US2345501A - High gain amplifier - Google Patents

Description

March 28, 1944. O w PINEO HIGH GAIN AMPLIFIER Filed May 2 n HIM INVENTOR u swk BY A I m ATTORNEY Patented Mar. 28, 1944 UNITED STATES PATENT oF lcE, 2,345,561 HIGH GAIN AMPLIFIER I Orrin Weston Pineo,.Milo, Maine. assignor to American Cyanamid Company, New York, N. Y; a corporation of Maine Application May 2, 1942, Serial No. 441,457

7 Claims. (Cl. 250-415) a My invention relates to high gain amplifiers and particularly to the amplifiers used in con- Junction with flickering beam spectrophotometers where the amplifier input signal is supplied by some converter of radiant energy into electric energy such as a photocell, bolometer, thermocouple, or the like.

One of the most serious problems in the high gain amplifiers used to amplify the A. C. component of photocell current in flickering beam spectrophotometers of the type described for example in my earlier Patent 2,107,836 is to eliminate spurious signals, because the proper signal is vanishingly small at balance of the photometer. In general, input signals of the order of 1 microvolt are large enough to drive the photometer nearer to balance, while smaller signals are lost below the level of unavoidable thermionic noise. Stray signals much larger than this amount can be caused by currents flowing through the capacities between the transformers and their cases, the motors and their frames, and the like. It is the object of my invention to eliminate, by means of suitable grounding connections, these stray signals which otherwise would cause the photometer to deviate appreciably from true balance. It is customary where it is desired to avoid the effects of capacities to shields and currents flowing in shields, to ground all points which are to be grounded by separate wires to a single point. This is common practice Where shielding must be extremely effective as for example, in radio signal generators where it is necessary to prevent radiation so that the only signal reaching the outside passes through the generator output wires. When, however, this standard practice is employed in the high gain amplifiers considered here, serious difficulties have arisen; amplifiers have been so constructed and have been operated, but their sensitivity, stability, and reliability have left much to be desired. Difficulties involved in these constructions used in the past have necessitated separate shielding of the photocell which makes it impossible to put the photocellv actually in the photometric integrating sphere. This results in a serious loss in sensitivitybecause the photocell outside the sphere does notreceive as large a portion of integrated light as is possible when it is actually placed inside the sphere.

According to the present invention the'stray capacities in the input circuit of the amplifier are shunted out by a low resistance conductor which carries 'a minimum of current and which is separatefrom' the ground wiresjcarrying much larger currents. from other portions of. the instrument. In other Words, the present invention adopts exactly the opposite procedure'from that which is normally. used where maximunishield-f ing isdesired. I M

The invention will be described more in detail in conjunction with the'drawinjg which shows a diagram. of a high gain amplifierwith photocell input and associated apparatus 7 used, in a; flicker-i ing beam spectrophotometer; ofithejtjgpejdd scribed my Patent morass, above referred to: v While the flickering. beamv spectrophotometers constitute the most important type of instrument in which amplifiers ofthe presentinvention are to, be used,.it should be understood that they are not limitedtothis use; I r In the drawing the frame of the sp'ectroph'o tometer itself is shown diagrammatically at "T, On this frame is mounted a 'balance motor2 and partof the frame forms anintegrating sphere'3 in which or 'immediately -below an openingein which is found the photocell shown at 4; The cathode of the photocell is connected'directly-to the control grid of "a p'entode 'vacuum tube- 5 forming the first stage of the high gain amplig fier; The plate "of *theamplifier is ied -from the source of positive directcurrent B-l-through' a high resistance l2'and :'the load resistance 13, While-the screen grid is fed through the same'resistance l2"and another resistance M. A- suitable filter condenser 20 leads from l-Z to the .low side of the D. CI supply voltage B."- A second con:- denser 21' by-passes alternatingcurrent fromth-e screen grid oftube 5 to the cathode 6-of the same tube to which the suppressor'grid is connected in the conventional" way; Operatingpotential-ior the photocell anode'is convenientlytakenirom thescreen' grid of the same tube. The rest of the amplifier isdlvided into stages marked S2, S3, S4 andSa; this portion of the-circuit being conventional. The grid of the'secon'd stage S2 is fed from the .plate of tube 5 through the small condenserlS; "The early stagesv of, the amplifier are shielded'by a shield'B to which the cathode 6 of tube 5 is connected throughconductor 9. The grid circuit of tube '5 has equivalent resistance R3 and capacitance Cg as shown, in dotted lines. i'-

. a; With the'photocell cathode directly coupled to the grid; as just" described, they assume a .fioating potential at which the electron stream from the photocell cathode-is equalled bya portion of the tube electronstream which is intercepted by the "grid; v.Other,plaths for the electrons, such as leakage pathsp-are made negligible by careful insulation. Thus a whiter sample gives more D. C. photocell current and the grid floats at a higher potential where it will attract more electrons from the tube stream. At the same time, the "space charge between grid and cathode becomes less and the consequent tendency for the tube {current to increase is offset by a lowering of the 1+R,.wC, "to give the input signal. Direct coupling or the cathode of the photocell to the control grid of the first stage tube is not new'in photocell fed amplifiers. In fact, it has been used in a commercial spectrophotometer which is widely used and which isfsold by the General Electric Company and described in my U. S. Patent 2,107,836 above referred to. In the General Electric amplifier, however, the screen grid potential is kept constant by means of a separate battery. This results in a wide variation in the resistive component Rg of the grid input impedance with variation of the D. C. photocell current. Since these variations in a practical spectrophotometer amount to about 1,000 to 1, at some point in the range the resistive component R; becomes equal to the capacitive component l/wC and at this point there is an abrupt change of phase in the first stage output signal amounting to 90. This causes serious difficulties in a spectrophotometer because the amplified signal and the field currents in the balance motor of flickering beam spectrophotometers should have a phase relationship of or 180.

.A more serious difficulty is encountered in the effect of the stray capacity between the photocell and the frame SJ This isshown on the drawing in dotted lines as an equivalent condenser 26 whose capacity is comparable with the total. input capacity-C of about micromicrofarads. Therefore an A. C. voltage at the signal freenemy of the amplifier, normally 60 cycles, which may appear :between the frame 3 and the first stage cathode 6 will cause a stray input signal. According to'the present invention this is avoided by directly connecting the cathode 9 to the frame of the integrating sphere 3 by a heavy conductor {which may, for example, be about 4 ft.- of No. l8 copper wire. The largest 60 cycle current allowed tofflow in'this wireshould be smaller than 40 microampere s, if the IR drop in the wire isto be less'llthan the one microvolt mentioned above; I

Theirarhel and the integrating sphere 3 are connected to ground through a conductor l9 and others. v It will be apparent, therefore, that instead of connecting cathode 8 to ground as directly as possible and to the same point as: all the other parts of the system which are to be grounded, its connection to ground is through conductor l9. This is important because the .A. C. currents flowing from the frame to ground,

for instance as a result of capacity 29 between currents may amount to several hundred microamperes.

By this arrangement, the only voltage across the stray capacity 26 is the voltage drop in conductor 1 due to currents which must pass through the high plate load resistance [3 and the higher internal plate resistance of the tube. The gain in the first stage is around times, and the ratio of plate resistance to load is at least 5, so that more than 100 times as much stray voltage can be tolerated between the first and second stage ground points, 5 and I5 respectively, than can be tolerated between the ends of conductor 1. Thus the first stage is effectively isolated from the rest of the amplifier and is tied to the photocell surroundings.

The B- lead is connected by H to the power amplifier chassis H at a point [6 between S: in the voltage amplifier and Si in the power amplifier. It is convenient to have this connection in the power amplifier so that the latter is properly grounded even when the voltage amplifier plug connector is opened. Also if the point I5 were closer to the end of the cathode resistor of stage 2, the current through the stray capacity 21 between the power transformer and its case might cause pick-up due to the IR drop in the B- lead between stages S2 and S3; and if 15 were closer to the rectifier, the signal currents in the lead between 55 and it might give enough voltage between it and I5 to cause oscillation. Besides that in lead 1 which is negligible. the other voltages appearing between B and I5 are the drops in leads ll, l8 andid. Lead ll carries the stray current through the capacity El, lead I 8 carries the stray current through capacity 28, and lead l9 carries the stray current through the capacity 29. In the amplifier of the present invention, leads ll and [dare reduced so that they have practically no resistance at all and lead i9 is of No. 16 wire and only about a foot long. It should be noted that lead I! connects to frame 3 at a point outside the path from the stray capacity 29 to the lead 19.

The present invention has been described in conjunction with an amplifier for a visible light using flickering beam spectrophotometer as this is of course the most important and widely used spectral region. The same difficulties are encountered, however, in spectrophotometer using the near ultraviolet or near infrared as in those using visible light and the present invention can be utilized therefor. Likewise it is adaptable with devices operating further into the infrared but in such cases other types of radiant energy converters such as bolometers or thermocouples are used in place of the photocell. Accordinglyit should be understood that the present invention is generally applicable to flickering beam spectrophotometers regardless of whether they are operating in the visible spectrum using a photocell as the transformer, or in other parts of the spectrum using other types of transformers of radiant energy into electrical energy.

What I claim is: a

1. A high gain, multi-stage vacuum tube amplifier having a screen grid vacuum tube in the first stage fed by a transformer into electrical current of radiant energy of frequency higher than radio waves, comprising a transformer of said high frequency radiation into electric current surrounded'by a grounded electric frame, the output terminal of said transformer being connected directly to the control and screen grids of ascreen grid vacuum tube of the first stage of the amplifier, the connection being in a direction such that increase in steady input of radiant energy to the transformer will result in impressing a voltage differential between the control grid and screen grid such that the latter becomes relatively more positive with respect to the control grid, means for directly grounding the cathode of the said vacuum tube to the metallic frame surrounding the energy transformer through a low resistancy conductor, a source of high D. C. potential feeding the anode of the screen vacuum tube and a high resistance connecting said source of D. 0. potential to the screen grid of the vacuum tube, said resistance constituting the only conducting path between the screen grid of the vacuum tube of other portions of the amplifier.

2. A high gain multi-stage vacuum tube amplifier having a screen grid vacuum tube in the first stage fed from the output of a phototube, comprising a phototube surrounded by a grounded metallic frame, the anode being directly connected to the screen grid of the screen grid vacuum tube of the first stage of the said amplifier, the cathode of the said tube being directly connected to the control grid of said vacuum tube and having no other conductive connection, a source of high D. C. potential feeding the anode of the vacuum tube and a high resistor connecting said D. C. potential source to the screen grid of the vacuum tube and constituting the only conductive path from said grid to any element of the amplifier, the cathode of the vacuum tube being directly connected to the metallic frame through a low resistance conductor.

3. A high gain multi-stage vacuum tube amplifier having a screen grid vacuum tube in the first stage fed by a transformer into electrical current of radiant energy of frequency higher than radio waves, comprising a transformer of said high frequency radio in electric current surrounded bya grounded electric frame, the out put terminals of said transformer being connected directly to the control and screen grids of the screen grid vacuum tube of the first stage of the amplifier, the connection being in a direction such that increase in steady input of radiant energy to the transformer will result in impressing a voltage differential between the control grid and screen grid, causing the latter to become relatively more positive with respect to the control grid, means for directly grounding the cathode of the said vacuum tube to the metallic frame surrounding the energy transformer through a low resistance conductor, a source of high D. C. potential feeding the anode of the screen vacuum tube and a high resistance connecting said source of D. C. potential to the screen grid of the vacuum tube, said resistance constituting the only conducting path from the screen grid of the vacuum tube of other portions of the amplifier, the cathode of the vacuum tube being ungrounded except through said low resistance conductor to the metallic frame.

4. A high gain multi-stage vacuum tube amplifier fed from the output of a phototube, comprising a phototube surrounded by a grounded metallic frame, the anode being directly connected to the screen grid of the screen rid vacuum tube of the first stage of the said amplifier, the cathode of the said tube being directly connected to the control grid of said vacuum tube and having no other conductive connection, a source of high D. C. potential feeding the anode of the vacuum tube and a high resistor connecting said D. C. potential source to the screen grid of the vacuum tube and constituting the only conductive path from said grid to any element of the amplifier, the cathode of the vacuum tube being directly connected to the metallic frame through a low resistance conductor, but otherwise ungrounded.

5. An amplifier according to claim 1 in which the screen grid of the vacuum tube is connected to the cathode through a condenser presenting an impedance path at audio-frequency which is low in comparison to the resistance connecting the screen grid of the vacuum tube to the source of D. C. potential.

6. An amplifier according to claim 3 in which the screen grid of the vacuum tube is connected to the cathode through a condenser presenting an impedance path at audio-frequency which is low in comparison to the resistance con necting the screen grid of the vacuum tube to the source of D. C. potential, the early stages of the amplifier being provided with a shield insulated from the shield of the latter stages, this stage shield being connected through a low resistance conductor to the cathode of the screen grid vacuum tube, but otherwise ungrounded.

7. An amplifier according to claim 4 in which the screen grid of the vacuum tube is connected to the cathode through a conductor presenting an impedance path at audio-frequency which is low in comparison to the resistance connecting the screen grid of the vacuum tube to the source of D. C. potential, the early stages of the amplifier being provided with a shield insulated from the shield of the latter stages, this stage shield being connected through a low resistance conductor to the cathode of the screen grid vacuum tube, but otherwise ungrounded.

ORRIN WESTON PINEO.

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