A sensing circuit for detecting a change in a normal characteristic of an object such as a change in a normal temperature characteristic, a change in a normal magnetic field characteristic, or a change in a normal light falling on the sensing element of the sensing circuit is provided. The circuit employs a four-terminal semiconductor device having alternate layers of conductivity type, which has an amplification characteristic of the collector current to the base-emitter grounded, wherein there is a sharp decrease in the h.sub.FE amplification characteristic at one value of gate to emitter voltage. This point represents the point in which the semiconductor device is biased for a normally desired characteristic being sensed. Whenever there is a deviation from this normal position, there is a sharp rise in the h.sub.FE irrespective of what the direction of deviation is. The h.sub.FE gain then substantially levels off.