why unity gain frquency of opamp>=30X singal bandwidth?

[maninnet]

unity gain frequency

I was told that in close loop application, the unity frequency of the opamp should be at least 30x the signal bandwidth (not always necessary, depend on application,
but just to be on the safe side), is there any theoretical orgin or is it just an Empirical value

 

[Fahmy]

Of course i t depends on the application , but usually you need the amplifier to amplify your signal. As your signal speed approaches the unity gain bandwidth of the amplifier the gain approaches 0dB . you can say that if your input signal BW is 30x smaller than the unity gain frequency then your gain is about 30dB (20dB/decadade *1.5 frequency decade) (you can calculate it in more accurate way of course). If this gain is still smaller than your requirements then you have to increase the factor 30 .