Operational amplifier characteristics

Irvine, R.G. 1994. Operational Amplifiers Characteristics and Applications, 3rd ed. Prentice-HaU, Englewood Cliffs, NJ. 

Fig. 11.1. Two basic types of current ampliflers. (a) Feedback picoammeter. It consists of two components, an operational amplifier (op-amp) A, and a feedback resistor 1 fb- a typical value of the feedback resistor used in STM is 10 fl. The stray capacitance Cfb is an inevitable parasitic element in the circuit. In a careful design, Cfb 0.5 pF. The input capacitance Cm is also an inevitable parasitic element in the circuit. Those parasitic capacitors, the thermal noise of the feedback resistor, and the characteristics of the op-amp are the limiting factors to the performance of the picoammeter. (b) An electrometer used as a current amplifier (the shunt current amplifier). The voltage at the input resistance is amplified by the circuit, which consists of an op-amp and a pair of resistors R, and R2. The parasitic input capacitance Cm limits the frequency response, and the Johnson noise on Rm is the major source of noise. Also, the input resistance for this arrangement is large.

The operational amplifier or in short, op-amp, is used so extensively in modem electronic circuits that it is called a panacea. Op-amps are always used with negative feedback so that the circuits are essentially determined by the feedback networks only. Within certain limits, the characteristics of the op-amps can often be neglected (Fig. H.2). 

These properties give the general characteristics of operational amplifiers, viz. ... 

The basis of analogue instrumentation is the operational amplifier (OA), an integrated circuit that exists in various forms and with different characteristics according to the applications and requirements19. 

For the purpose of this text, an operational amplifier consists of a series of solid-state components designed to have certain fimctional characteristics. A schematic representation of an operational amplifier, given in Figure 6.1(a), shows 5 leads attached to the operational amplifier. The vertical leads, marked Vs+ and Vs-, provide power to the amplifier and are connected to a direct-current power supply. The two leads on the left, termed the noninverting (-I-) and the inverting (—) input, have potentials V+ and VL, respectively. The output potential is Vq. 

As Aop is very large, the linear region of operation is correspondingly very small. The characteristics of an ideal operational amplifier are that ... 

As shown in Figure 6.1(b), the output of the operational amplifier under open-loop conditions tends to be in the sahiration region. Operation within the linear region is made possible by inclusion of feedback loops, leading to operational characteristics important for electrochemical instrumentation. This is termed operation under closed-loop conditions. 

Example 6.1 Negative Feedback Find the electrical characteristics of an ideal operational amplifier xvith negative feedback, shown schematically in Figure 6.3. 

The characteristics of operational amplifiers are discussed in many electronics texts (2) and in the manufacturers literature. The following are of importance ... 

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 basic corrosion instrumentation requirement involves the measurement of potential difference. Currents are measured as the potential across a resistor (R ) as shown in Fig. 1.2, where the potential difference is again determined with an operational amplifier. More sophisticated measurements such as polarisation characteristics and zero resistance ammetry involve the use of potentiostats which again use operational amplifiers in a differential mode. The potentiostat is an instrument for maintaining the potential of an electrode under test at a fixed potential compared with a reference cell, and the basic circuit is similar to that for potential measurement with the earth return circuit broken to an auxiliary electrode in the electrochemical cell. Such a circuit would maintain the potential of the test electrode at the reference cell potential. This potential may be varied by inserting a variable potential source (V ) in the input circuit as shown in Fig. 1.3. The actual cell potential (V ) and the current required to polarise the test electrode to this potential may be measured using the basic circuits shown in Figs. 1.1 and 1.2 respectively. 

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. 

The disadvantage following from output signal dependence on the MOSFET characteristics is overcome in the circuit depicted in Fig. 7C (constant charge method). The negative voltage feedback of an operational amplifier is used here to compensate a source-potential (Ug) drift. The value of Ur is controlled to obtain a constant value of Ug (here 0 V) and consequently constant values of Uos Id = Uds/Rs ... 

With continued improvement in the performance of operational amplifiers, operational characteristics of potentiostats have also advanced. The original vacuum tube op-amps were surpassed by the introduction of solid state devices, first discrete transistor based devices and then by integrated circuits. Increasing the level of integration of electronic circuits has resulted in the... 

In previous considerations of operational amplifier analysis, we have assumed that the device was ideal in the sense that it can respond instantly to any change in response at the inputs. This is equivalent to assuming that the open loop gain is independent of frequency. The characteristic dependence of open loop gain for a real operational amplifier is shown in Fig. 11.8. The roll-off frequency determines the time constant for the potentiostat, Ti, through the relationship... 

Brown ER, Smith DE, Booman GL (1968) Operational amplifier potentiostats employing positive feedback for IR compensation I Theoretical analysis of stability and bandpass characteristics. Anal Chem 40 1411-1423. doi 10.1021/ac60266a024... 

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

Potentiostatic circuits make use of operational amplifiers (OA) to control voltage and to measure current. Operational amplifiers have a number of inputs, but in circuit diagrams the power supply connections are understood and only three connections are considered (Figure 2-28). Based upon their action, these connections are the inverting input (—), the noninverting input (-h), and the output. In normal use, OAs are used in a feedback mode that is, feedback loop is introduced fi-om the output or from some other source to the inverting input. The action of the OA in electrical currents can be understood largely on the basis of a few characteristics. 

Figure 1.21. Gain characteristic for a typical operational amplifier.

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