Figure 2. Theoretical and experimental dependence of the output frequency of
the autogenerator transducer on methane concentration
Calculations showed that according to the formula (13), the sensitivity of the
device was in the range from 300 ppm to 1000 ppm 270 Hz/ppm, and in the range from
1000 ppm to 5000 ppm – 120 Hz/ppm. The necessary parameter values for theoretical
calculations were obtained from [14, 15].
The circuit of a microelectronic gas concentration sensor with a frequency output
is shown in Figure 3. The proposed scheme of a microelectronic gas concentration
sensor is built on a transistor structure of three bipolar transistors on a single crystal
HFA3046. The scheme on electrodes of the collector-emitter of the bipolar transistor
VT3 and the base-emitter of the bipolar transistor VT2 have a volt-ampere
characteristic with a declining area corresponding to the appearance of a negative
differential resistance. The operating point of the autogenerator from direct current is
selected on the falling area of the volt-ampere characteristics.
The oscillating system of the autogenerator (Fig. 3) consists of the capacitance
existing on the collector-emitter VT3 electrodes, capacitance C1 as well as the external
inductance L1. The resistances R1-R3 provide the mode of operation of transistors
VT1, VT2 and VT3 from direct current.
Figure 3. Electrical circuit of the gas sensor with frequency output
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