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Chapter 3 Operational Amplifiers in Chemical Instrumentation Most modern analog signal-conditioning circuits owe their success to the class of integrated circuits known as operational amplifiers, which can be referred to as op amps. Operational amplifiers are everywhere. If you open any instrument or piece of electronic equipment, it would be likely to find one or more op amps. This fact, along with the ease of complex functions that may be accomplished, makes clear the importance of understanding their principles of operation. In this chapter, we will briefly discuss some operational amplifier circuits and applications. Op Amps: Systems-on-a-chip Op amps are key analog building blocks that condition signals throughout a system. Many systems, especially more sophisticated ones, use more than one op amp because different types fulfill various requirements. Operational amplifiers derive their name from their original applications in analog computers, where they were used to perform mathematical operations. Op amps also find general application in the precise measurement of voltage, current, and resistance, which are measured variables in transducers that are used in chemical instruments. They are also widely used as constant- current and constant-voltage sources. Symbols for Operational Amplifiers Figure 3-1 is an equivalent circuit representation of an op amp. In this figure, the input potentials are represented by v+ and v-. The input difference voltage vs is the difference between these two potentials.
The power supply connections are labeled + PS and – PS and usually have values of +15 and –15 V dc. The open-loop gain of the operational amplifier is shown as A; thus the output voltage vo is given by vo= - A vs. Finally, Z i and Zo are the input and output impedance of the operational amplifier. Realize that the input signal may be either ac or dc and the out put signal will correspond. Note that all the potentials of op amps are measured with respect to the circuit common shown in Figure 3-1. Circuit common is also referred to as ground. Figure 3-1 (Principles of Instrumental Analysis) Operational Amplifier Circuits
Feedback Circuits It is often advantageous to return the output signal or some fraction of the output signal of an op amp to one of the two inputs. When an output signal of an op amp is connected to one of its inputs, the signal is called feedback. Figure 3-2 is an operational amplifier with a feedback loop consisting of the feedback resistor Rf that is connected to the output S , which is called the summing point. Note that the feedback signal is opposite in sense to the input signal vi as a result of the characteristics of the inverting input and is called negative feedback. Figure 4
Figure 4 (Principles of Instrumental Analysis) Applications of Operational Amplifiers Op amps are easily used to generate constant-potential or constant- current signals. Constant-voltage sources include several instrumental methods that require a dc power source whose potential is precisely known and from which reasonable currents can be obtained without alteration of this potential. A circuit, which satisfies such qualifications, is a potentiostat. Constant-current dc sources, called amperostats, find applications in several analytical instruments. These devices may be used to maintain a constant current through an electrochemical cell, for example. An amperostat reacts to a change in input power or a change in internal resistance of the cell by altering its output potential in such a way as to maintain the current at a predetermined level. Mathematical operations with operational amplifiers include addition or subtraction, multiplication or division, integration, and differentiation. Operational Amplifiers for Signal Detection & Processing
dB point. Because the scope’s time base setting will be low compared to the signal frequency, the signals will be displayed as an envelope of the signal, making peak measurement and comparison of the input and output signals easier. Once you have determined the amplitude in this way, alter the time base of the scope so you can determine the phase.) Plot the results in a Bode plot with log (gain) vs. Log (frequency) and determine the -3 dB point. Make a similar plot for the phase vs. Log (frequency). Does the amplifier behavior meet the manufacturer’s specifications? B. Active Low and High Pass Filter. The passive RC filters are affected by changes in the impedance of circuits following them unless the impedances are very high. If the input impedance of the subsequent circuit is not high, loading affects both the efficiency and cutoff frequency of the filter. The use of an operational amplifier as part of an ACTIVE filter dramatically reduces the problem. Active filters may be high pass, low pass, selective (amplify a specific frequency) or notch (reject a specific frequency). Many types of active filters are now available as integrated circuit (IC) components. You can construct the first order, low pass filter shown in Figure 5 in your lab and measure the response as a function of
frequency using a 1 Vp-p sine wave input from 50 Hz to 20kHz for each. Now build another, identical filter, and connect the output of one to the input of the other. Repeat the measurements. Plot gain vs. Frequency on a log-log plot for each filter. Is the roll-off the same? Which is the better filter? Figure 5 Construct the high pass filter shown in Figure 6. Characterize its frequency response in the same way. Figure 6 C. Higher order filter. Construct the single stage, second-order low-pass filter shown in Figure 10. Characterize its frequency
operational amplifiers (op-amps) as devices with a large number of uses in the measurement of electrical signals. In the current market, solid-state operational amplifiers of high quality are readily available from commercial sources at quite modest cost. Some of their applications include voltage gain, impedance matching, integration and analog computation. Another important and widespread application of operational amplifiers is switching. Such circuits are found in a wide variety of application in which signals levels must be monitored and compared to reference voltages, such as sampling circuits, peak detection circuits, analog timers, circuits designed to produced limited signal levels, and circuits at the interface of the boundary between the digital and analog domains. REFERENCES: “Operational Amplifiers.” http://192.215.107.101/ebn/942/tech/techfocus/1071main.html “Operational Amplifiers.” http://bolongo.ee.queensu.ca:8000/www/dept/courses/elec221/ opamps.htm “Operational Amplifiers.” http://www.chem.usu.edu/~sbialk/Classes/565/opamps/opamps.html Skoog, Holler, and Neiman. Principles of Instrumental Analysis. 5 th^ ed. Orlando: Harcourt Brace & Co., 1998.
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