I'm learning about the Miller capacitance effect when a capacitor is tied to both the output and input of an amplifier. This is specifically about learning to deal with the parasitic capacitances of a transistor amplifier circuit. I understand the derivation to get 
$$
C\_\mathrm{Mi} = (1-A\_v)C\_f, 
$$
and it makes sense that you will get a positive miller input capacitance when Av is negative (or I guess also when Av is positive but less than 1?)

In the case that you DO have a positive gain amplifier, and you calculate a negative miller capacitance, how should I interpret this for the transistor frequency analysis? Would a negative capacitance result in a low-pass break frequency like a positive capacitor? Do you just pretend the feedback capacitor isn't there?