Differentiator, current feedback

Figure 6.20 illustrates a circuit that has been widely used. SW1 affords choice of anodic or cathodic current, SW2 initiates the experiment, and then SW1 may be used for current reversal. OA-3 is available for differentiating E with respect to t. A more advanced circuit (Fig. 6.21) incorporates an additional feedback loop and a comparator to perform cyclic chronopotentiometry with automatic switching. Operation of this circuit is perfectly analogous to the cyclic voltammetry circuit discussed in Section II.E. 

A photoinduced discharge apparatus includes a corona charging unit, an exposure system, and a noncontacting voltmeter. A grid is usually positioned between the corona wires and the sample to control the corona current. The apparatus is normally provided with a beam splitter and radiometer. The surface potentials are determined by a capacitively coupled probe and FET amplifier. The time derivative of the voltage can be obtained electronically or by numerical differentiation on a computer. When a time resolution greater than 10 ms is adequate, commercial voltmeters, for example, Monroe Electronics Isoprobe or Trek Electrostatic voltmeters, can be used. These are feedback devices, based... 

In-situ SPV measurements seem possible with minor modifications (1) the tip potential (versus the reference) is set at a value close to the rest potential of the semiconductor in darkness (this must be compatible with the electrochemical response of the tip), and (2) the tip current is quenched by adjusting the sample voltage (versus the reference) with the second feedback system. With p-type materials the method seems more obvious than with n-type specimens, since illumination promotes surface electrons. At n-type materials SPV measurements will induce corrosion since holes are driven to the interface. If absolute measurements of the SPV seem difficult, because they depend on the adjustment of the tip potential, differential measurements appear accessible to experiment. 

We have already noted that a negligibly small voltage differential at the inputs will drive a practical amplifier to its limit thus we almost never use the amplifier to deal with an input signal without elaboration of the circuit. Normally the amplifier is stabilized by feeding back part of its output to the inverting input. The manner in which the feedback is accomplished determines the operational properties of the whole circuit. Here our concern is with circuits involving the routing of a current from the output to the input (1-7). 

Fig. 10. Differential amperometric enzyme sensor. (1) glass plate, (2) Au-layer (150nm) or Cr-layer (20 nm), (3) deactivated GOD-layer, (4) common cathode, (5) active GOD-layer. Abbreviations Up, polarization voltage I, measured current 1, compensation current Rf, feedback resistors of current-to-voltage converters R, subtractor-circuit resistors.

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