I have heard a lot of Bob. = I have heard of Bob a lot. = I have often heard of Bob.
hear of sb. 听说过某人。
a lot 是表示程度的副词,意思是“量大,多,经常”,修饰 hear of。表示经常听到关于某人的情况。
还有一种分析:
a lot 是代词,作heard 的宾语,of = about。I have heard a lot about Bob. 意思是我听到关于某人的情况有很多。
第二种分析更合理。