19．（2021•大同模拟）已知各项都为正数的等比数列$\{a_{n}\}$的前$n$项和为$S_{n}$，且满足$a_{1}=1$，$S_{3}=7$，若$f(x)=S_{n}x+a_{2}x^{2}+a_{3}x^{3}+\ldots +a_{n}x^{n}(n\geqslant 2)$，$f'(x)$为函数$f(x)$的导函数，则$f'$（1）$-f'(0)=($　　$)$