from 1mA to 8.5 mA. Negative differential resistance transforms the energy of the
constant electric field into the energy of the alternating electric field, which allows to
compensate the energy losses in the vibrating circuit of the transducer.
Conversion function, describes the dependence of the output frequency of the
transducer on the change of gas concentration, is determined on the basis of nonlinear
equivalent circuit of alternating current, based on the electrical scheme (Fig. 1). From
the system of Kirchhoff equations, which are made on the basis of nonlinear equivalent
circuit of the transducer by alternating current, the total output resistance on the
electrodes of the collector-collector of transistors
1
VT and
2
VT is calculated. The
system of Kirchhoff equations was solved with the help of Matlab 9.2 [13], which
allowed to obtain the value of total output resistance, the active component of which
has a negative value, and the reactive component has a capacitive character. From the
equation, when the reactive component is equal to zero, we determine the dependence
of the output frequency
0
()
Fw
of the transducer on the gas concentration, which looks
like
1
2
2
0
()
()
1
()
1
2
()
()
g
ekv
g
ekv
RwC
w
Fw
RwC
w
L
=
−
, (12)
where
()
g
Rw−
the differential negative resistance of the oscillatory circuit,
()
ekv
Cw
is equivalent to the capacity of the oscillatory circuit, L−
the value of the
active inductance,
w
is the concentration of the measured gas. In fig. 2 the dependence
of the output frequency of the autogenerator transducer on the change in the
concentration of methane gas
4
CH is presented. The sensors of the firm Figaro (Japan)
were sensors.
Sensitivity of the transducer is determined on the basis of expression (12) by
differentiation of its argument
w
and is described by the formula
0
1
2
2
2
1
2
2
2
()
()
()
()
()
1
()
()
()
2
()
()
()
()
4
()
()
1
g
ekv
g
g
ekv
g
ekv
g
W
F
g
ekv
g
ekv
g
ekv
RwC
w
dRw
dRw
dC
w
RwC
w
Rw
L
dw
dw
dw
S
RwC
w
RwC
w
RwC
wL
L
−
+
=
−
−
−
1
2
2
2
()
()
()
1
.
2
()
()
g
ekv
ekv
g
ekv
RwC
w
dC
w
L
dw
RwC
w
−
−
(13)
- 490 -