A Transistorized Power Amplifier for Controlled-Potential Coulometers R. C. Propst, Savannah River Laboratory, E. 1. du Pont de Nemours & Co., Aiken, S. C. HE DESIGN
of the original controlled-
Tpotential coulometer as described by Booman (1) has been modified by
Kelley et al. (3, 4),and by Connally and Scott (2, 6). These authors have eliminated the load-compensating (vacuum tube power) amplifier from the original design and have replaced the remaining vacuum tube power amplifier with a transistor. While these design changes hare simplified the circuitry and eliminated the heat dissipation problem that is associated with the vacuum tubes, these instruments have a limited frequency response which has been attributed to the transistor amplifier and to the mode of operation of the George A. Philbrick Researches, Inc. GAP/R USA-3 amplifier (6) in the potential control circuit ( 2 ) . Kelley (4) has indicated the desirability of evaluating transistors that have a high frequency response and the advantages to be gained in utilizing P K P and KPN transistors in the amplifier to simplify the switching problem. A transistor amplifier was developed that has a high frequency response and does not have t o be switched in changing from the oxidation to the reduction mode. These units have been incorporated into a controlled-potential coulometer that follows the original design of Booman (1). The performance of this instrument has been satisfactory, and no evidence of instability has been detected. The instrument in which the transistorized amplifiers are utilized will be the subject of a later publication. A schematic diagram is shown in Figure 1 for the amplifier coupled with a computer amplifier in a feedback loop. Transistors V z and V3 are connected for Complementary operation as emitter followers, and the output is taken from the junction of the emitters. Transistor VI serves as an emitter follower driver for V? and Vt. Since the collectors of VZ and V3 are connected to identical supply voltages of opposite polarity, the output voltage a t the junction of the emitters will be zero when the resistances of both transistors are equal. By the introduction of current flow into the base emitter junction of the appropriate transistor, the effective resistance of that transistor can be reduced. Thus V zand Va act as variable resistors, and the output voltage a t the junction of the emitters can be of either polarity, depending on the effective resistances of V s and VI. Xormally 588
ANALYTICAL CHEMISTRY
for complementary operation no forward bias is required; however, since V pand ' v 3 do not have identical characteristics, a small forward bias is applied to each so that each transistor draws 5 ma. of collector current when the output voltage is zero. This forward bias minimizes cross-over distortion and is obtained from the voltage drop across Rs. Transistor VI supplies the drive and bias voltages for Vz and V3. V I is operated as a class A amplifier to eliminate the need for a fourth transistor. A forward bias sufficient to produce a collector current of 50 ma. is applied to V I ,and the value of Ra is chosen so that the emitter of VI is approximately a t ground potential for zero output voltage. The voltage drop across Re is the sum of the base emitter voltages of V z and V 3 required to produce collector currents of 5 ma. If other types of transistors are to be utilized in this circuit, then the value of Re must be recalculated. The correct forward bias for VI t o give an output voltage of zero with the input a t ground potential is obtained by adjusting R2. The value of C1was selected t o give negligible distortion on the basis of the response of the amplifier t o a 20-kilocycle square wave.
The amplifier is designed to be utilized with the GAP/R K2P-K2X computer amplifiers (5) and will deliver an output voltage of h 1 5 volts a t 300 ma. with an input voltage swing of h 100 volts a t 2 ma. Experience in this laboratory has indicated that this output voltage range is sufficient to supply 200 ma. to a titration cell; however, in certain cases where high cell resistances are encountered a larger voltage swing may be necessary. In these cases, higher output voltages can be obtained by increasing the supply voltage. The amplifier was constructed as a plug-in unit with the transistors mounted on a 5 X 2l/4 X 2 V 4 inch aluminum box. The aluminum box has proved to be an adequate heat sink. In normal service, the surface temperature of the box does not exceed 32' C. In controlled-potential coulometric service where the transistor amplifier is utilized in a feedback loop with a K2P stabilized K2X amplifier, any change in the characteristics of the transistors due to temperature variations is effectively nullified by the high gain of the stabilized amplifier. Hom-ever, to ensure 3 symmetrical drive voltage from the stabilized amplifier, the voltage to
9UN
tout
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, I
I I
I I 2w
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Figure 1.
054
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Transistorized power amplifier
t25V
the input of the transistorized amplifier must be adjusted to zero after a warmup time of approximately 30 minutes. This adjustment is made by Rz with the feedback network in the balance mode and is independent of the stabilized amplifier balance adjustment. The stabilized amplifiers are balanced in the conventional manner (6). The amplifier was evaluated by constructing a coulometer which followed the basic design of Booman (1) and in which the vacuum tube power amplifiers were replaced with the transistorized units. The frequency response was determined by comparison with that of a similar coulometer which utilized vacuum tube amplifiers. The frequency responses were identical from 0 to 100 kilocycles. The least squares calibrations for the transistorized version showed a root mean square error of the full scale integrator of =tO,OlyG reading for both the anodic and the cathodic curves. A full scale integrator reading corresponded to 21.535 coulombs. il relative standard deviation of &O.O9yG ( n = 12) was deterniined for the titration of from 1 to 16
mg. of uranium, and a relative standard deviation of 0.09% ( n = 12) for the titration of NBS-13Ga potassium dichromate. Bias was absent in the titration of the uranium, while in the case of the potassium dichromate the recovery was 99.95%. Several versions of the transistor amplifier are possible, and less expensive transistors such as the 2x242 and the 2N32G may be utilized with some sacrifice in power capabilities and gain. Temperature compensation through the utilization of thermistors will result in improved stability. However, this feature was not considered necessary for the present application. Since the completion of this work, the K2X amplifiers have been discontinued by Philbrick and replaced by model K2Xa; also, a number of transistorized booster amplifiers have become available commercially. The K2Xa amplifier is essentially identical to the old model R2X but has a higher gain and is less tolerant of a capacitive load. In some circuits where long cables are utilized between the coulometer and the titration cell, a
5GO-ohm, I/*-watt resistor must be incorporated between the output pin 6 of the K2Xa and the input to the transistorized amplifier. The incorporation of this resistor did not affect the performance of the instruments currently in use a t this laboratory. While commercial booster amplifiers are now available and may be preferred by some workers, no unit combining complementary symmetry operation nith high current output could be found in the literature. LITERATURE CITED
(1) Booman, G. L., ANAL.CHEV. 29, 213
(1957).
( 2 ) Connally, R. E., Scott, F. d.,U. S.
Atomic Energy Comm., Rept. HW65919 (1960). (3) Kelley, M. T., Jones, H. C., Fisher, D. J., ANAL.CHEM.31, 488 (1959). (4)Ibid., p. 956 (1959). ( 5 ) Philbrick, George -4., Researches, Inc., Boston 16, Mass., “GA4P/RElectronic Analog- ComDuters” (catalog data eheets). ( 6 ) Stromatt, R . W., Connally, R . E., ANAL.CHEW33, 345 (1961). Work performed under contract hT(072)-1 with the U. S. Atomic Energy Commission.
Multiple-Column Gas Chromatograph Utilizing a Single Detector and Recorder Bernard
VERY
M. Mitzner and Philip Gitoneas, International Flavors & Fragrances, Inc., Union Beach, N. J. simple gas chromatographic
A apparatus makes possible the utiliza-
tion of a multiplicity of chromatographic columns of varying diameters in one instrument employing a single detector and recorder. One can easily proceed from one column to another by merely opening and closing a valve. -FROM
Columns. Several large diameter columns !%-ereinstalled in the instrument a t the same time ‘/4-inch and 3/16-inch columns were present. The larger columns may be used for trapping, and, by flipping a switch, the purity of the collected material can be monitored on one of the analytical columns. Approximately 2 to 3 minutes is required to switch from one column to another
GAS T A N K
-INJECTORS
----EXIT
No special parts are required and the columns can all be maintained a t the same temperature or be used a t different temperatures. This system has been used for well over a year and found to be both useful and effective for analytical as well as preparative gas chromatography. A flow diagram is shown in Figure 1. The apparatus consists of four parts.
MANIFOLD
Figure 1. Flow diagram of five-column chromatograph
Inlet Manifold. The gas supply leading from the tank is split into the number of columns that vie wish to employ. (For the present discussion, it is assumed that five columns are required.) The inlet manifold consists of a l/r-inch “cross” a i t h extra fittings soldered in front and back to give a total of six “ports.” One port is connected to the gas line and the other five are connected to the valves. (We have found Hoke 1252 toggle valves ideally suited for this purpose.) Injectors. The tubing size leading from the valves is reduced to inch O.D. and connected to the injectors. The injectors can be simply constructed of a Swagelok 3 / 1 ~ X L/4 inch reducing union as shown in Figure 2. They are wrapped with Nichrome resistance wire and may be individually controlled via a Variac or joined either in series or parallel and controlled by a single Variac.
-
-SILICONE
L
I I B ’ s s TUBING SILVER SOLDERED I N HOLE DRILLED
REDUCINO
UNION
l N T o S I D E o F
Figure 3/1,j
X
l
l
SEPTUM
4
k
’
SWAGELOK NUT
3 1 6 ’ S S TUBINO FILLED WITH 5. WOOL
3/16
SWLGELOK N U T
2. Injector, constructed from ‘/4
inch Swagelok reducing
union VOL. 34,
NO. 4,
APRIL 1962
589
