Operational Amplifier Circuit

In order to appreciate the principles of analog semi-integration, let us first review the classical operational amplifier circuits shown in Fig. 30. The output of circuit (a) is a voltage proportional to the instantaneous value of the current input... 

Fig. 30. Classical operational amplifier circuits, (a) A current follower (b) a current integrator.

D. Stout and M. Kaufman, Handbook of Operational Amplifier Circuit Design, McGraw-Hill, New York, 1976. 

Rotating platinum screens have been used for electrogravimetric analyses. Commercial instruments employ two concentric cylindrical platinum screen electrodes with one or both electrodes rotating to increase convection. The cell itself is usually a beaker with a sample volume of about 150 mL. Typically, no cell top is used, and when running multiple analyses, there should be adequate ventilation to prevent accumulation of hydrogen. A simple operational amplifier circuit can be constructed for the instrument. The commercial instruments available are expensive given the simplicity of this type of experiment. 

Electrical measurements with both types of cells were made by using two devices as the gain-controlling elements of an inverting operational amplifier circuit which was driven with a constant input voltage (E. ). The ratio of the resistance of the reference device,... 

Detection of the particles is accomplished by means of their attenuation of a light beam provided by a halogen lamp and collimated by two slit assemblies. A silicon photodiode and an operational amplifier circuit are used to monitor the light intensity. Adjustable gain and offset potentiometers are located on the control module. The signal can be displayed on a stripchart recorder and is monitored by means of a digital panel meter. 

The rapid development of solid-state electronic devices in the last two decades has had a profound effect on measurement capabilities in chemistry and other scientific fields. In this chapter we consider some of the physical aspects of the construction and function of electronic components such as resistors, capacitors, inductors, diodes, and transistors. The integration of these into small operational amplifier circuits is discussed, and various measurement applications are described. The use of these circuit elements in analog-to-digital converters and digital multimeters is emphasized in this chapter, but modern integrated circuits (ICs) have also greatly improved the capabilities of oscilloscopes, frequency counters, and other electronic instruments discussed in Chapter XIX. Finally, the use of potentiometers and bridge circuits, employed in a number of experiments in this text, is covered in the present chapter. 

Operational amplifier circuit to deliver current at a standard Weston cell voltage. 

The characteristics of present amplifiers are such that one can easily obtain accurate, reliable performance on time scales of 10 piS or greater (i.e., bandwidths less than 100 kHz). Time scales below 10 )ns (bandwidths above 100 kHz) can be reached with care in circuit design and choice of components. Building reliable operational amplifier circuits like those described below for time scales under 3 )ns is very difficult. 

The voltage limits of the amplifier are controlled by the power supplies. They usually are quite close to the supply values. For most devices, the limits are 13-14 V. Currents will be supplied freely to a load until the current limits are reached, typically at 5-100 mA. Special devices with larger current or voltage output limits are available, but high output power in operational amplifier circuits is usually obtained by booster stages, as described below. 

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... 

Devise an operational amplifier circuit that will integrate the sum of two input signals. Only one amplifier is required. 

Several different methods have been used to obtain derivative spectra. For modern com iuter-controlled digital spectrophotometers, the differcjuiation can be performed numerically using procedures such as derivative least-squares polynomial smoothing, which is discussed in Section. SC 2. With older analog instruments, derivatives of spectral data could be obtained electronically with a suitable operational amplifier circuit (see... 

Most transducers converting chemical concentration into an electrical signal have a nonlinear response for example, electrode potential and optical transmission are not directly proportional to concentration. In general, this nonlinearity is easily and simply corrected in equilibrium analytical measurements. However, it is considerably more difficult to instrumentally correct the response-versus-concentration function in reaction-rate methods, and often the correction itself can introduce significant errors in the analytical results. For example, the simple nonlinear feedback elements employed in log-response operational-amplifier circuits are not sufficiently accurate in transforming transmittance into absorbance to be used for many analytical purposes. 

Fig.4. Data plot for a 20 nA current pulse. The pulse width was 16.5 fis. The circuit used for this data consisted of a single transistor in a common emitter configuration with a light emitting diode as the load element. This circuit was used instead of the operational amplifier circuit in Fig.l because it provides improved response at low current levels.

Fig.3. Automatic compensation for decreasing sensitivity. The sensor is periodically exposed to standard solutions and the response compared to stored values. The expert system calulates the amount of compensation required and adjusts a digital potentiometer accordingly. The potentiometer controls the gain in a simple non-inverting operational amplifier circuit. 

Figure 8.12 Operational amplifier circuits. The details shown in (a) are always present but...

R. Brown, D. E. Smith, and G. Booman [1968] Operational Amplifier Circuits Employing Positive Feedback for IR Compensation. I. Theoretical Analysis of Stability and Bandpass Characteristics, Anal. Chem. 40, 1411. 

An operational amplifier circuit for performing the mathematical operation of integration. 

In a potentiostatic system, the current flows between the auxiliary and the working electrodes while the third, or reference, electrode operates under essentially zero current conditions and controls the potential of the working electrode through the feedback arrangement of the operational amplifier circuit. 

Kennedy, E.J. 1988. Operational Amplifier Circuits. Holt, Reinhart, and Winston, New York. 

Table 11.4 —Various configurations for operational amplifier circuits (a) inverting amplifier, (b) summing inverting amplifier, (c) current follower, (d) differential amplifier, (e) integrator, (f) differentiator.

A completely different concept is based on the idea to connect the working electrode not to ground but to a potential —Uq [48, 49]. This means, the potential drop in the electrolyte Uq and this potential offset Uq compensate each other completely. This requires a special operational amplifier circuit, a so-called negative resistance (Fig. 4) with a transfer function ... 

A Properties of Operational Amplifiers 59 3B Operational Amplifier Circuits 62 3C Amplification and Measurement of Transducer Signals 65... 

Several transducers produce output voltages related to concentration or to a physical quantity of interest. For example, ion-selective electrodes produce voltage outputs related to pH or the concentration of an ion in solution. Thermocouples produce voltage outputs related to temperature. Similarly, Hall effect transducers produce output voltages proportional to magnetic field strength. Operational amplifier circuits, particularly those based on the voltage follower (see Section 3B-2), are used extensively for such measurements. 

An important characteristic of operational amplifier circuits, such as the difference amplifier described... 

The vesicle assay previously used ves a good survey of transporter activity under a fixed set of conditions. However it is not suited to exploration of transport activity as a function of the sign and magmtude of the transmembrane potential. Bilayer conductance, or single channel recording techniques are required (21). In this techmque, a lipid bilayer is formed across a small hole in a Teflon barrier, by direct application of the lipid in decane to the hole. Under favorable conditions, Ae lipid tl s to a bilayer membrane which electrically isolates the two halves of the cell. T icdly KCl is used as an electrol, and electrical contact is made via Ag/AgQ wires directly inserted into the solutions. A high impedance operational amplifier circuit (bUayer clamp) can then be used to apply a fixed transmembrane potential and to monitor the current which flows as a function of time. The two sides of the membrane are independently accessible, so different sequences of transporter addition, control of pH, and other variables are in principle j ssible. 

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