Incorrect stability criteria for non-foster circuits

The successful design of non-Foster circuits requires tests for stability that are accurate and reliable. The loop-gain and Llewellyn or Rollett tests have been used as the basis for stability analyses in papers on non-Foster circuits. However, these tests are unreliable and can report that a non-Foster circuit as stable when it is not. Simple circuits are shown that serve as counter examples to such tests. Consequently, loop-gain and Llewellyn-Rollett tests should not be relied on for evaluating the stability of non-Foster circuits. Any demonstration of circuit stability from the use of these tests should not be accepted.