Command Block outputs a signal continuously after /testfor command has been used

When a command block is set to have a /testfor command and the command block is activated and outputs a positive signal (it tests true for the /testfor) the signal is constant and the command block remains powered. The command block will continue to output a signal until it is broken. Even when the comparator is updated this issue persists. The command block's output should stop once the command block becomes un-powered however this does not happen either.