Amplifiers - ACS Publications

REPORT FOR ANALYTICAL

CHEMISTS

The Electrical and Electronic Tools of the Analytical Chemist: Amplifiers by S. Z. LEWIN Department New York 3, Ν. Υ.

of Chemistry,

r p i i E USE of sophisticated electrical -*- and electronic instrumentation in all types of analytical work is at the present time so widespread, fre quent, and, indeed, indispensable that the chemist cannot afford the luxury of a lack of understanding of the principles of circuitry and de sign which these instruments in volve. This article and subsequent ones on this general topic will under take to survey and assess the role of these modern tools of the analytical chemist, and to provide the nonspecialist in electronics with a qual itative insight into their design and functioning. It is hoped that this will be useful in increasing the ef fectiveness of some chemists in choosing and applying instrumenta tion in their work, and in providing a basis for, as well as a stimulus to, further detailed study of this sub ject. The electrical and electronic tools that will be treated in this fashion include: amplifiers, power supplies, trigger and control circuits, oscil lators, and computational circuits. In general, an analytical instrument comprises a transducer combined with and acted upon by one or more of these types of circuitry in order to yield a read-out that can be eval uated quantitatively, or utilized to control or program a process.

Basic C o n s i d e r a t i o n s on A m p l i f i e r s in G e n e r a l

The distinction that is made be tween the terms "electrical" and

New

York

university,

Washington

"electronic" in the context of in strument design is based upon the nature of the electron transport around the circuit. If any portion of the complete circuit involves the conduction of electrons through a vacuum or a gas-filled region be tween electrodes, the process is des ignated as "electronic" in char acter. If the entire path involves only electron flow through metallic conductors, the circuit is termed "electric." Thus, such circuits or devices as voltage dividers, potenti ometers, Whcatstone bridges, in duction motors, galvanometers, batteries, etc., are electrical in na ture, whereas any circuit or device involving vacuum tubes, gas dis charge tubes, photoelectric tubes, ionization chambers, etc., is elec tronic in nature. In the case of semiconductor instrumentation, the behavior of the circuits and devices is so analogous to that of the older vacuum and gas tube circuits that these are generally designated as electronic too, although often with the qualifying adjective "solidstate" or "physical" appended. Some amplifiers are all-electric in nature, but the majority are elec tronic. The former are typified by the saturable reactor device, and are called "magnetic amplifiers." The latter are based upon the spe cial properties of vacuum tubes, gas tubes, and transistors. Classification of Amplifiers. The purpose of an amplifier is to trans form an input signal into another form that is more useful for the pur poses of the user. This may in

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volve increasing the magnitude of either the voltage or the current, of freeing the signal from limitations

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Dr. S. Z. Lewin is professor of chemistry at New York University, Washington Square, Ν. Υ. He was born in New York City and received a B.S. from the College of the City of New York in 1 9 4 1 . He then moved to the University of Michi gan for graduate study, receiving an M.S. in 1942, and Ph.D. in 1950, following a three-year inter val in the Army Chemical Corps during the war. He came to N.Y.U. as an instructor in 1950, was promoted to assistant profes sor in 1 9 5 1 , associate professor in 1958, and full professor in 1960. His research interests are in in strumentation, physical measure ments, and molecular structure. His monthly feature series on "Chemical Instrumentation" has appeared continuously in the Jour nal of Chemical Education since January 1 9 5 9 .

VOL. 34, NO. 2, FEBRUARY 1962

25 A

REPORT FOR ANALYTICAL CHEMISTS

imposed by its source so that it can interact more efficiently with a measuring device, or of performing a mathematical operation upon the signal to change its wave form or frequency. In addition to this variety of applications of ampli fiers, a multiplicity of circuit de signs may be employed to produce amplifiers for any given use. As a consequence, there are many de scriptive terms in use with respect to amplifier types. The designa tions which have been adopted are based upon the following independ ent categorical schemes: (A) na ture of electron transport; (B) pur pose, or end use; (C) magnitude of grid bias; (D) circuit configuration ; (E) signal frequency; (F) form of interstage coupling; and (G) circuit design principle. Table I sum marizes the terminology stemming from these several classification schemes. Role of Grid Bias. The terms employed in classification schemes A and Β can be understood in the light of the foregoing paragraphs. Scheme C applies to a.c. amplifiers —i.e., amplifiers designed to ac cept alternating current input sig nals—and is based upon the magni tude of the static—i.e., d.c.—grid

Table I. Classification Scheme Based on A. Electron transport mechanism B. End use C. Grid bias D. Circuit configuration E. Signal frequency F. Interstage coupling G. Design Principle

RANGE; ADJUST^

Amplifier Magnetic; electronic; solid-state Voltage; current impedance matching; operational Class A (—1 or — 2 ) ; Class AB (—1 or — 2 ) ; Class B ; Class C Grounded cathode; grounded grid; grounded plate D . c ; audio (or, AF) ; intermediate (or, I F ) ; radio (or, R F ) ; pulse, or video (or, VF) Direct; resistance-capacitance; impedance; transformer Electrometer; balanced; feedback; chopper; carrier; pushpull; tuned; servo

potential relative to the amplitude of the signal (a.c.) superimposed upon this potential. If the grid potential is such that the tube is never cut off during the entire cycle of the signal input, the type of amplifier is Class A. The suffix — 1 is attached if no grid current flows ; —2 if some grid current flows during at least part of the signal cycle. In the other categories of amplifiers, the grid bias is such that the signal drives this electrode nega tive enough during part of its cycle to cut off the tube current. In Class AB this occurs during less than 50% of the cycle; in Class Β it occurs 50% of the time; and in Class C more than 50% of the time. In general, the grid bias used in a given instrument is chosen to give the

best compromise between freedom from distortion and efficiency of power output; the closer to Class A-l the amplifier is, the smaller is the distortion introduced, but the lower is the power amplification. Location of Grounded Part of Circuit. Scheme D emphasizes the role of circuit configuration on the effective interelectrode capacitances associated with the vacuum tube. With the grounded cathode arrange ment, the interelectrode capacitance between grid and plate tends to be high enough to limit severely the amplification at high frequencies. In the grounded grid configuration, the gain of the amplifier is lower than in the case just described, be cause the grid-plate interelectrode capacitance is effectively in parallel

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Designation of Amplifier Types

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and i? 3 . If the bridge is balanced at the outset of the titration, it will be markedly unbalanced at the end point, and the unbalance signal may be read, or employed to activate a relay and stop the addition of titrant. This circuit is not intended for precise measurements of the light transmission of the solution. The only requirement is that the characteristics remain fairly stable during the several minutes required for the titration to be completed. There is no problem about the input signal to the grid of the tube, since it has a moderate impedance, and is of large magnitude, and all the power needed is available from the 20-volt power supply. Hence, almost any commercial radio-type vacuum tube may be used, and no special circuit designs need be invoked. Single-Stage Electrometer. If it is necessary to obtain stable, reproducible measurements of the current flowing through a vacuum tube in order to sense the magnitude of the input signal impressed on the grid, this may be achieved by very careful selection and stabilization of the components of a simple circuit, or through a sophistication of design that eliminates the effect on the read-out of variations occurring in the characteristics of the components. The latter approach will be described shortly; the former is exemplified by the circuit shown in Figure 2 [Andreatch, A. J., Feinland, R.,

ANAL. CHEM. 32,

1021

(I960)]. The heart of this circuit is an electrometer-type tube, which is a triode vacuum tube that is especially designed to provide the maximum insulation between the electrode structures, so that the leakage current that flows between grid and

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VOL 34, NO. 2, FEBRUARY 1962

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31 A

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cathode when there is a potential difference impressed is as small as possible. It is this leakage current (along the glass surfaces where the electrode contacts emerge from the tube envelope, through the glass it self, and within the tube due to ionization of the residual gas) that limits the magnitude of the input impedance of the amplifier. In a good electrometer tube the leakage current is less than 10~12 ampere (input impedance is greater than 10 12 ohms). The electrometer tube is also designed for high mechanical rigidity of the electrode structures, which is conducive to electronic stability and low noise. Fluctuations in the tube charac teristics are avoided by the use of conservative—i.e., relatively low— RU

IOOK

voltages applied to plate and fila ment, and by the utilization of stable batteries as the power sup plies. The flame detector may be thought of as a variable resistance (the magnitude of which depends upon the degree of ionization in the flame) that is in series with Rlr and the applied voltage, Elr divides between these in proportion to their relative resistances. Alternatively, one may say that battery Εχ drives a current through 2?i that is limited by the conductivity of the flame; this current flowing through Rx de velops an IR drop, or potential difference, across i?i. Thus, the po tential impressed on the grid of the electrometer tube is related to the degree of ionization in the flame. »Bf

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32 A

ANALYTICAL CHEMISTRY

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Β C Figure 5. A. Capacitance-resistance coupled a.c. amplifier used for control of servomotor rotation in response to degree of unbalance of an a.c. Wheatstone bridge. The bridge signal undergoes two steps of amplification, and a push-pull inversion Figure 5. B. Illustrates the function of the cathode resistor bypass condensers. The condenser diminishes the influence of the a.c. component of the tube current on the cathode potential, and hence on the degree of amplification of the stage Figure 5. C. Illustrates the principle of the phase-sensitive servomotor. The two sets of motor field coils are excited by a.c. signals which are 9 0 ° out of phase with each other. This creates the effect of a rotating magnetic field with respect to the motor armature, the direction of rotation depending upon the sense of the phase difference

Battery E2 drives current through the branch composed of Rs + R2, and simultaneously through the branch in parallel with this, com posed of: R6 -\- [the parallel com bination of R5 and (M + R*) ] + the electrometer tube. Battery E3 sends the heating current through the tube filament, and also sends a current through the path: R5 + M + R4. Thus, the meter, M, feels a component of electron flow in one direction from battery E2, and a component in the opposite direction from E3 (note battery polarities shown in the figure) — that is, E 3 "bucks out" part of the tube current, and the meter dis plays only the net effect of these opposing forces. This has the ad vantage of permitting a meter of high sensitivity to be used to dis play relatively small changes in a large tube current. That is, sup pose the background tube current is 100 μ&., and the increase in tube current due to flame ionization is 10 μα. Without this current-buck-

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Figure 6. The variation with load resistance, RLr of the voltage amplifi cation and power output for a singlestage amplifier involving a triode hav ing a plate resistance, rp, of 10K, a transconductance of 1000 micromhos, and an amplification factor of 10

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ing feature, a meter having a fullscale deflection (f.s.d.) of 200 μ,Ά. would have to be used, and the signal of interest would have to be read as the small difference be tween two large deflections. With the current-bucking arrangement, a higher sensitivity meter can be used to display the lO-^a. signal directly. The "zero adjust" sliding contact permits the grid potential to be varied when the flame is burning, in order to adjust the read-out meter to zero deflection when no sample is being introduced into the flame. Two-Stage Electrometer Ampli fier. If a higher gain is required than is available with the singlestage electrometer just described, its output may be direct-coupled to a second stage of amplification, as shown in the circuit of Figure 3 [Hansen, R. E., Buell, M. V., ANAL.

31, 878 (1959)]. In this case, the transducer is a multiplier photo tube, the anode of which is connected to the top of the grid input (or, dropping) resistor, Β.Λ. Photoelectrons reaching the anode flow down through i?i and create an 772 drop across it, making the grid of the CK5889 electrometer tube more negative, the greater the photocurrent. The more negative this grid is, the smaller is the tube current flowing through the elec trometer and the smaller is the 772 drop through the plate load resistor, 72i4- A decrease in this IR drop is equivalent to an increase in the po tential at the plate of the electrom eter tube. This plate is directly coupled, through Ε is, to the grid of the 6AU6 tube. Hence, a negativeCHEM.

TO

m\\

Figure 7. Differential cathode follower circuit, used to couple a sig nal from a high internal impedance source to a low input impedance read out device Circle No. 33 on Readers' Service Card

34 A

·

RECORDER


REPORT

going signal on the grid of the electrometer tube produces a posi tive-going effect on the plate of that tube and also on the grid of the next stage, causing the tube current of the 6AU6 to increase. This increases the IR-drop through Rxi, which is equivalent to a de crease in the potential of the plate of the vacuum tube. A reference voltage is applied to the grid of the second 6AU6 tube (right-hand tube in the figure), which determines the magnitude of the tube current and hence of the plate potential (the latter differs from that of -\-EB by the IR drop through R12)- The read-out is the difference in plate potentials be tween the sensing and reference tubes; for greater sensitivity of de tection, that potential difference is amplified in a transistor amplifier. In each branch of the latter, battery E2 drives a current through the transistor, the magnitude of which is controlled by the potential in jected through the third electrode (coming from the plates of the

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36 A

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6AU6's). These currents, flowing through load resistors Ble and R17, respectively, create IB drops, the difference between which is read out on the 0- to 20-/ia. meter. If the circuit is always balanced to give a zero output by appropri ate adjustment of the reference grid potential, the setting of the latter (read in terms of the posi tion of the sliding contact on the "%T" variable resistor) can be calibrated to read directly in per cent transmittance of the sample positioned between the multiplier photo tube and the light source. Balanced Amplifier. T h e sta bility of the output of a single-tube (single-ended) stage of amplifica tion depends upon the constancy with which the circuit conditions— viz., filament heating current and plate voltage—are maintained. I t is possible to decrease the sensitivity of the amplifier to fluctuations in the power supply by making use of a balanced, double-ended circuit design, such as is shown in Figure 537 ( I 9 6 0 ) ] . The input stage consists of a balanced pair of electrometer tubes, Vi and F 2 , the outputs of which are direct-coupled to a balanced pair of second-stage tubes. All tubes are connected to the same power sup ply, so any variations in the applied voltages affect the tubes practically equally, and the difference in out puts remains substantially unaf fected. In this circuit, the e.m.f. de veloped between a pair of potentiometric electrodes is applied to the grid of one electrometer tube, while a reference voltage is maintained on the grid of the other electrometer. RC-Coupled A.C. Amplifier. In m a n y cases, it is possible to excite a transducer with a periodically fluctuating signal, so t h a t an a.c. output can be generated for intro duction to t h e amplifier. T h e a.c. amplifier has the advantage of rela tive freedom from sensitivity to variations in the power supply volt ages, similar to the balanced ampli fier just described. If the transfer characteristic (variation of tube current as a function of grid po tential) is linear, drifts in the d.c. potentials applied to the electrodes


the amplifier is sharply tuned to the carrier frequency. The tuning is accomplished by the incorporation of capacitor-inductor combinations in the input or coupling circuits that are of such values as to be at resonance at the carrier frequency. Figure o,A shows the circuit diagram of an RC-coupled a.c amplifier that illustrates several important principles [Simmonds, D. H.,

cause the d.c. component of the tube current to drift, but do not affect the a.c. component. Since a capacitor passes a.c, but acts as a break in the line to d.c, it is possible to pass the a.c. components of each tube current to the grid of the next stage, without any interaction occurring between the d.c. components in these stages. The a.c. amplifier permits high gains to be achieved with low distortion and low noise, and the power supply can be simple and inexpensive. Whenever its use is possible, an a.c. amplifier is preferable to the types of d.c. electrometer amplifiers described above. The exciting frequency is called the carrier frequency, and an amplifier designed to be used for the manipulation of this signal is called a carrier amplifier. The optimum signal-to-noise ratio is obtained if

INPUT CONVERTER

Rowlands, R. J., ANAL. CIIEM.

32,

256 (I960)]. The transducer is a Wheatstone bridge which is excited by an a.c. voltage, and the unbalance voltage is applied through input capacitor, C\ to the grid of the first stage. This signal causes the tube current to vary, increasing as the grid becomes more positive than its static, d.c. potential, and decreasing as the grid becomes more negative. To prevent these changes in tube current from affecting the

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Figure 9. Circuit diagram of a chopper-type d.c. amplifier (Leeds and Northrup Model 7 6 6 4 pH indicator). The input signal is converted to a.c. This is passed through a cathode follower for impedance matching to the subsequent voltage amplification stages. The output of these is demodulated back to d.c. so that it may be fed back in opposition to the original signal. The feedback voltage provides the read-out signal

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1935

REPORT

cathode potential as a consequence of t h e variations in t h e IB drop through the cathode resistor, Rit a bypass capacitor, Ci, is employed. The role of this bypass capacitor is illustrated schematically in Figure 5,R. If the capacitive reactance, Xc, is small, the a.c. component of t h e tube current develops only a small voltage across i t ; if the bypass capacitor were absent, this a.c. com ponent would have to flow through R and would generate a much larger voltage drop, since R is much larger than Xc. T h e voltage drop devel oped across the parallel R C com bination has the effect of making the cathode more positive, which is equivalent to a negative-going volt age on t h e grid. Hence, t h e cathode resistor serves to feed back a signal to the grid t h a t opposes the input signal. T h e bypass capacitor di minishes this negative feedback as far as the a.c. component is concerned. In the amplifier of Figure 5,A, the output stage employs a pair of triodes in push-pull arrangement to deliver an a.c. signal to one set of the field coils of a servomotor. In t h e push-pull stage, t h e plates of the two tubes are connected t o opposite ends of a transformer sec ondary winding, so t h a t when one plate is + and this tube is capable of conducting, the other plate is —, and t h a t tube is nonconducting. Hence, current passes alternately through these two tubes. This a r rangement is used when i t is desired to have a relatively undistorted a.c. output t h a t is capable of delivering appreciable power to a load. The principle of the servomotor which is t h e load of this amplifier is illustrated in Figure 5,C. T h e a.c. signal t h a t the amplifier sends through coils L i (Figure 5,A) is 90° out of phase with the signal from t h e utility line t h a t passes through coils L 2 . This phase dif ference creates t h e effect of a rotat ing magnetic field in t h e motor, and the armature is swept along with t h e field. If the armature is mechani cally linked to the sliding contact of the Wheatstone bridge, it is pos sible to have the amplifier auto matically and continuously rebal ance the bridge to the point where

REPORT

the input signal (and hence the output which drives the motor) is reduced to zero. This is the principle of the servomechanism, which is now so widely used in chemical instrumentation. The Cathode Follower. In voltage amplification, an input signal on the grid of the amplifier first stage is converted into a larger signal appearing across the load resistor, RL, in the plate circuit. As Figure 6,A, shows, the voltage amplification increases as this load resistance increases. However, when RL is very large, the tube current (and, hence, the load current) is small, and the ability of the current to deliver power to the load (P = PRL) is ever smaller. Figure 6,B, shows that the maximum output power is obtained from an amplifier when the internal resistance of the latter, rv, is equal to the resistance of the load, RL- Thus, in power amplification it is important to match the impedance of the source of a signal to that of the load that is to be energized. In many chemical problems the signal source has a very high internal impedance, and it is necessary to employ special impedance matching circuitry in order to couple this

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Employs palladium-silver alloy for diffusion Source gas with 7 5 % or more hydrogen*, such as cylinder hydrogen or dissociated ammonia is passed through a special palladium-silver alloy t h a t selectively separates hydrogen from all impurities. Result is hydrogen so pure that modern analytical techniques cannot detect impurities. And, the palladium alloy never becomes brittle, needs no purging.

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The unit is simply plugged into any convenient 115-volt, a-c outlet, the impure hydrogen connected to the inlet...pressure and flow adjusted to the proper operating range and the ultra-pure, dry hydrogen collected a t the outlet.

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D.

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cx

Figure 10. Operational amplifier circuits. In each case the output voltage is related to the input function by the following operation: A. multiplication by a constant; B. summation of the individual inputs; C. integration; D. differentiation

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Controlled Volume Pumps Colorimetric Analyzers Hydrogen Purifiers Laboratory Instruments

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VOL. 34, NO. 2, FEBRUARY 1962

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39 A

REPORT

source to the devices used for meas urement or control purposes. The type of circuit utilized for this func tion is the cathode follower, an ex ample of which is shown in Figure 7 [Gucker, F. T., Jr., Peterson, A. H., ANAL. CHEM. 25, 1577 (1953)].

The input signal is applied to the grid of an electrometer tube if very high input impedance is required, or of a radio-type triode vacuum tube for less critical applications. It affects the tube current, and con trols the IR drop across a resistor in the cathode branch of the circuit. This voltage is used as the output, rather than the IR drop across a plate load resistor, as in several of the cases discussed previously. In the circuit shown in the figure, the cathode resistor drop in one cathode follower is compared with a similar drop in a reference circuit, so that the circuit has the advantage of the balanced amplifier design. The effective output impedance is that of the cathode resistor, shunted by the triode and its power supply, and can be adjusted to small values. Negative Feedback-Stabilized Amplifier. Negative, or inverse, feedback, reduces the gain of an amplifier but makes the perform ance of the circuit independent of any drift or noise occurring in the amplifier itself [for an explanation of the mechanism of this effect, see Lewin, S. Z., ANAL. CHEM. 33, 23 A

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Circle No. 169 on Readers' Service Card 40 A

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(March 1961)]. A schematic dia gram of a negative feedback ampli fier is given in Figure 8,A. If a negative-going signal passes through the input capacitor, d , to the grid of Vu the tube current de creases, and the plate potential in creases. The positive-going effect passes through the coupling capac itor, C2, to the grid of V2, causing the current in that tube to increase. This lowers the plate potential of V2, and a negative-going signal is passed through the feedback capac itor, Cf, to the cathode of Υλ. This cathode going negative is equivalent to its grid going positive; thus this chain of events constitutes a feed back in opposition to the original input signal. In many feedback-stabilized am plifiers, the feedback signal is made approximately equal to the input signal, and the feedback current

REPORT provides t h e power for operation of t h e r e a d - o u t device. T h i s is il lustrated symbolically in Figure 8,B, which p o r t r a y s the t y p e of cir cuit design used in several com mercial direct-reading p H meters [Keegan, J. J., M a t s u y a m a , G.,

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Chopper Amplifier. T h e desir able features of t h e RC-coupled a.c. amplifier can be employed in t h e amplification of a d.c. signal if t h e latter is converted, or "chopped" into an alternating voltage. T h e design principle of t h e very widely used chopper-type amplifiers is (A) conversion of t h e p r i m a r y d.c. signal into a . c , (B) amplification of t h e a . c , a n d (C) rectification, or de modulation, of t h e a.c. back to d.c. Figure 9 shows t h e circuit dia g r a m of a commercial chopper a m p lifier used as a p H meter. I t has, in addition to the features described above, t h e provision of feedback stabilization by returning t h e de modulated signal coming from t h e "electronic converter" in opposition to t h e input signal. T h e read-out is t a k e n from t h e feedback loop. Operational Amplifiers. One of the most recent developments in commercial amplifier instrumenta tion is t h e availability of a series of feedback-stabilized, d.c. amplifiers which a r e of modular—i.e., "build ing block"—construction, and are designed for t h e performance of v a r ious analog computation operations. These are called operational ampli fiers, a n d t h e symbolic representa tions of t h e principal types are given in Figure 10. These amplifier mod ules are extremely useful in t h e con struction of experimental a p p a r a t u s , and h a v e been exploited in a n u m ber of t h e electroanalytical studies cited in T a b l e I I .

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NEW! MOO RE-MIL FORD IR M O I S T U R E A N A L Y Z E R Easy, accurate readings regardless of sample density or weight. No preliminary weigh-in—simply spread thin layer of sample on t h e p a n and t h e balance automatically gives you the moisture content read t o t h e nearest 0 . 1 % . Extensive testing shows amazing repeatability of results. W h e n drying time is set a n d heater positioned, operation is automatic. Analyzer shuts itself off—answer remains until you change it. N o possibility of reabsorption or careless technique giving incorrect answers. I n stock a t your nearest Will warehouse. Write for technical literature; free demonstration on request. N o obligation.

BIBLIOGRAPHY Bair, E. J., "Introduction to Chemical Instrumentation," McGraw-Hill, New York, 1962. "Basic Electronics," Navpers 10087, U. S. Government Printing Office, Washing ton, D. C , 1955. Donaldson, P. Ε. Κ., "Electronic Ap paratus for Biological Research," Butterworths Scientific Publications, London, 1958. "Radar Electronic Fundamentals," Navships 900,016, U. S. Government Print ing Office, Washington, D. C., 1944. Stacy, R. W., "Biological and Medical Electronics," McGraw-Hill, New York, 1960.

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Amplifiers - ACS Publications

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