PA的增益控制是怎么实现的？

12-09

我看PA一般都分为低躁放,驱动,末级放大三级.
那一般VGA的功能是在哪一级?
而为了实现低增益,是不是需要关闭其中的某一级?
thx

If there is one stage is shut down,the PA can not deliver the RF signal,since all the three stages(for example) are connected in series.One solution is putting the VGA in front of PA,and use the power detection point to control the VGA to have the proper gain.