BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 1

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BIOPOTENTIAL AMPLIFIERS

In general, signals resulting from physiological activity have very small amplitudes and
must therefore be ampli fi ed before their processing and display can be accomplished. The

speci fi cations and lists of characteristics of biopotential ampli fi ers can be as long and con-

fusing as those for any other ampli fi er. However, for most typical medical applications, the

most relevant ampli fi er characterizing parameters are the seven described below.

1. Gain. The signals resulting from electrophysiological activity usually have amplitudes on
the order of a few microvolts to a few millivolts. The voltage of such signals must be ampli ed

to levels suitable for driving display and recording equipment. Thus, most biopotential
ampli

fi

er is measured
in decibels (dB). Linear gain can be translated into its decibel form through the use of

fi ers must have gains of 1000 or greater. Most often the gain of an ampli fi

Gain(dB) 20 log 10 (linear gain)

er should be
such as to amplify, without attenuation, all frequencies present in the electrophysiological
signal of interest. The bandwidth of any ampli

2. Frequency response. The frequency bandwidth of a biopotential ampli fi

fi er, as shown in Figure 1.1, is the di ff erence

between the upper cuto ff frequency f 2 and the lower cutoff ff frequency f 1 . The gain at these

cuto frequencies is 0.707 of the gain in the midfrequency plateau. If the percentile gain

is normalized to that of the midfrequency gain, the gain at the cutoff

ff

ff frequencies has

decreased to 70.7%. The cuto points are also referred to as the half-power points , due to

the fact that at 70.7% of the signal the power will be (0.707) 2

ff

0.5. These are also known

as the 3-dB points, since the gain at the cutoff ff points is lower by 3 dB than the gain in

20 log 10 (0.707).
3. Common-mode rejection. The human body is a good conductor and thus will act as
an antenna to pick up electromagnetic radiation present in the environment. As shown in
Figure 1.2, one common type of electromagnetic radiation is the 50/60-Hz wave and its
harmonics coming from the power line and radiated by power cords. In addition, other
spectral components are added by

the midfrequency plateau: 3dB

fl fluorescent lighting, electrical machinery, computers,