It is very difficult to build an amplifier with 120 dB of gain and maintain the correct DC level through out. If the DC level changes at the input, it will also shift at the output by the gain of the receiver. To overcome this the DC between amplifier stages are blocked by a capacitor often in conjunction with a filter to reduce the bandwidth of a receiver. In real life most signals do not need their true DC levels to be received, and that can often be reconstructed at the output of a receiver, by say clamping to a known voltage
Frank