Amplifiers are an important part of modern instrumentation systems for
measuring biopotentials. Such measurements involve voltages that often are
at low levels, have high source impedances, or both. Amplifiers are required to
increase signal strength while maintaining high fidelity. Amplifiers that have
been designed specifically for this type of processing of biopotentials are
known as biopotential amplifiers. In this chapter we examine some of the basic
features of biopotential amplifiers and also look at specialized systems
6.1 BASIC REQUIREMENTS
The essential function of a biopotential amplifier is to take a weak electric
signal of biological origin and increase its amplitude so that it can be further
processed, recorded, or displayed. Usually such amplifie rs are in the form of
voltage amplifiers, because they are capable of increasing the voltage level of a
signal. Nonetheless, voltage amplifiers also serve to increase power levels, so
they can be considered power amplifiers as well. In some cases, biopotential
amplifiers are used to isolate the load from the source. In this situation, the
amplifiers provide only current gain, leaving the voltage levels essentially
unchanged.
To be useful biologically, all biopotential amplifiers must meet certain
basic requirements. They must have high input impedance, so that they
provide minimal loading of the signal being measured. The characteristics
of biopotential electrodes can be affected by the electric load they see, which,
combined with excessive loading, can result in distortion of the signal. Loading
effects are minimized by making the amplifier input impedance as high as
possible, thereby reducing this distortion. Modern biopotential amplifiers have
input impedances of at least 10 M
The input circuit of a biopotential amplifier must also provide protection
to the organism being studied. Any current or potential appearing across the
amplifier input terminals that is produced by the amplifier is capable of
affecting the biological potential being measured. In clinical systems, electric
currents from the input terminals of a biopotential amplifier can result in
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