case of impurity samples of a gas sensor semiconductor, the dependence
.
spch
C
on the
surface potential
()
s
yw is of a similar nature.
Substitution of formulas (8) and (11) for a semiconductor with an electronic type
of gas sensor conductivity into expression (1) allows to get dependence of its total
resistance on surface potential, which, in its turn, unambiguously depends on
concentration of measured gases. The theoretical dependence of specific capacity and
conductivity on the surface potential for various semiconductors with different types
of conductivity are presented in [10]. The theoretical and experimental dependence of
the output frequency on changes in the concentration of gases can be obtained on the
basis of an autogenerator device, the scheme of which is presented in Fig. 1 [11, 12].
Figure 1. Electrical scheme of an autogenerator gas transducer based
on two bipolar transistors with an active inductive element
Creating a gas concentration transducer with a frequency output signal in the form
of an integrated circuit requires the use of film technology for the manufacture of a
passive inductive element in the form of a spiral, but its quality is of little importance
and, secondly, its dimensions at frequencies up to
6
10
Hz are incompatible with the
size of the integrated circuit transducer. Therefore, to solve this problem it was
proposed to use the inductive nature of the total resistance of a bipolar transistor with
a
RC− circuit, which is easily performed in the form of an integrated circuit [8].
Bipolar transistors
1
VT,
2
VT and
3
VT implement an autogenerator of electrical
oscillations, in which the oscillating circuit is formed by the capacitive component of
total resistance on the electrodes collector-collector of bipolar transistors
1
VT,
2
VT
and the inductive component of total resistance on the electrodes emitter-collector of
bipolar transistor
3.
VT
On the electrodes of the collector-collector transistors
1
VT
and
2
VT there is a
negative differential resistance, which is realized on the falling section of the volt-
ampere characteristic of the transducer. The volt-ampere characteristic itself is
calculated on the basis of an equivalent device circuit from the Kirchhoff equation
system. The falling section is in the range from 3V to 16V with the change of currents
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