The microelectronic gas concentration sensor operates as follows. The choice of
a constant voltage source U1 is achieved by generating electrical oscillations of the
auto generator. With the subsequent action of the gas concentration on the gas-sensitive
resistor R2, its resistance changes, which results in the change in the equivalent
capacity of the oscillatory circuit of the auto generator, which in turn changes the
generation frequency.
The resonant frequency, which depends on the change in gas concentration, is a
sensor transformation function. It is determined on the basis of the zero reactive
component of the complete input resistance of the circuit (Figure 2) and has the form
1/2
2
()
1
()
1
2
()
g
ekv
g
ekv
RC
C
FC
RC
C
L
=−
. (14)
where
g
R is differential resistance at the operating point of the circuit; R is loss
of resistance in the oscillatory system;
ekv
C
is equivalent capacity of the oscillatory
system; L is inductance of the oscillatory system.
The sensitivity of the frequency sensor of the gas is determined on the basis of
expression (14) and is described by the equation
2
2
2
()
()
()
1
1
1
2
()
2
()
1
p
g
ekv
ekv
ekv
F
C
g
ekv
g
ekv
RC
C
dC
C
dC
C
L
dC
dC
S
RC
C
RC
C
L
L
−
=−
+
−
. (15)
In Figure 4, the transformation function is given, that is the dependence of the
resonance frequency on the concentration of gas C
3
H
8
. As can be seen from the graph,
the transformation function has a nonlinear character.
Figure 5 presents the dependence of sensitivity of the sensor on the change in gas
concentration. The analysis of the graph shows that the highest sensitivity of the device
lies in the range of 100 ppm to 200 ppm and is 110 Hz/ppm, and in the range from 500
ppm to 1500 ppm is 65 Hz/ppm, the least sensitivity is 61 Hz/ppm in the range zone
1500 ppm to 2500 ppm.
The construction of the frequency transducer of gas concentration in the form of
an integrated circuit requires the use of a film technology for the manufacture of a
passive inductive element in the form of a spiral, but its quality factor is of little
importance, and secondly, its dimensions at frequencies up to 10
6
Hz are incompatible
with the dimensions of the integrated circuit of the transducer. Therefore, to solve this
problem, it is suggested to use the inductive character of the impedance of a bipolar
transistor with an RC circuit, which is easily implemented in the form of an integrated
circuit [8].
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