Biopotential Amplifiers

BIOPOTENTIAL AMPLIFIERS

·        These are very important part of modern medical instrumentation. We need to amplify biopotentials which are generated in the body at low levels with high source impedance.

·        Biopotentials amplifiers are required to increase signal strength while maintaining fidelity

Basic Requirements of Biopotential Amplifiers

Essential functions of a bioamplifier are:

• To take a weak biopotential and increase its amplitude so that it can be processed, recorded or displayed

• To amplify voltage, but it could be considered as a power amplifier as well.To amplify current since in some cases a biopotential amplifier is used to isolate the load from the source current gain only

Input Impedance (Zin)

• All biopotential amplifiers must have high input impedance minimize loading (remember the characteristics of biopotential electrodes resulting into loading and distortion if input impedance of the amplifier is not high enough) – typical values of Zin over the frequency range of the measure and = 10 MΩ (remember the loading rule)

Protection & Isolation

• The input circuit of a biopotential amplifier must provide protection to the live measure

Vbio

• Any potential or current at amplifier’s input terminals can affect

Vbio

• Electric currents produced by the biopotential amplifier can result in microshock and macro shock

• The bioamplifier must have isolation and protection circuitry so that the current through the electrodes can be kept at safe levels and any artifact generated by such current can be minimized

Output Impedance (Zout)

·        The output circuit does not present any critical problems, all it needs to do is to drive the load

·        Output impedance must be low with respect to the load impedance and it must be capable of satisfying the power requirements of the load

Bandwidth (BW)

Frequency response

• The biopotential amplifier must be sensitive to important frequency components of the biosignal

• Since biopotentials are low level signals, it is important to limit bandwidth optimize signal-to-noise ratio

Gain (G)

• Biopotential amplifiers have a gain of 1000 or greater

Mode of Operation

• Very frequently biosignals are obtained from bipolar electrodes

• Electrodes symmetrically located with respect to ground need differential  amplification 

• High CMRR required because:

1. Common mode signals much greater than the biosignal appear on bipolar electrodes

2.  Symmetry with respect to ground is not perfect (mismatch between electrode impedances) – more on this later

Calibration Signal

·        Medical and clinical equipment require quick calibration. The gain of the biopotential amplifier must be calibrated to provide us with an accurate indication of the signal’s amplitude 

·        Push button to apply standard signal to the input of the biopotential amplifier

·        Adjustable gain switch carefully selects calibrated fixed gains.