The difference amp (Howland portion) converts the signal source on the left to a constant current. The bipolars boost the current through the common emitter configuration. That output goes through the load resistors located at the bottom right corner near ground (4 ohm & 20 ohm resistors). The vertical resistor (3 ohm) sets the current for the load. If you set this resistor lower than 3 ohms you will latch one of the transistors and break the oscillation.
