- Bio Amplifier Definition: A bio amplifier is a device used to increase the amplitude of low-level bioelectric signals for analysis.
- High Input Impedance: Bio amplifiers need a high input impedance (2 MΩ to 10 MΩ) to reduce signal distortion.
- Isolation and Protection: They include isolation circuits to protect patients from electrical shocks.
- Types of Bio Amplifiers: Includes differential amplifiers, operational amplifiers, instrumentation amplifiers, chopper amplifiers, and isolation amplifiers.
- High CMRR: A bio amplifier should have a Common Mode Rejection Ratio (CMRR) greater than 80dB to minimize interference.
Why is Bio Amplifier Required?
Biological amplifiers, also called bio amplifiers or biomedical amplifiers, raise the amplitude of low-level bioelectric signals so those signals can be analysed. Bioelectric signals usually have low amplitude and low frequency. The amplifier outputs are used for further analysis and they appear as ECG, EMG or any bioelectric waveforms.
Basic Requirements for Biological Amplifiers
- The biological amplifier should have a high input impedance value. Teaching notes often quote 2 MΩ to 10 MΩ depending on the application; many modern ECG inputs are higher still. Higher impedance value reduces distortion of the signal.
- When electrodes pick up biopotentials from a patient-connected device, the input circuit should be protected. Patient-connected bio-amplifiers include isolation and protection circuits, to limit leakage current and electrical shock.
- Since the output of a bioelectric signal is in millivolts or microvolt range, the voltage gain of the amplifier is large. Teaching notes often quote more than 100dB; many ECG channels use a lower analogue gain and later digital scaling.
- Throughout the entire bandwidth range, a constant gain should be maintained.
- A bio-amplifier should have a small output impedance.
- A bio-amplifier should keep drift and noise low enough that they do not hide the bioelectric waveform.
- Common Mode Rejection Ratio (CMRR) of the amplifier is typically specified greater than 80dB to reduce interference from the common mode signal.
- The gain of the bio-amplifier should be calibrated for each measurement.
Types of Bio Amplifiers
- Differential Amplifier
- Operational Amplifier
- Instrumentation Amplifier
- Chopper Amplifier
- Isolation Amplifier
Instrumentation Amplifier
In biomedical applications, instrumentation amplifiers are used to achieve high gain and high input impedance. These circuits usually have a 3-amplifier setup. The transducer output is fed into the instrumentation amplifier, which has high CMRR and high input impedance to reduce loading.
To each input of the differential amplifier, the non-inverting amplifier is connected. From the figure, the amplifier on the left side acts as non-inverting amplifiers.
The two input amplifiers form the input stage of the instrumentation amplifier. The third op-amp is the difference amplifier, and it is the output of the instrumentation amplifier.
The output from the difference amplifier Vout is the difference between two input signals given at the input points. VO1 is the output from op-amp 1 and VO2 is the output from op-amp 2.
Isolation Amplifier
Isolation amplifiers, also known as pre-amplifier isolation circuits, increase the input impedance of patient monitoring systems and isolate the patient from the device. Isolation reduces leakage current from hospital power lines. Isolation resistance on the order of 10^12 Ω is a typical isolation-amp figure.
Electrical signals are obtained using electrodes and then sent to the amplifier block for amplification. After this, the signal enters the modulation block. If using an optical cable, the modulated signal goes to an LED, converting it into light energy. With a transformer, the modulated signal connects to the primary winding, transferring energy to the secondary winding through mutual induction. The secondary output then goes to the demodulation block, providing the final amplified demodulated signal.
ECG Isolation Amplifier
During ECG measurements, signals from all leads are sent to a low pass filter, called an Electro surgery filter, to reduce interference from electrosurgery and radio frequencies. Next, the high voltage and overvoltage protection block withstands large voltages during defibrillation.
Proceeding further, the signal goes to the Lead Selector Switch block, which selects the required configuration. Lead selection output goes to the DC amplifier. We have a transformer, whose primary winding is connected to the oscillator and secondary to rectifier and filter. The ECG signal is modulated with the Synchronous modulator. The second transformer delivers the output from the synchronous modulator to the synchronous demodulator. The output from the demodulator is fed as input to the power amplifier.
