This example shows the ability of a Peltier Module, or Thermo-Electric Cooler (TEC), to actively transfer heat away from a "laser" or other electronic device, during fast-changing power dissipation conditions. A thermistor, with resistance that is sensitive to temperature, is used in a Wheatstone Bridge configuration. It produces a differential voltage that is amplified by an op-amp circuit. The op-amp output voltage is approximately proportional to temperature, and 180 degrees out of phase.
The rest of the control loop is modeled here using ideal mathematical control blocks. This abstraction allows the designer to focus on the overall performance of the regulator, and to assess the choice of PID gains during actual transient operation. These gains were selected using a nominal operating point, to ensure stability of the loop at that point. This was done in the companion design https://www.systemvision.com/design/laser-temperature-regulator-tdfs. But because many of the components in this loop are non-linear in nature (e.g. the TEC, thermistor, even the op-amp with rail voltage limiting), it is good practice to use simulation to verify performance during large-signal transients.