Stability of a negative feedback amplifier

Consider a negative feedback amplifier whose open loop gain is A(jw) and whose feedback factor is F(jw).

• We all know that the Oscillation Criterion is A(jw)F(jw)=-1

To evaluate the stability, the frequency Phase-Crossover Angular Frequency fp is defined as the frequency at which the angle of A(jw)F(jw) is -pi

and the frequency Gain-Crossover Angular Frequency fg is defined as the frequency at which the amplitute of A(jw)F(jw) is 1

I read in the text book that if fp < fg, for the frequency between fp and fg, the amplifier is not stable.

• my question is for frequency between fp and fg A(jw)F(jw) is not -1, how could the amplifier be not stable? thanks!