Signal amplification and conditioning. Reduction of noise

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 I_c flows into the collector and out of the emitter. This current is controlled by the current I_B flowing into the base. Small changes ΔI_B in the base current causing large changes ΔI_C in the collector current. Thus there is current amplification.

Current amplification is situation when small changes ΔI_B in the base current is caused large changes ΔI_C in the collector current

The current gain A_Iis defined as A_I = ΔI_C/ΔI_E where ΔI_Eis the input signal and ΔI_C 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 ΔV_B at the input will cause a large voltage change ΔV_C at the output.

The voltage gain A_Vis defined as A_V = ΔV_C/ΔV_B. We will call ΔV_B the input signal V_in and ΔV_C the output signal V_out.

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.

E_bis a DC voltage supply, which provides the energy for the circuit to operate. A resistor R_L is placed in the collector lead. When the collector current changes by an amount ΔI_C, due to a small change ΔI_B, the voltage across R_L changes by an amount ΔV_L = R_LΔI_C. The base signal current ΔI_B can be produced by applying a voltage of input signal ΔV_B , 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 A_V 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 R_E is shown in Fig.8.3.

(a)                                                (b)

Fig.8.3.Schematic diagram of an amplifier with input resistance R_in and stray capacitance C_S: (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 A_I, or A_V, the input resistance R_in = V_in/I_in, the frequency dependence of the amplification and the time constant.