Filter group delay

s20192s20192

最近在看Filter相關書籍
有些疑惑想請教版上各位大大
1.
The time for the step response to reach the value 0.5 is an approximate measure of the average group delay
and ringing in the step response indicates that the group delay varies strongly in the passband
想請問為何從step response中就可以知道關於group delay
有什麼相關數學公式來驗證這些關係嗎

2.
為何group delay (in passband)為何會隨頻寬減少而上升阿

3.
voltage mode and current mode
為何voltge mode較不適用在高頻阿
書中是說voltage mode 用 feedback
所以較不適用在高頻
但是用回授
不是頻寬增加!?

先謝拉~~