Sensing circuits

Claim 1

A sensing circuit for detecting a change in a characteristic of an object comprising a semiconductor device having a first semiconductor region of one conductivity type forming an emitter region, a second semiconductor region of the opposite conductivity type forming a base region adjacent said first region with a first semiconductor junction therebetween, a third semiconductor region of the same conductivity type of said first region forming a collector region adjacent said second region with a second semiconductor junction therebetween, an insulating layer on the outer surface of said emitter region, a gate electrode on said insulating layer forming a gate over said emitter region only, said semiconductor device having an emitter grounded current amplification variation in response to changes in voltage between said emitter region and said gate, said variation exhibiting a minimum value at a first voltage between said emitter region and gate and an increasing value for voltages higher and lower than said first voltage, an output circuit connected to said collector region, a base bias circuit connected to said base region, a gate bias circuit connected to said gate, one of said bias circuits including a sensing element for detecting the change in characteristic of said object and the other bias circuit including a bias adjustment element, whereby a change in the object characteristic creates a variation in the current amplification which changes a voltage level in the output circuit.