File:Negative resistance vs loop gain.svg

English: Graph of input current-voltage (IV) curve of typical amplifier with positive feedback for different values of feedback loop gain ${\displaystyle A\beta }$ showing it has negative resistance when ${\displaystyle A\beta \;>\;1}$.

• ${\displaystyle A\,}$ is the voltage gain of the amplifier
  
• ${\displaystyle \beta (j\omega )\,}$ is the tranfer function of the feedback loop
  
• ${\displaystyle R_{i}\,}$ is the input resistance of the amplifier.
  

The slope of the central linear segment of the graph is

${\displaystyle R_{f}={R_{i} \over 1-A\beta }\,}$

In electronic oscillator the negative resistance of a feedback amplifier is used to cancel the positive resistance in a tuned circuit, to excite oscillations in the tuned circuit.