Biopotential Amplifiers also called Bio-Amplifiers are specifically designed for processing of Bio-electric signals as they are low in amplitude. This post will discuss what is Biopotential Amplifier (also called Bio-Amplifiers), it’s types, how it works, applications, advantages and Disadvantages.
What is Biopotential Amplifier
Amplifiers are an integral part of Electronic devices and modern Instrumentation for measuring Bio-potentials. As the name indicates, Amplifiers are used to increase the signal strength while maintaining high fidelity. The measurements include voltages that are at low levels and high source impedance.
Fig. 1 – Introduction to Biopotential Amplifier
Bio-electric signals are generally low in amplitude. Amplifiers that are specifically designed for processing this type of Bio-potentials are known as Biopotential Amplifiers. The outputs from such amplifiers are used for analysis and they show up as ECG, EMG or other Bio-electric waveforms. These amplifiers typically process voltages but in some cases they process current.
The amplifier provides high impedance, high CMRR and thereby minimizes loading effects. This is the vital functionality of Biopotential Amplifiers. For Biomedical applications, Bio-amplifiers must meet the below mentioned requirements in order to work incessantly.
To measure Biopotential, electrodes are placed on Human skin as shown in the Fig. 2. The signals from the Electrodes pass on to the Amplifier stage. Amplifier helps in minimizing, eliminating most of the signals interfering with the measurement of Bio-potentials and final readout is obtained.
Fig. 2 – Schematic Representation of Biopotential Measurement
Basic Requirements for Biopotential Amplifier
The basic requirements for Biopotential Amplifiers include:
- High input impedance. Typically, they range between 2 MΩ to 10 MΩ. Greater the impedance value, lower the distortion of the signal.
- Every Bio-amplifier must contain Isolation and Protection circuits for safety purposes especially to prevent patients from macro and micro-electric shocks.
- Output impedance should be low to drive an external load with minimal distortion.
- Most Biopotential Amplifiers are differential.
- Signals are recorded using bipolar electrodes that are symmetrically allocated.
- CMRR (Common Mode Rejection Ratio) must be high as they ride on a large offset signal and to reduce interference from common-mode signals.
- The gain must be calibrated for each measurement.
- An ideal Bio-amplifier must be free from noise and distortion.
- A constant gain must be maintained throughout the entire bandwidth range.
Types of Biopotential Amplifier
There are different types of special circuits used as Biopotential Amplifiers or Bio-Amplifiers. They are:
- Differential Amplifier
- Operational Amplifier
- Instrumentation Amplifier
- Chopper Amplifier
- Isolation Amplifier
These are used to amplify the difference between the voltages applied to its inputs. The circuits are of two types.
- Amplifiers built using Op-Amps.
- Amplifiers built using either FET’s (Field Effect Transistors) or BJT’s (Bipolar Junction Transistors).
Fig. 3 – Differential Amplifier Circuit
These are multistage amplifiers which are interconnected and occupies minimal space even though it consists of many Transistors, Resistors, and FET’s. They are available in the form of an Integrated Circuit (IC).
Fig. 4 – Symbol of Operational Amplifier
It consists of 3-amplifiers in the circuit. The input to the amplifier is the output from the Transducer. A non-inverting amplifier is connected to each of the input of the Differential Amplifier. Non-inverting amplifiers are the ones on the left side of the diagram. The non-inverting amplifiers together form the input of the instrumentation amplifier.
Fig. 5 – Instrumentation Amplifier Circuit
The third op-amp is called the difference amplifier and is the output of the instrumentation amplifier. The difference between the two input signals forms the output Vout. V1 and V2 are the inputs to Op-amp 1 and Op-amp 2 respectively. A typical application of these amplifiers includes Biomedical applications such as Biopotential Amplifier. High gain and high impedance are attained using these amplifiers which are crucial in medical instruments to determine the health condition of an individual.
Noise and drift are the two major issues encountered when recording Bio-potentials. Noise is caused by the movement of the patient or due to the recording device. A DC Amplifier hits a sudden peak in the output when the input is zero. A Chopper Amplifier samples the problem of drift in DC amplifiers.
Fig. 6 – Chopper Amplifier Circuit
These are also known as Pre-amplifier isolation circuits. It provides electrical isolation for the safety of the patient. It prevents accidental shocks and increases the input impedance of the patient’s monitoring system.
Fig. 7 – Symbol of Isolation Amplifier
How does Biopotential Amplifier Work
Various stages which represents Biopotential Amplifier is shown in the Fig. 8. The Electrodes (Bipolar) are placed on the patients skin which provide transition between the ionic flow of currents in biological tissue and electronic flow of current in the Amplifier. The measurement of Bio-potentials is critical and due to relative movements of electrode and tissue, it gives rise to electrode offset potential and electrode/tissue impedance.Thus, two interference signals are generated which are successfully eliminated at later stages of the amplification.
The signal from the electrodes pass on to the pre-amplifier stage which helps in minimizing, eliminating most of the signals interfering with the measurement of Bio-potentials. High Pass Filter and Low Pass Filter eliminates interference signals like electrode Half-cell potentials and Pre-amplifier offset potentials. It also reduces noise amplitude. Bio-signal should not be distorted or attenuated and hence Filters are used.
Fig. 8 – Block Diagram of Stages of Biopotential Amplifier
In the Isolation Amplifier stage, galvanic decoupling of the patient from the measuring equipment is served. It prevents Galvanic currents from deteriorating Signal to Noise Ratio and provides safety to the patient from electrical hazards. Transformer, Optical or Capacitive Couplers are used in Analog Isolation Amplifiers, to transmit signal through the isolation barrier. On the other hand, Digital Isolation Amplifiers use Voltage and frequency converter to digitize the signal before it is transmitted.
Recording of the Bio-potentials in the last stage is done with electrical systems which produce strong electrical and magnetic fields. Hence the system is capacitively coupled and the current flows to the ground electrode.
Applications of Biopotential Amplifier
The applications of Bio-Amplifiers include:
- They are majorly used in medical instrumentation systems such as ECG, EMG, CT scan equipment, Patient hospital monitor.
- They are also used in Electromyogram integrator’s, Cardio tachometers, Vector Cardiograph.
- They are used in Bio-telemetry, Holter Recorder and other devices to determine the specific health condition of a patient.
Advantages of Biopotential Amplifier
The advantages Bio-Amplifiers are:
- Monitored to understand heart health.
- Displays ECG waveform.
- Instrumentation amplifiers give accurate testing and measurement. They do not require input impedance to be matched. This is the reason for using these amplifiers for testing and measuring a wide variety of equipment.
- Biopotential Amplifiers are very easy to use and stable. These are ideal for long term usage.
- They don’t necessarily depend too much on various factors that influence the output at the later stages. The Instrumentation Amplifiers work with just input.
- The Biopotential Amplifiers are highly scalable.
- Even a small input can be amplified to a greater extent at the input level.
Disadvantages of Biopotential Amplifier
The disadvantages of Bio-Amplifiers are:
- Sometimes, there could be minor distortion or noise in the output.
- The system often depends on special cables to remove the noise.
- Superimposing of original is the only concern when the noise gets transmitted for a long-range.