Hi vinaych_22,
It would really depend on the underlying cause of your sore back.
If a mattress is too firm then it can cause you to twist away from pressure points or may not fill in the gaps in the sleeping profile (generally the waist or small of the back) which can cause back issues. In this case where a mattress that has comfort layers that are too firm or thin for good pressure relief a topper can be effective and the suggestions in post #8 here can be helpful to add some additional softness and pressure relief.
It sounds to me though that your mattress is probably too soft which can result in the heavier parts of your body sinking down too far and putting your spine out of alignment. Unfortunately it’s not really effective to use a topper to firm up a mattress that is too soft because the topper will just bend into the softer foam below it and in some cases can even make alignment issues worse. In cases where the mattress is too soft the most effective solution is to remove the softer foam in the comfort layers and replace it with either firmer foam or thinner layers.
Having said that … there are a few suggestions in post #4 here that may be worth a try.
Phoenix