Selection of our books indexed in the Book Citation Index in Web of Science™ Core Collection (bkci)

Figure 30. Schematic design of the main stages of a biopotential amplifier. Three electrodes connect the patient to a
preamplifier stage. After removing DC and low frequency interference, biological signal is connected to an output low-
pass filter through an isolation stage which provides electrical safety to the patient, prevents patient loops, and reduces
the influence of interference signals.
A typical design of the various stage of a biopotential amplifier is shown in figure 30. The three
electrodes which provide the transition between the ionic flow of currents in biological tissue
Figure 29. Typical configuration for the measurement of biopotentials
Biomedical Sensor, Device and Measurement Systems
http://dx.doi.org/10.5772/59941
213

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and electronic flow of currents in amplifier represent a complex electrochemical system. To a
large extent, these electrodes determine the composition of the measured signal. The pream‐
plifier represents the most critical part of a amplifier since it sets the stage for the quality of
the biosignal. With proper design, the preamplifier can eliminate, or at least minimize, the
most signal interfering with the measurement of biopotentials. In addition to electrode
biopotentials and electromagnetic interference, noise which is generated by the amplifier and
the connection between biological source and amplifier has to be taken into account when
designing a preamplifier.
(a) One-order low-pass filter for its circuit and logarithmic amplitude-frequency characteristics 
(b) One-order high-pass filter for its circuit and logarithmic amplitude-frequency characteristics 
(c) Band-pass filter principle diagram 
(d)Band elimination filter priciple diagram 
Figure6.30 Four types of filters and its amplitude-frequency characteristics 
Figure 31. Four types of filters and its amplitude-frequency characteristics
Advances in Bioengineering
214

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After biosignals are preamplified, some unuseful signal have to be eliminated or filtered to
highlight the useful biosignal. Such function can be realized by all kinds of filters. In circuit,
according to the frequency range of signals there are four classes of filters: low-pass filter (LPF),
high-pass filter (HPF), band-pass filter (BPF) and band elimination filter (BEF). These four
types of filters are shown in figure 31.