已知\(f\left( x \right)={lo}{{{g}}_{2}}\left( {{4}^{x}}+1 \right)-kx\left( k\in R \right)\).
\((1)\)设\(g\left( x \right)=f\left( x \right)-a\),\(k=2\),若函数\(g\left( x \right)\)存在零点,求\(a\)的取值范围;
\((2)\)若\(f\left( x \right)\)是偶函数,设\(h\left( x \right)={lo}{{{g}}_{2}}\left( b\cdot {{2}^{x}}-\dfrac{4}{3}b \right)\),若函数\(f\left( x \right)\)与\(h\left( x \right)\)的图象只有一个公共点,求实数\(b\)的取值范围.