Abstract: A circuit for detecting hydrocarbons comprises an input circuit receptive of an input signal representative of the electron movement in an unknown sample to be detected for producing a first amplified signal and a signal processing circuit receptive of the first amplified signal from the input circuit for producing a first output signal when the input signal represents the presence of hydrocarbons. The input circuit comprises four transistors connected as common base amplifiers with the base of a first of the four transistors receptive of the input signal, the base of a second of the four transistors connected to the emitter of the first transistor, the base of a third of the four transistors connected to the emitter of the second transistor, the base of a fourth of the four transistors connected to the emitter of the third transistor to produce the first amplified signal at the collector of the fourth transistor and wherein for the first transistor 70<hfe.ltoreq.120, for the second transistor 150.ltoreq.