PURPOSE: To perform fine adjustment to reduce a temperature drift by using field effect transistors and fixed resistances together.

CONSTITUTION: A fixed resistance (R1) 2 of high wattage is connected to drains (D) of transistors TRs 1a and 1b. Control resistances (R2) 3 and (R3) 4 are connected to gates (G) of TRs 1a and 1b respectively. A voltage is applied between points A and B to control gate currents of TRs 1a and 1b. Thus, fine adjustment is performed more easily than a conventional method even if the constant of a load is severely rated, and the temperature drift is reduced.