The Essential Requirements of Biopotential Amplifiers for Medical applications

Biopotential amplifiers are also termed to as Bioamplifiers. Bioelectric measurements are normally low-level voltages with high source impedances therefore signal amplification is essential part of biomedical measurement systems. The signal amplification is needed to boost or increase the strength of the input signal to match the requirements of recording/display systems. We have specialized amplifiers designed to do signal amplification in biomedical measurement applications and are known as biopotential amplifiers.

Biopotential amplifiers are usually in the form of voltage amplifiers because they are capable of increasing the voltage levels of a signal. However, voltage amplifiers also serve to increase power levels so they can be considered power amplifiers too. In some circumstances biopotential amplifiers are employed in isolating the load from the source. In this case, the amplifiers provide only current gain, leaving the voltage levels principally unchanged.

Examples of biopotential amplifiers include Chopper amplifier, Differential amplifier, Instrumentation amplifier, etc. These examples and more are covered in details in this article: Types of Amplifiers used in Biomedical Measurement Applications

Let’s look at some of the essential features and requirements of biopotential amplifiers as discussed below:

To be used in medical measurement applications, all biopotential amplifiers must meet the necessary requirements.

Biopotential amplifiers must have high input impedance so that they provide minimal loading of the signal being measured. The characteristic of biopotential electrodes can be affected by the electric load they see; which in addition to signal loading, can result in distortion of the signal. Loading effects are minimized by making the amplifier input impedance as high as possible thereby reducing the signal distortion. Modern bioamplifiers have input impedances of at least 10 Mꭥ.

Biopotential amplifiers must operate in that portion of the frequency spectrum in which the bioelectric potentials that they amplify exist. Because of the low level voltages of these kinds of signals, it is important to limit the bandwidth of the amplifier so that is just great enough to process the signal adequately. This way, we can obtain optimal signal-to-noise signal (SNRs).

Bioelectric potentials normally have amplitudes of the order of a few millivolts or less, therefore such signals must be amplified to levels compatible with recording and displaying devices. This implies that most biopotential amplifiers must have gains of the order of 1000 or greater.

The input circuit of a biopotential amplifier must provide protection to the organism being studied. The current or potential appearing across the amplifier input terminals which is produced by the amplifier is capable of affecting the biological potential being measured. These electric currents from input terminals of a biopotential amplifier can result in microshocks or macroshocks in the patient being studied. To prevent these problems, the amplifier should have an isolation and protection circuitry so that the current through the electrode circuit can be kept at safe levels and any artifact generated by such current can be minimized.

The main function of the amplifier output circuit is to drive the amplifier load i.e. an indicating instrument or recording device with maximum range and reliability in the readout. Hence the output impedance of the amplifier must be low with respect to the load impedance, and the amplifier must be able to supply the power required by the load.

In most cases, biopotential signals are obtained from bipolar electrodes, these electrodes are often symmetrically located, electrically with respect to the ground. Under such conditions, the most proper biopotential amplifier is a differential one. Since such bipolar electrodes frequently have common-mode voltage with respect to the ground, which is much larger than the signal amplitude and as the symmetry with respect to ground can be distorted; such biopotential differential amplifiers must have high common mode-rejection ratios to minimize the interference due to the common-mode signal.

Lastly, for biopotential amplifiers that are employed both in medical applications and in the laboratories, they should make quick calibration possible.