Filter Group delay 和 step response關係

s20192s20192

最近在看Filter相關書籍
) s- d4 T7 c- D: f, i$ i書中提到
1.
5 U8 \* X5 L5 W% J+ O/ K" nThe time for the step response to reach the value 0.5 is an approximate measure of the average group delay
$ G5 f5 @" I. [/ z9 ^& Sand 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.
: x0 {6 U# g3 m: w, Y, g% Vvoltage mode and current mode
為何voltge mode較不適用在高頻阿
書中是說voltage mode 用 feedback
' J$ m5 s3 Y/ p3 v2 q4 E' O' R所以較不適用在高頻
但是用回授
9 E8 g% B$ p0 v! b7 E3 i8 Y不是頻寬增加!?
, B7 \# V/ e  B
想請教版上各位大大這兩個問題

6 V. A  M% \# X: Y1 w) j* s' v9 c8 Z5 ?感謝

s20192s20192

再多補充一個問題
" w$ L+ ~1 ], k. J0 l  z為何group delay (in passband)為何會隨頻寬減少而上升阿

感謝