I believe this is the basis for opamp gain. There is also a pushpull at the output of the opamp for current gain. Any comments? Well, no, it isn't. Every op-amp has three basic subsystems: a differential amp as the input stage. This provides a moderate amount of gain. The next stage is the high gain interstage. This provides the lion's share of the open loop gain. Frequently constructed as a BJT stage with constant current loading, as a constant current source has a theoretical infinite impedance. The output is some variation on a complementary symmetry unity gain amplifier, as this provides the desireable low output impedance. The design for a complementary symmetry amp in that PDF you linked is fuxxored. I fixed it. In the correct version, The two RE resistors are usually chosen to be small, compared to the load resistance, and serve to set the no-signal idling current. The diodes provide the necessary bias to establish that no-signal idling current. The RB resistors determine the maximum base current. This circuit won't have a very high input impedance, and so is often driven with either an op-amp or some other driver that can operate into a low impedance, such as a constant current driven voltage amp, or sometimes a bootstrapping arrangement can be used.