Sensor Circuits - The Electronic Nose Artificial Olfaction Technology - page 187

Voltage Comparator
A comparator compares the voltages at the (+) and (-) inputs. If the (+) input is at
a higher voltage than the (-) input the comparator output will be high. If the (-) input
is at a higher voltage than the (+) input the comparator output will be low (Fig. 7.10 ).
The circuit diagram of an integrator using op-amp is shown in Fig. 7.11 . The
output voltage is the time integral of the input voltage as shown in the waveform
figure.
In multiple sensor array analysis buffer op-amp provides isolation between two
parts of the circuit or works as buffer in between two circuit analysis with
impedance matching problem. These buffer circuits as shown in Fig. 7.12 typically
have a gain of 1 but have very large input impedance and very small output
impedance. Because their gains are 1, they are also called voltage followers.
Rectifier circuits are mostly used in the design of power supply circuits. Further
applications are also there, i.e., in instrumentation applications, the signal to be
rectified can be of incredibly small amplitude, making it impractical to use the
conventional rectifier circuits. Also it is used for very precise transfer character-
istics. Hence, it is also called as precision rectifier.
A half-wave precision rectifier is constructed with the use of an op-amp, and
includes the diode in the feedback loop (as shown in Fig. 7.13 ). This efficiently
terminates the forward voltage drop of the diode, so very low-level signals (well
below the diode's forward voltage) can still be rectified with minimal error.
Full-wave rectifiers are more difficult, compared to the half-wave circuits. Full-
wave rectifiers output one polarity of the input signal and reverse the other. A
circuit for a full-wave rectifier is shown in Fig. 7.14 .
3. Filter
Filtering is the process of removing certain portions of the input signal in order
to create a new signal. Filters can be broadly grouped into four classes according to
their frequency response.
• Low-pass,
• High-pass,
• band-pass, and
• band reject (notch).
These analog filters can be implemented using passive or active circuits. Pas-
sive filters consist of networks of resistors, capacitors, and inductors, whereas
active filters contain active components (e.g., op-amps, transistors), resistors, and
Fig. 7.10 Voltage
comparator