Input impedance, operational amplifiers

Several types of reference electrodes are convenient for use in analytical electrochemistry. The use of high-input-impedance operational amplifiers in the reference electrode inputs of potentiostats ensures that very low levels of current are drawn from the reference electrode (see Chap. 6). This permits the use of reference electrodes that do not have to contain a large number of redox equivalents in order to ensure a constant reference potential and are therefore very small. Three reference-electrode designs that are convenient for use in analytical electrochemistry are shown in Figure 9.4. Saturated calomel and silver-silver chloride (of various concentrations of chloride) are among the most common commercially available or conveniently fabricated reference electrodes. 

Fig. 1.6 Schematic diagram of the voltmeter (pH meter), which consists of high-input-impedance operational amplifier. The cell (pH probe) of the meter is ccnmected to the non-inverting input. The output voltage, which is directly proportional to pH, is read with a voltmeter (in this and the following figures the two power supply connections to the operational amplifier are omitted for the...

Electrometer amplifier— An electronic amplifier with an extremely high -> input impedance (Rln > 1014 Q). The device allows measurements of electrical voltages (potentials) at practically zero current. Early devices employed specially designed and selected vacuum tubes (electrometer tubes) operated in a mode with very low grid current. The development of field effect transistors of various types allowed the application of solid-state devices. Electrometer amplifiers are employed in - pH meters (and generally in so-called pi meters, where I stands for ion), all types of instruments for po-tentiometric measurements and in the reference electrode input of -> potentiostats. Because of the high input impedance electrometer amplifiers are sensitive towards electric interferences, consequently some potentiostats have their -> reference electrode input circuitry (essentially an electrometer amplifier) mounted in a separate housing to be attached as close as possible to the reference electrode in order to minimize external interference. 

Operational amplifier. A linear, high-gain DC voltage or current amplifier with high input impedance, low output impedance, and the capability of producing a bipolar output from a bipolar input. 

Electronic instrumentation is available for the measurement of D.C. and A.C. voltage, current and power as well as impedance. Such instruments usually have higher sensitivities, operating frequencies and input impedance than is normally found in the electromechanical instrumentation described above. However, they may need to incorporate amplifiers and they invariably need power to operate the final display. Hence, an independent power source is needed. Both mains and battery units are available. The accuracy of measurement is very dependent on the amplifier, and bandwidth and adequate gain are important qualities. 

Operational amplifier— An electronic device (available in numerous different forms, built with discrete components, in thick film or thin film technology, but mostly as an integrated solid state circuit IC). It is a an amplifier with ideally infinite input impedance, zero output impedance, response behavior independent of the rate of change of the input signal (amplification constant from DC to high frequency AC). It is schematically plotted as a triangle ... 

The operation of an oscilloscope can best be described by reference to Fig. 5, which shows a simplified layout of the controls of a commercial (Tektronix) digital instrument. The signal to be measured is applied to the input connector (BNC) of one of the vertical amplifier channels and must not exceed an upper limit of, typically, 400 volts if the scope input impedance is one megaohm and 5 volts for 50-ohm input impedance. The latter impedance is necessary for signal changes that occur rapidly, such as in the fluorescence decay measurements of Exps. 40 and 44. The lower limit of sensitivity is about 1 mV/division, so preamplification is sometimes needed if very low signal levels are to be measured. 

The input parameters of the noise generator (Ri = IMQ Ci = 130 pF) are not optimised for a current-free measurement of the voltage drop across the resistors Rm Rin is too small and Cj Ls too high. Therefore we use an operational amplifier (TI 081C) as impedance converter with a MiniLab 603B as voltage source. 

For most potentiostats, the assumption Av >> 1 becomes invalid at frequencies above 1 to 10 kHz. In this case, the noise terms are still additive, but the interaction between the gain of the operational amplifier jmd the cell impedance results in additional correlation between the input and output chaimels. 

A refined philosophical approach toward the use of impedance spectroscopy is outlined in Figure 23.1, where the triangle evokes the concept of an operational amplifier for which the potential of input channels must be equal. Sequential steps are taken until the model provides a statistically adequate representation of the data to within the independently obtained stochastic error structure. The different aspects that comprise the philosophy are presented in this section. 

If the cable core terminates at a high impedance device such as the input channel of an operational amplifier, then Ro is large when compared with Rc, R, Re and R. ... 

Since most electrochemical measurements relating to corrosion of metals are satisfied with a sensitivity of 1 jiV or 1 xA, modern instrumentation usually employs electronic operational amplifiers where the noise limits control the range of measurements. The function of the operational amplifier is to amplify the potential (Vg) applied at the input so that it can be displayed on a low impedance analogue or digital meter (V ) as shown in Fig. 1.1. The output potential of the operational amplifier is proportional to the source potential and is required to have sufficient input impedance to avoid polarisation of the potential source. 

FIGURE 9.11 Passive isolation amplifier can be operated without the need for an isolated power supply (a). The biological source provides the power to modulate the load impedance of an inductive transformer. As an easy realization shown in (b), an FET can be directly connected to two electrodes. The source-drain resistance changes as a linear function of the biopotential that is then reflected by the input impedance of the transformer. 

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