Types of Amplifiers used in Biomedical Measurement Applications

Signal amplification is essential part of any biomedical measurement. Bioelectric measurements are usually low-level i.e. microvolt level measurements, therefore amplification is required to boost the level of the input signal to match the requirements of recording/display systems or to match the range of the analog-to-digital convertor, thus increasing the resolution and sensitivity of the measurement.

We discuss some of the amplifiers that are used in biomedical measurement applications.

Carrier Amplifiers

The carrier amplifiers are used with transducers which require an external source of excitation. They are characterized by high gain, negligible drift, extremely low noise and the ability to operate with resistive, inductive or capacitive transducers.

A carrier amplifier is made up of a carrier oscillator, a bridge balance, with a calibration circuit, a high gain ac amplifier, a phase-sensitive detector, and a dc output amplifier.

Chopper Amplifiers

The chopper amplifier is used to amplify very small dc signals of the order of microvolts. To avoid the drift problem that is characterized by the direct coupled amplifier, the chopper amplifier is used. The amplifier uses a chopping device that converts a slowly varying direct current to an alternating voltage (the dc is chopped into a square wave with a chopper modulator). 

The resulting alternating voltage has amplitude that is proportional to the input direct current and with phase dependent on the polarity of the original signal. The resulting ac square wave is amplified with an ac amplifier and then demodulated to get an amplified dc.

Chopper amplifiers are available in both single-ended as well as differential input configurations.

Chopper amplifiers are used in medicine in amplification of small dc signals of a few microvolts. They are used with transducers such as temperature sensors (thermistors, thermocouples), strain gauge, etc.

You can also read: Biomedical Recorders

Isolation Amplifiers

Isolation amplifiers are used to provide protection against leakage currents. They break the ohmic continuity of electric signals between the input and output of the amplifier.

We have 3 methods of isolation that can be used:

• Optical isolation
• Transformer isolation
• Capacitive isolation

Optical Isolation

From the above diagram, as the input signal varies, the light intensity of the LED shown in the last stage of amplification also varies. An optocoupler is used to couple this light to the phototransistor. This light falls on a phototransistor. The collector current of the phototransistor is proportional to the light intensity. For stabilization purposes, a feedback from the output may be provided.

Electrical isolation is used to ensure patient protection against electrical hazards. Biomedical instruments such as pacemakers, electrocardiographs, pressure monitors, pressure transducers, etc. are designed to electrically separate the portion of the circuit to which the patient is connected from the portion of the circuit connected to the ac power line and ground.  

Differential Amplifiers

Biomedical amplifiers employed in the input stage of a biomedical measurement system are mostly of the differential type. Differential amplifier has three input terminals out of which one is arranged at the reference potential and the other two are live terminals.

The differential amplifier is used when it is necessary to measure the voltage difference between two points, both of them varying in amplitude at different rates and in different patterns.

Heart-generated voltages that are picked up by means of Bioelectrodes on the arms and legs and brain-generated voltages picked up by the Bioelectrodes on the scalp are typical examples of signals whose measurements needs the use of differential amplifier.

Reasons why Differential amplifier is preferred over other electronic amplifiers

• Its ability to reject common-mode interferences which are invariably picked up by electrodes from the body along with the useful bioelectric signals.
• As a direct coupled amplifier, it has good stability and versatility.
• High stability is achieved because it can be insensitive to temperature changes which is often the source of excessive drift in other configurations
•  It is versatile in that it may be adapted for many applications e.g. applications requiring floating inputs and outputs or applications where grounded inputs and or outputs are desirable.

The ability of the differential amplifier to reject common voltages on its two input leads is known as Common-mode rejection and is specified as the ratio of common-mode input to differential input to derive the same response. It is abbreviated as CMRR (Common-mode rejection ratio).

CMRR is an important specification with regard to differential amplifiers and is usually expressed in decibels.

CMRR for the input stage of biomedical instrumentation systems should be as high as possible so that only the wanted signals find a way through the amplifier and all unwanted signals get rejected in the preamplifier stage.

A high rejection ratio is normally achieved by the use of a matched pair of transistors in the input stage of the preamplifier and a large ‘’tail’’ resistance in the long-tailed pair to provide maximum negative feedback for in phase signals.

In order to minimize effects of changes occurring in the electrodes impedances, it is necessary, to use an input stage amplifier or preamplifier with a high input impedance. It has been established that a low value of input impedance give rise to a considerable distortions of the data recordings.

High gain integrated dc amplifiers, with differential input connections and a provision for external feedback are termed to as operation amplifiers because of their ability to perform mathematical operations. They come in integrated circuit form.

The common-mode rejection for most op-amps is typically between 60 dB and 90 dB. This may not be enough to reject common-mode noise that is usually encountered in biomedical measurements. In addition, the input impedance is not very high to handle signals from high impedance sources. One way to increase the input impedance of the op-amp is to use Field effect transistors (FET) in the input differential stage. Alternatively, the best solution is to use an Instrumentation amplifier in the preamplifier stage.

Limitations of Differential amplifiers

Although the differential amplifier is well suited for most of the applications in biomedical measurements, it has the following limitations:

• The amplifier has limited input impedance and therefore, draws some current from the source and loads them to some extent.
• The common-mode rejection ratio (CMMR) of the amplifier may not exceed 60 dB in most of the cases, which is inadequate in modern medical measurement systems.

The Instrumentation amplifier, which is an improved version of a differential amplifier, overcomes the limitations of the differential amplifier. In fact connecting a buffered amplifier to a basic differential amplifier makes an instrumentation amplifier!

Instrumentation Amplifiers

Instrumentation amplifier is a differential voltage gain device optimized for operation in an environment that is hostile to precision measurements. It is consists of 3 op-amps and 7 resistors.

The instrumentation amplifier is made up of 2 parts: a buffered amplifier (OP1, OP2) and a basic differential OP3.

The differential amplifier part is essential for biomedical sensors; this is because a sensor produces a signal between its terminals however, in some applications neither terminal may be connected to the same ground as your measuring circuit hence the terminals may be biased at a high potential or might be riding on a noise voltage. The differential amplifier fixes this problem by directly measuring the difference between the sensors terminals.

The buffered amplifier OP1 and OP2 provides gain and also prevents the sensor resistance from affecting the resistors in the op-amp circuit.

Key Characteristics of Instrumentation Amplifiers

• Voltage gain from differential input (V₁-V₂) to single ended output is set by one resistor (RG).
• The input resistance of both inputs is very high and does not change as the gain is varied.
• V_o does not depend on common-mode voltage but only on their difference i.e. output voltage is proportional to the difference between the two input voltages.

Related: The Essential Requirements of Biopotential Amplifiers for Medical applications

Reasons why Instrumentation Amplifiers are Preferred in Biomedical Applications

• They have high input impedance
• They have high common mode rejection ratio (CMRR)
• Low bias and offset currents
• Low power consumption
• High slew rate
• Less performance deterioration if source impedance changes
• Possibility of independent reference levels for source and amplifier