BIOPOTENTIAL AMPLIFIER
INTRODUCTION •
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.
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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.
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In this chapter we examine some of the basic features of biopotential amplifiers and also look at specialized systems.
TYPES OF AMPLIFIERS • Current Amplifier: An amplifier that makes the given input current higher. It is characterized by a low input impedance and high output impedance.
• Voltage Amplifier: An amplifier that amplifies given voltage for a larger voltage output. It is characterized by a high input impedance and low output impedance.
• Transconductance Amplifier: An amplifier that changes output current according to changing input voltage.
• Transresistance Amplifier: An amplifier that changes output voltage according to changing input current. It is also known as a current-to-voltage converter
• Power Amplifiers: Power amplifier is a general term that refers to the amount of power provided by the power supply circuit or the amount of power delivered to the load. It is usually used in the last output stages of a circuit. Examples include: audio power amplifiers, servo motor controllers, push-pull amplifiers and RF power amplifiers.
TYPES OF AMPLIFIERS •
Current Amplifier: An amplifier that makes the given input current higher. It is characterized by a low input impedance and high output impedance.
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Voltage Amplifier: An amplifier that amplifies given voltage for a larger voltage output. It is characterized by a high input impedance and low output impedance.
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Transconductance Amplifier: An amplifier that changes output current according to changing input voltage.
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Transresistance Amplifier: An amplifier that changes output voltage according to changing input current. It is also known as a current-to-voltage converter
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Power Amplifiers: Although not technically a type, power amplifier is a general term that refers to the amount of power provided by the power supply circuit or the amount of power delivered to the load. It is usually used in the last output stages of a circuit. Examples include: audio power amplifiers, servo motor controllers, push-pull amplifers and RF power amplifiers. Again, we’ll look at the classifications of power amplifiers specifically in a little bit, since they’re very important.
BIOPOTENTIAL AMPLIFIER • Biosignals are recorded as potentials, voltages, and electrical field strengths generated by nerves and muscles.
• The measurements involve voltages at very low levels, typically ranging between 1 µV and 100 mV, with high source impedances and superimposed high level interference signals and noise.
• The signals need to be amplified to make them compatible with devices such as displays, recorders, or A/D converters for computerized equipment.
• Amplifiers adequate to measure these signals have to satisfy very specific requirements. • They have to provide amplification selective to the physiological signal, reject superimposed noise and interference signals, and guarantee protection from damages through voltage and current surges for both patient and electronic equipment.
• Amplifiers featuring these specifications are known as biopotential amplifiers. Basic requirements and features, as well as some specialized systems, will be presented.
BASIC AMPLIFIER REQUIREMENT The basic requirements that a biopotential amplifier has to satisfy are:
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The physiological process to be monitored should not be influenced in any way by the amplifier
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The measured signal should not be distorted
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The amplifier itself has to be protected against damages that might result from high input voltages as they occur during the application of defibrillators or electrosurgical instrumentation
The amplifier should provide the best possible separation of signal and interferences The amplifier has to offer protection of the patient from any hazard of electrical shock
• 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.
• 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 MΩ.
• 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.
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In clinical systems, electric currents from the input terminals of a biopotential amplifier can result in microshocks or macroshocks in the patient being studied—a situation that can have grave consequences
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To avoid these problems, the amplifier should have 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
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Biopotential amplifiers must operate in that portion of the frequency spectrum in which the biopotentials that they amplify exist.
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Because of the low level of such signals, it is important to limit the bandwidth of the amplifier so that it is just great enough to process the signal adequately.
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In this way, we can obtain optimal signal-to-noise ratios.
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Biopotential signals usually have amplitudes of the order of a few millivolts or less.
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Such signals must be amplified to levels compatible with recording and display devices. This means that most biopotential amplifiers must have high gains—of the order of 1000 or greater.
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Can be used both in medical applications and in the laboratory is that they make quick calibration possible.
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In recording biopotentials, the scientist and clinician need to know not only the waveforms of these signals but also their amplitudes.
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Frequently biopotential amplifiers have a standard signal source that can be momentarily connected to the input, automatically at the start of a measurement or manually at the push of a button, to check the calibration.
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Biopotential amplifiers that need to have adjustable gains usually have a switch by which different, carefully calibrated fixed gains can be selected, rather than having a continuous control (such as the volume control of an audio amplifier) for adjusting the gain. Thus the gain is always known, and there is no chance of its being accidentally varied by someone bumping the gain control
• A typical configuration for the measurement of biopotentials is shown in Fig. 1. Three electrodes, two of them picking up the biological signal and the third providing the reference potential, connect the subject to the amplifier.
FIGURE 1 Typical configuration for the measurement of biopotentials.
DIFFERENTIAL AMPLIFIER • Operational Amplifiers: The operational amplifier is a direct-coupled high gain amplifier usable from 0 to over 1MH Z to which feedback is added to control its overall response characteristic i.e. gain and bandwidth. The op-amp exhibits the gain down to zero frequency.
• Differential Amplifiers: Differential amplifier is a basic building block of an op-amp. The function of a differential amplifier is to amplify the difference between two input signals
Figure 2 Emitter biased circuit
• The two transistors Q1 and Q2 have identical characteristics. The resistances of the circuits are equal, i.e. RE1 = RE2, RC1 = RC2 and the magnitude of +VCC is equal to the magnitude of -VEE.
• These voltages are measured with respect to ground. To make a differential amplifier, the two circuits are connected as shown in Figure 2. The two +VCC and -VEE supply terminals are made common because they are same. The two emitters are also connected and the parallel combination of RE1 and RE2 is replaced by a resistance RE. The two input signals V1 & V2 are applied at the base of Q1 and at the base of Q2. The output voltage is taken between two collectors.
• The collector resistances are equal and therefore denoted by RC = RC1 = RC2. Ideally, the output voltage is
zero when the two inputs are equal. When V1 is greater then V2 the output voltage with the polarity shown appears. When V1 is less than V2, the output voltage has the opposite polarity.