A=1+2=3.
 
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