How to add I/Q signal after the mixer?

In an I/Q modulator,we use two seperate mixers to do up-conversion of I/Q signal,then how do we add these two paths together and then drive a PA?

You need a simple combiner to add the two paths.

Most people would combine the I/Q modulated IF signal BEFORE the upconversion. If you modulate the I channel, and then upconvert; and modulate the Q channel and upconvert with a 2nd mixer, you then have to worry a lot about the Amplitude and Phase balance between the two paths.

simple, you connect in your IQ modulator the I and Q outputs from these two separate mixers to the same load(=inductor or resistor). then you have I+jQ...