limits - Find the value of the series $sumlimits_{n=1}^ infty frac{n}{2^n}$

Find the value of the series $\sum\limits_{n=1}^ \infty \dfrac{n}{2^n}$

The series on expanding is coming as $\dfrac{1}{2}+\dfrac{2}{2^2}+..$

I tried using the form of $(1+x)^n=1+nx+\dfrac{n(n-1)}{2}x^2+..$ and then differentiating it but still it is not coming .What shall I do with this?