John T. Stock University of Connectcut Stows. 06268
I
A Versatile Operational Amplifier Assembly
Anv introduction to the electronic aspects of chemical instrun&ation must includt. the more ihvinus properties of. the ouerational amplifirr (OAI. F i ~ u r e1 is a view of the panrl of a; easily constrkted assemhfi that allows many df the properties of the OA to he examined in a single 3-hr laboratory period. Overall dimensions are 11 X Ell2 X 5 in. high. Construct,ional details are similar to those of the previously described digital systems ( 1 , Z ) . Jumper cables terminating in color-coded miniature insulated alligator clips are used to make interconnections to pins on the panel. Most of these pins are 6-32 X 1-in. machine screws that are isolated from the olvwood nanel hv insulatine washers. Separate strios are ., prnvided ;or G C . H ~ ~coni6ctions ~ND and fo'r unconnectr.d nr OPKN pins that can be used as tie points. Parallel lug strios support' the 1-Kohm, 1-Mohm, and the two 10- ohm resis-1 tors. Those shown are of 1%tolerance. hut common resistors are acceptable if .'actual values" arr given. In fact, an external resistor substitution box (Heath IN-31371. hased on *l(N tolerance components, i s used when variable values are needed. A calibration chart is prepared and affixed to the side of this box. The common and inexpensive integrated-circuit (IC] type 741 OA is held in an 8-pin socket that is mounted on a small rectangle of perforated insulating hoard. A 2N2218 or equivalent transistor is mounted in a socket and connections are brought out to jumper pins, as shown. To demonstrate control of outnuts hevond the uower of the unaided OA. an internal lx)osteramplifierwith i I U O - m A capability is provided. This tx,oater is availnl~leas an IC. the National Semiconductor LH0002. The amplifier actually used was assembled from discrete components, following the circuit schematic of this IC (3).A 50-ohm LOAD resistor simulates a device that may need up to 100 mA for operation. Regulated +15Vfor the OA ( 4 ) and unregulated approximately f 12V for the booster are supplied by line-operated internal power units that have separate switches and pilot lamps. Binding posts allow supply voltages to he checked and, when needed, limited current takeoff for external devices. The IC pins are not directly connected to the three OA jumper pins. Instead, connections are made through the BALANCE-RUN function switch, as shown at (a) in Figure 2. In the RUN position, the IC is electrically connected to the jumper pins. When switched toBAL, the IC becomes a "gain
of 10" inverting amplifier with both inputs grounded. With a vacuum-tube or other high-impendance voltmeter (VM) connected to the OA OUT, the 10-Kohm balance control, which is connected to -15 V and to pins 1and 5 of the IC (S), is rotated to bring the VM reading to zero. Except when making actual observations, the function switch is kept in the BAL position. Any drift from the balanced state is then obvious. Furthermore, hookups can he safely made or altered, because the input pins on the panel are disconnected and the output pin is a t or very near ground potential. Although the assembly is also used for demonstrating AC applications such as signal generation and active filtration, student experiments are normally confined to a sequence of dc hookups and measurements (6-8). These are (1) voltage
1%
.
NONIPiV
3g
OUT
Figure 2. (a)Function switching arrangement, shown in RUN position. R,, R,. 10 K% RJ 100 KR; S. Cpole 2-positionrotary switch: IC. type 741 minidip OA. (b)circuitof integrator subunit.R,.. 1 MR: STSPSTswitch:S2.SPST pushbUnm switch, normally open; C, see text.
-x
Figure 1. Operational amplifierassembly
Figure 3. Integrator subunit. Volume 55. Number 7, July 1978 / 471
follower, with estimation of the IC input current (2) controlled potential source, with estimation of the maximum OA output voltage (3) constant current source, with estimation of the maximum OA output current (4) inverting voltage amplifier (5) voltage to large current converter, using booster amplifier (6)integrator, including ramp generation and measurement of the value of a capacitor and (7) non-linear (logarithmic) response. Apart from the jumper cables, the only external components are the millammeter, VM, resistor substitution box, voltage reference source (Heath EU-80A) and the small integrator subunit shown in Figure 3. The wiring of this suhunit is shown at ( b ) in Figure 2. The attached "unknown" capacitor is an unmarked non-electrolytic component that has a value very close to 1fiF. Although not depicted on the panel, the transitor, used
472 1 Journal of Chemical Education
principally for experiment (I), carries the internal stabilization and polarity-guarding capacitor-diode-resistor arrangement described by Strohel (9). Literature Cited (L) Stock, J.T.. J. CHEM.EDUC..49.516 (1972). 121 Staek, J.T., J. CHEM. EDUC.. 51,357 119741. (3) RadioShack,"Linesr App1ications:Tsndy Corp., Fort Worlh.Texas,Vol. 1.1958, p. diN.31 ....
( I ) Ewing,G. W.J.CHEM.EDUC.,S3.A1911976J. (51 Texas Instruments, he.. "The Linear and Interface Circuits Data Book," Texas Inatruments. Dallss.Terss. 1973, pp. 1 ~ 6 and 7 4~71. c W. B. S s u n d e ~ (6) Diafenderfer, A. J.. ''Basic Techniques in E l ~ t r o n i Instrumentation." Co., Philadelphia, 1972, Chs. 19-22. 17) Malmstadt, H. V., Enke, C. G.. and Crouch. S. R.,"Elelectronic Measurements for Scienti8ts;'W. A. Benjamin, Inc., Reading, Masn., 1973, Section 3-2. 16) Msnn, C. K., Viekern. T. J., and Gulick, W. M.,"Bsrie Concepts in Electronic Instrumenution: Harper & Row, New Yark. 1974, Ch. 8. 19) Sfrobel. H. A,. "Chemical Instrumentation: A syrtematie Approach.). 2nd Ed.. Addison-Wesley Publishine Co.. Resdina. Mass.. 1973. o. 171.
