# Doubt on bandwidth of a control system

Consider this image of a loop shaping design:

here which is the bandwidth of the system? I have a doubt because until now I have considered as bandwidth to the system the one of the complementary sensitivity function, but I heard someone sayong that it is not a good measure for the bandwidth, so it is better to consider as bandwidth the one of the sensitivity function.

Can somebody clarify this?

Assuming a SISO feedback loop, with plant $$P$$ and controller $$C$$, a common definition of bandwidth is the 0 dB crossover frequency of the open loop transfer function $$PC$$. Assuming that the sensitivity can be written as $$\frac{1}{1 + PC}$$ this is also (roughly) the frequency where the sensitivity peaks.