Nyquist stability criterion for positive feedback
This criterium (Nyquist) can be applied to all feedback systems. 

Note that positive feedback does not necessarily mean "instability". Furthermore, for real systems (with parasitics) negative feedback will always turn to positive feedback for rising frequencies. Hence, for this purpose (stability check) it makes no sense and it is not necessary to distinguish between "positive and negative" feedback. 

Remember, this criterion is applied to the loop gain - that means: The feedback loop is opened!