Operational amplifier galvanostat

It is usual in electrochemical measurements to control the potential of the working (or indicator) electrode or the electrolytic current that flows through the cell. A potentiostat is used to control electrode potential and a galvanostat is used to control electrolytic current. Operational amplifiers play important roles in both of these. 

Fig. 5.44 Current control circuit with the aid of an operational amplifier (a) and a circuit of galvanostat (b).

A galvanostatic perturbation, in principle, can be applied by means of a rather simple electrical circuit, as is represented in Fig. 3(a). (More sophisticated instrumentation, employing operational amplifiers, has been described in the literature see ref. 22). It is only required that the galvanostat resistance, Rg, be large compared with the equivalent cell resistance, so that the current forced through the cell is independent of the cell properties. If the source of electricity is a d.c. source, as in Fig. 3(a), a constant current I — jA will start to flow after the time t = 0 at which the circuit is closed [see Fig. 3(b)]. The effect of this action will... 

Three-electrode control systems are widely available in the market and there are also four-electrode systems for double working electrodes. The construction is either integral or modular. It is perfectly possible to construct the necessary electronics in-house and, in this case, modular construction is suggested as being more flexible. Operational amplifiers and other components of high quality should be used, particularly for kinetic applications. The elements of a bipotentiostat (independent control of two working electrodes) and a galvanostat are described in ref. 139. 

The galvanostatic current-pulse procedure was used in early works (74,117) for evaluation of the extents of UPD H coverage and initial stages of surface oxidation of noble metals (117-119). An improved diflerential procedure using differentiation of the potential response, by means of an operational amplifier, was described by Kozlowska and Conway (120). 

Operational amplifiers provide the fotmdation for electrochemical instrumentation. The aim of this chapter is to describe the main properties of an operational amplifier so as to imderstand the principles of potentiostats and galvanostats and to imderstand how they can be used for impedance measurements. 

Electrochemical interfaces consist of potentiostats and galvanostats. These devices can be described in terms of combinations of operational amplifiers and resistors. 

Two different galvanostats can be derived from operational amplifier circuits that we have considered above (6, 7). The device shown in Figure 15.5.1 is strongly reminiscent... 

Figure 3 Standard galvanostat circuitry for constant current coulometry consisting of a voltage source, a resistor, and an operational amplifier.

The introduction of operational amplifiers, starting after 1950, made galvanostatic setups... 

Fig. 6. Block scheme of the galvanostat with the microcomputer control OA -operational amplifier, R - 1 kQ resistor, SG - pulse generator, DAS - data aquisition system, pC-microcomputer...

The section of Fig. 24 to the left is marked Polari2ation Control and is involved in electrical control of the cell potential or current. Note that the instrument can operate as either a potentiostat or galvanostat and the potential or current applied to the cell is programmed by summing the internal sources, marked DC Ref and Sweep, with an external polarity input. The box marked Feedback/Bandwidth control represents actual control circuitry similar to that in Figs. 8 and 11. Relays and electronic switches, controlled by the internal microprocessor, allow switching between potentiostatic or galvanostatic mode. Tbe microprocessor also sets the control loop bandwidth, which allows the experimenter to trade bandwidth for increased control loop stability. Other inputs to the feedback control circuitry are the RE potential and IR compensation (if necessary). A power amplifier is inserted at the counter (auxiliary) electrode connection. This allows currents of up to 2 A to be applied to the cell. 

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