Hi,
The OpAmps U1, U2 and U3 form an ordinary differential amplifier which is an amplifier with very high input impedance and very low output impedance; I think its familier, this part has output ±24 Vpp if the output is from U3 to ground.
U4 acts as an inverting amplifier with unity gain. ie. its output is shifted by 180° from the input or simply it inverts (multiplies by -1) the input. This way if the output of U3 is Vo, the output of U4 is -Vo relative to ground. Now, if we take the output between the terminals of U3 and U4 making U3 the positive terminal, the ouput will be Vo-(-Vo)=2Vo, this way the output will be in the range of ±48 Vpp.
In other words, the output is differential between two terminals, which are opposite in polarity, if one is 1V the other is -1V and hence the differential output voltage is 2 V. Hence if the maximum is 24Vpp for each OpAmp alone, the differential will have output between 48 Vpp
Added after 5 minutes:
Hi,
The OpAmps U1, U2 and U3 form an ordinary differential amplifier which is an amplifier with very high input impedance and very low output impedance; I think its familier, this part has output ±24 Vpp if the output is from U3 to ground.
U4 acts as an inverting amplifier with unity gain. ie. its output is shifted by 180° from the input or simply it inverts (multiplies by -1) the input. This way if the output of U3 is Vo, the output of U4 is -Vo relative to ground. Now, if we take the output between the terminals of U3 and U4 making U3 the positive terminal, the ouput will be Vo-(-Vo)=2Vo, this way the output will be in the range of ±48 Vpp.
In other words, the output is differential between two terminals, which are opposite in polarity, if one is 1V the other is -1V and hence the differential output voltage is 2 V. Hence if the maximum is 24Vpp for each OpAmp alone, the differential will have output between 48 Vpp.