SENSORS AND TRANSDUCERS FOR SIGNAL CONDITIONING

Types of amplifiers:

The amplifiers, on the basis of principle of working may be categorised as following:

1. Mechanical amplifiers 2. Fluid Amplifiers 3. Optical amplifiers 4. Electrical and Electronic amplifiers 1. Mechanical Amplifiers: The mechanical amplifiers may be further classified as follows: i) Simple and compound : The compound has tow or more levers linked together so that output from one lever provides the input to the other. Example: The huggenberger externsomerter is one of the most popular and accurate mechanical . It uses a system of compound levers to give very high magnification to the order of 2000 or even more. ii) Simple and compound : The simple and compound trains are used quite frequently to provide mechanical amplification of either angular or rotary . A compound gear train gives greater modification with the additional advantage of no change in the direction of input signal. Example: The gear trains are used fro the magnification of displacement in the Bourdon tube pressure gauge and in the dial test indicator where linear movement is translated into rotation by means of rack and pinion. Limitation s of mechanical amplification: 1. Internal loading 2. Friction at the mating parts 3. Elastic deformation 4. Backlash.

2. Fluid Amplifiers : Fluid amplifiers may be classified as follows:

i) Hydraulic amplifier: when a small displacement is applied to a piston operating inside a cylinder containing some liquid, there occurs a large displacement of the liquid in the output tube which has a small diameter. Example: This principle is employed in the mercury-in- glass thermometer and the single-column manometers. SENSORS AND TRANSDUCERS ii) Pneumatic amplifier: Pneumatic methods are extensively used and can be applied to any type of measurement.

3) Optical amplifier: In optical amplification, a ray of light strikes a mirror with an angle of incidence I and gets reflected with an angle of reflection equal to the angle of incidence. When mirror rotates through an angle ‘Ө’, the angle of incidence change to (i+Ө). Before rotation of the mirror, the angle between the incident ray and reflected ray is 2i and after rotation it is 2(i+Ө). Obviously there is a angular magnification of 2Ө between the incident and reflected rays. In order to get a greater magnification, more number of mirrors surfaces may be used. Examples: This principle to amplify the input signals is used in the following cases. i) optical levers ii) U.V. Galvanometers iii) Mechanical pointer galvanometers.

4. Electrical and Electronic amplifiers: The electrical and electronic amplifiers are used to increase the magnitude of weak voltage or current signals resulting from electromechanical transducers. The desirable characteristics of electrical and electronic amplifiers are: i) High input impedance so that its loading effect on the transducer is minimum. Ii) Low output impedance so that the amplifier is not unduly loaded by the display or recording device. Iii) Frequency response should be as good as that of the transducer.

AC and DC amplifiers: Instrumentation system employ the following two types of electrical and electronic amplifiers and they are i) AC amplifier ii) DC amplifier.

For an AC amplifier bandwidth is the range of frequencies between which gain or amplitude ratio is constant to within -3dB. This corresponds to the frequencies at which the voltage output amplitude falls by 29.3% to 70.7% of the maximum value. The AC amplifiers are only capable of dealing with rapid, repetitive signals but are usually simpler and cheaper when compared with their DC DC counterparts. In an AC amplifier system the amplifier drift and spurious noise are not significant; the mains frequency pick-up rejection is very high.

The DC amplifiers are capable of amplifying static, slowly changing or rapid repetitive input signals. The DC amplifier systems are easy to calibrate at low frequencies and have athe ability to recover rapidly from overload conditions.