The opamp is a Howland current pump which is controlled by the 10ms voltage source. The current pump sets the tail current for the transistor pair. The input on the left transistor must be very small, the two transistors and their collector resistors must be closely matched. The voltage difference between the two collectors is the output. Since it’s the difference, the DC component introduced by the control current is cancelled out.
In practice, this circuit works, but it is very susceptive to temperature variations.
