Signal amplification and conditioning. Reduction of noise

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8. SIGNAL AMPLIFICATION AND CONDITIONING

The purpose of the signal conditioner is to amplify the small signals derived from the transducer and to remove unwanted signals.

Each organ of human body has own small electric field. For the registration of electric potentials the special methods of signal conditioning are used:

Biosystem®Transducer®Amplifier®Registration of signal

Transducer is device, which converts any activity into electric signal.

          Amplifier is device, which amplifies any weak electric signal.

Basic element of signal amplifier is transistor. A transistor consists of three slabs of semiconducting material (Fig.8.1b) called the emitter (E), base (B) and collector (C). When a voltage V is established across the collector-emitter terminals a large current Ic flows into the collector and out of the emitter. This current is controlled by the current IB flowing into the base. Small changes ΔIB in the base current causing large changes ΔIC in the collector current. Thus there is current amplification.

Current amplification is situation when small changes ΔIB in the base current is caused large changes ΔIC in the collector current

The current gain AIis defined as AI = ΔIC/ΔIE where ΔIEis the input signal and ΔIC the output signal.

(a)                                         (b)

Fig.8.1.(a) A schematic diagram of a bipolar transistor. (b) The symbol for a transistor

Voltage amplification is situation when a small voltage change ΔVB at the input will cause a large voltage change ΔVC at the output.

The voltage gain AVis defined as AV = ΔVC/ΔVB. We will call ΔVB the input signal Vin and ΔVC the output signal Vout.


The circuit for a simple transistor amplifier is shown in Fig.8.2a.

(a)                                                               (b)

Fig.8.2. (a) A signal stage transistor voltage amplifier. (b) Two integrated circuits

An amplifier consists of an arrangement of active device (transistor), passive circuit element (resistors and capacitors) and source of energy.

Ebis a DC voltage supply, which provides the energy for the circuit to operate. A resistor RL is placed in the collector lead. When the collector current changes by an amount ΔIC, due to a small change ΔIB, the voltage across RL changes by an amount ΔVL = RLΔIC. The base signal current ΔIB can be produced by applying a voltage of input signal ΔVB , across the base-emitter terminals.

The current and voltage amplifications produced by a single amplifier stage are typically 200 and 100 respectively. Often this is insufficient and it is necessary to arrange several stages in series. An integrated circuit (1C) consists of a number of amplifying stages constructed on the same slab of semiconducting material. The physical appearance of two types of IC is shown in Fig.8.2b. The inherent amplification AV is of the order of 105. The 1C is used with various combinations of external components (resistors, capacitors and inductors) to produce an amplifier with the desired properties.

The way in which an amplifier operates is not important in the analysis of a measurement system. The important factor is how the amplifier affects the signal. An amplifier can be represented as a 'black box', which has certain properties. A schematic diagram of an amplifier DC coupled and ac coupled to a transducer with resistance RE is shown in Fig.8.3.


(a)                                                (b)

Fig.8.3.Schematic diagram of an amplifier with input resistance Rin and stray capacitance CS: (a) DC coupled, and (b) AC coupled, to a transducer

The apex of the triangle used to represent the amplifier indicates the direction of signal flow. The properties of the amplifier that determine the signal at its output are the amplification AI, or AV, the input resistance Rin = Vin/Iin, the frequency dependence of the amplification and the time constant.

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