US2273150A - Alternating current operated direct current amplifier circuits - Google Patents

Description

1942- v F. 'H. SHEPARD, JR 73,150

ALTERNATING CURRENT OPERATED DIRECT CURRENT AMPLIFIER CIRCUITS Original Filed June 29, 1938 2 Sheets-Sheet 1 a mourn/r ac/A/Pur I FRANCIS H.5H RDA '1 7K5? Amman.

represent the line conductors of the A. C.

Patented Feb. i7, 1942 ALTERNATING CURRENT OPERATED DI- RECT CURRENT AMPLHIER CIRCUITS Francis H. Shepard, Jn, Merchantville, N. 3.

Original application June 29, 1938, Serial No.

216,448. Divided and this application Decemher 7, 1940, Serial No. 369,060

7 Claims. (on. 179471) This application is a division of my U. S. application Serial No. 216,348, filed June 29,1938, Patent No. 2,224,199, dated December 10, 1940.

The invention relates generally to D. C. amplifier circuits in which the energy for the operation of the amplifier is derived from a source of alternating current of the usual low frequency, and more particularly to such amplifiers of the balanced type, and is an improvement over my invention described and claimed in co-pending application Serial No. 727,968, filed May 28, 1934, and issued as Patent No. 2,137,419 on November 22, 1938. In that patent I have disclosed various types of A. C. operated D. C. amplifiers some of which are adjustable to provide compensation for all supply voltage variations. In order to avoid the inconvenience of a separate and rather critical compensation adjustment, such as is necessary in the circuits of the above patent, I have conceived and tested several types of self-biased, balanced, A. C. operated D. C. amplifiers that tend to be inherently compensated for all supply voltage variations including plate, bias and heater or filament voltage variations. The amplifiers according to my present invention may be degenerative or regenerative, and any number of the circuits may be cascaded without the necessity of cascading the power supply voltages.

My invention will be better understood from the following description when considered in connection with the accompanying drawings and its scope will bepointed out in the appended claims.

In the drawings Fig. 1 shows a single'stage A. C. operated D. C. amplifier utilizing a pair of tubes in balanced relation. Figs. 2 to 8illustrate various modifications of the amplifier shown in Fig. 1. Fig. 9 shows a pair of coupled balanced amplifiers. Fig. 10 shows a modification of one of the balanced stages of Fig. 9, and Fig. 11 shows a balanced amplifier according to my invention utilizing a single, multi-grid tube.

Referring now specifically to Fig. 1, Li, L2 power supply, between which are connected in parallel relation a pair of tubes T, T of the triode type,

although it will be understood that multi-grid, multi-purpose, or other suitable tubes may be used in place thereof. The cathode K of each tube is directly connected for example to one side L2 of the A. C. power supply and the anodes are each connected to the other side Ll of the power supply through resistor-capacity networks put or signal potential to be measured is applied to the terminals 3, 4 which'are connected respectively to anode A of tube T through the resistor 5 and to the control grid G. A condenser 6 connected between the cathode side of the line L2 and the side of the resistance Swhich is connected to input terminal 3 forms with said resistance 5 a filter network which removes the A. C. component from the plate circuit and supplies biasing potential from the plate A to the control grid G of tube T and also opposes to a large extent the effect on the grid of the signal. voltage input. A .similar resistance-capacity network 5', 6 is connected as shown to provide bias for the control grid of tube '1". The output terminals 1 and 8 are connected to the respective low potential ends of the load resistors, l, l of tubes T and T. In the circuit just described the output of one tube is balanced against the output of the second, these tubes having preferably substantially similar characteristics so that changes due to variations in the supply voltage will tend to be cancelled in the common out- In considering the operation of this single stage amplifier it should be understood that both sides of an A. C. power supply line are at the l-2, l'-2' and the potentiometer P. The insame D. C. potential, that is, a D. C. voltmeter placed between the opposite sides of the A. C. line will read zero. The current due to the electron fiow from cathode K to anode A in tube T is in such direction that the IR drop in the plate load resistor i causes the plate to be at a negative potential with respect to the voltage of supply line Ll. The plate A of the tube T will go positive with respect to the cathode each time the upper side L! of the A.,C. line swings positive. During this time electron current flows from the cathode to the plate and, as explained above, the plate drops to a potential negative with respect to the upper side Llof the A. C. line. The condenser 2 in shunt to the plate load resistor l is of such value as to hold its charge without appreciable lossfrom one cycle to the next of the power supply. Since the plate is negative with respect to theline Ll, it will also be negative with respect to the opposite side of the line L2 andhence after theremoval of the A. C. component of the potential by means of the filter network comprising resistor 5 and condenser 6, this potential can be applied in series with the signal input or else used as bias for the grid of tube T. It will be seen that any change in input signal voltage results in a change of grid potential which in turn changes the plate 8S 8, vacuum part of the output voltage is'nsed for degenera tube. In Fig. 3 differences in current, plate potential and hence the potential of point 3 which is one terminal of the input circuit, and hence will react through the input circuit to tend to return the grid to its original grid to cathode potential. In other wordsthe action will be degenerative. Tube T operates in the same manner as tube T, and potentiometer P is adjusted to a point such that with no input signal the output terminals 1 and 8 will be at the same potential.- In this condition the amplifier is said to be balanced, and subsequent application of an input signal voltage will be indi-' cated or measured in the output across the terminals I and 8.

If the gain of tube T were infinite, the control grid would be returned to its initial potential. However, in practice the gain of tube T is finite .hence the grid will not be restored to quite its original potential. This means that the change in the potential on the plate is not quite as great as the input signal, hence the gain of this stage is not quite unity. In the tube T there is no input signal applied to its grid, hence the drop in resistor I and across condenser 2 in the plate circuit of said tube will be identical to that in tube T if. the D. C. input voltage to tube T is zero. An indicating meter connected between the output terminals I and 8 will read zero. A change in grid voltage of tube T changes its plate current only and hence results in a difference in the IR drops in the two plate load resistors and hence causes the output meter or indicator to read. In order to compensate for practical differences in the plate resistances 'of tubes T and T there is provided the potentiometer P. If the movable contact of P is so adjusted that the output meter will read zero when the input signal is zero, variations of the A. C. supply voltage will cause changes in the plate current, plate voltage and normal grid bias voltage of each tube to practically the same extent so that these variations will not appear in the D. C. output.

In the circuit above described no voltage amplification is obtainable, the circuit being useful tube voltmeter where it is desirable to place little or no load on the measured circuit. The output of this circuit is essentially linear for D. C. input voltages and for A. C. input voltages in phase with the power supply.

The circuit of Fig. 2 is practically the same as that'shown in Fig. 1 except that variable portions of the voltage developed across the respective load resistors l and l aife applied to th respective control grids G aiirig'jG' i. e. only tion. The voltage amplification of the single stage shown in Fig. 2 is roughly proportional to the ratio of the whole of the potentiometer to the upper part, within certain practical limits set by the gain of the tube itself. The circuit is balanced by adjusting the slider 9 on the load resistance l for tube T.

Figs. 3 and 4 are variations of the preceding circuits in which tube- T is self-biased as in Fig. 1 and the tube T'-is biased from the self-bias resistor of the tube T. The voltage gains obtainable in these circuits approach the mu of one the characteristics of tubes T and T are compensated for by adjusting the plate load resistor N! of the latter, and the bias on tube T is obtained from the D. C. plate potential of tube T, and because of this,

.changes in the potential between terminal 3 and terminal 4 result in signal being applied to the grid of tube T. This applied voltage is such that the potential of the' plate of tube T varies a .and T is obtained by means of the slider on potentiometer P and the entire signal is applied to the grid of tube T and no change in plate current of tube T will be occasioned by D. C. input signal.

Fig. 5 shows a modification of Fig. 2 in which the tubes are cross self-biased so that the circuit becomes regen'erative. Balance and regeneration are controlled by adjusting the sliders on plate resistors I and I. This method of biasing is such that a change in the plate potential of tube T causes a change of grid potential of tube T which in turn causes a change in plate current and hence plate potential of tube T which in turn reacts upon the grid of tube Tin the same direction as the original grid change potential of tube T. It will therefore be seen that this circuit is regenerative and that if the sliders on the plate resistors are set too near the plates the circuit will become unstable, that is, it will plop in one direction or the other. If, however, the sliders are set at a point just above this unstable point the gain of the circuit over a portion of this characteristic can theoretically be made infinite. In Figure 5 adjustment of the slider on the plate resistors results in a change in balance of the circuit as well as a change in regeneration, and because of this it is extremely diificult to make the appropriate adjustment. The circuit shown in Fig. 6 is designed to eliminate this difilculty.

In the circuit of Fig.6 in addition to the potentiometer P, I have provided a second potentiometer P2 connected between the anodes of tubes T and T. The filter R.C is connected between the mid-point of P2 and the cathode side of the line L2, and the grid of T is connected to a point between R and C. A variable tap H on P2 is connected through resistor R to the input terminal 3. If the circuit is properly balanced by the adjustment of the slider on potentiometer P, regeneration can be adjusted by means 'of slider II on potentiometer P2 without aflecting the balance of the circuit, thus greatly simplifying operation of the circuit. As the slider ll is adjusted to the lefttowards the plate of tube T the gain approaches from a large value to a value of slightly less than twice the input while as slider H is moved to the right toward the plate of tube T the gain approaches infinity tending to make the circuit unstable. The regeneration or degeneration of this circuit may be, controlled by adjusting slider ll without disturbing .the balance. Likewise the balance may be made by P without disturbing the regeneration. This circuit is easily adjustable and provides an excellent null indicator for Wheatstone bridges, etc.

In the circuit shown in Figure 7 operation is identical to that of Figure 4 with the exception that the grids are returned to an A. C. potential intermediate the A. C. potential of the two sides L'I and L2 of the A. C. power supply. This introduces an A. C. bias into the grid circuit so that the drop in the plate resistor may be. in-

creased to a value equal to the normal grid bias plus the peak value of the applied A. C. The

power transformer TR shown in the figure may be utilized for this purpose, or else an autotransformer or a divider. The provision of the A. C. bias enables the current through the plate resistors to increase and hence the normal D. C..

bias assumes a value in excess of th peak applied voltage-between points L2 and I2. This in turn enables the tubes T and T to operate at or near the center of their operating characteristic rather than relatively near cut-off as is the case in the circuits shown in Figs. 1 to 6.

- It will of course be understood that this principle may be applied equally as well to the foregoing circuits.

Fig. 8 shows a circuit in which the principles involved in the circuits .of Figs. 6 and 7 are combined, the former embodying regeneration, and

the latter the feature of operating the tube at or near the center of its characteristic.

In Fig. 9 I have shown an improved A. C. operated self-biased balanced bridge circuit cascaded with a second identical balanced bridge circuit. Each of said bridge circuits is essentially identical regarding operation and component parts to the circuit shown in Fig. 7. It will be noted that the A. C. and D. C. loads have been rearranged without changing the fundamental operation of the bridge circuit so that one side of the output can be connected to one side of the A. C. line. In each stage the plate of one tube (T in the first stage and T1 in the second) is held at ground D. C. potential while'the amplified T. C. output is measured by the assumed D. C. potential on the plate of the other tube (T in the first stage and T'z in the second) of the respective stages. Both grids of each stage are normally biased at ground D. C, potential. Hence, the output of one bridge type amplifier is capable of being used to drive the grid of a following stage providing a suitable A. C. filter such as R-C is used to transfer the D. C. from the plate of one stage to the grid of the next. The grids of the tubes have an A. C. bias applied to them to enable the potential drops in the plate resistors to be great enough to allow the tubes to operate at or near the centers of their operating characteristic curves.

Electrically the only difierence between Figs. 7 and 9 is that the plate load of tube T has been placed on the other side of transformer TR. The drop across this load resistor P3 still supplies bias for both tubes T and T. The D. C. load on tube T returns to the cathode in both cases. In Fig. 7 the load resistance transformer winding is included in the return path while irf Fig. 9 the return is direct. The grid of tube T receives its D. C. bias direct from the plate of tube T in both cases. In Fig, 7 however an additional filter 5-6 is necessary to supply the correct A. C. bias. It will be noted that in both cases the elements of tubes T and T' receive identical 'A. C. and D. C. voltages.

The circuit shown in Fig, 10 shows how. the circuit of Fig. 9 can be made regenerative. Balance adjustment is made by meansof P3 independently of the regeneration, and the regeneration adjustment is made independently of balance by adjusting P5.

Any of the above described circuits will measure A. C. of the same frequency as, and in phase with the power supply voltage. Thus these circuits provide excellent balance or null indicators for Wheatstone bridges where the bridge current can be supplied from D. C. or from the same A. C. source as the amplifier.

The circuits shown in Figs. '7, 8. 9 and 10 are normally operated on the straight line portions of their characteristics, hence A. C. input voltages of frequencies other than the supply voltage produce no variations in the D. C. output voltages or currents. If the frequency of either the input or supply voltage is varied until a slow beat is obtained the output of the bridge will beat slowly, the maximum reading being a direct measure of the amplitude of the input voltage. The supply voltage amplitude will have practically no effect on the accuracy of the reading. If an oscillator is used to supply the plate or supply voltage to the bridge there is a slight tendency for the oscillator frequency to look into step with that of the signal. This tendency is an advantage in that as soon as the oscillator frequency is brought near that of the signal the frequency of the oscillator will look into step with that of the signal. The proper phase relations between the signal and oscillator frequencies will automatically be held for reading. This greatly facilitates any measurements to be made and it has no apparent effect on the accuracy of the readings. In other words if the above mentioned power supply voltage is from a variable frequency oscillator this device will measure only D, C. or A. C. voltages of the same frequency as the oscillator. This means that the device can be certain changes or variations will result in contact potential. To eliminate this difliculty, I have devised the circuit shown in Fig. 11 which is el'ectrically equivalent to and operates in much the same manner as one of the balanced stages, T and T for example, of the amplifier circuit shown in Fig. 9. The circuit of Fig. 11 utilizes a pentagrid tube T3 which may be of the type commonly known as 6L7, although it will be understood by those skilled in the art that a simple pentode such as the 6J7 having in addition to the cathode K and the anode A the three grids Gfl,

G2 and G3 will be entirely suitable for the purpose. In the circuit utilizing the pentagrid tube T3 the grid G! acts as the grid of tube T in Fig. 9 while the plate A acts as the plate of tube T of Fig. 9. The grid G3 of the pentagrid tube acts as the signal grid of tube T shown in Fig. 9 and the grid G2 of the tube acts as. th plate of the tube T in- Fig, 9. The pentagrid tube shown is provided with the additional shield grid G4 connected to the grid G2 and with a suppressor grid G5 between the shield grid G4 and the plate A, although as stated above the latter two grids are not essential to the operation of the circuit. It will be noted that in the circuit just described the cathode K serves to supply space current to both the grid G2 and the plate. A of the pentagrid tube as shown or of the pentode if the grids G4 and G5 were omitted. Control is obtained by changing the ratio of currents between the grid G2 and the anode A. It will be understood that variations or changes in contact potential at the surface of the cathode affect the total cathode current but do not necessarily afiect the ratio of currents between the grid G2 and the plate A. Thus, since the output-is taken between said grld and plate, changes in the cathode current, when the circuitis balanced, do not cause any change in output. The circuit just described will be balanced not only for line voltage variations but also for contact potential variations.

While I have disclosed and described several without departing from the scope of my invention I as set forth in the appended claims.

What I claim is:

1. A system for the amplification of direct current voltages or alternating current voltages in phase with the power supply, comprising means for emitting electrons, a pair of control grids and a pair of electron collecting electrodes, a power supply source of alternating current having a pair of line conductors, a conductive connection from one of the line conductors to one of the collecting electrodes, a capacitive connection from the other line conductor to the electron emitting means, a load resistor connected between the second collecting electrode and said other line conductor, means for adjustably connecting the electron emitting means to said load resistor, an input circuit connected to said pair of control grids, and an output circuit connected to said collecting electrodes.

2. A system for the amplification of direct cur.- rent or alternating current voltages in phase with the power supply, comprising means for emitting electrons, a pair of control grids and a pair of electron collecting electrodes, a power supply source of alternating current including a transformer having a pair of line conductors connected thereto, a conductive connection from one of the line conductors to one of the collecting electrodes, a connection including a resistance from the other line conductor to the other collecting electrode, a connection from the electron emitting means to an intermediate point on said resistance, a source of input potentials connected to one of the control grids and to a mid-tap of the transformer secondary, and, an output circuit having terminals each connected to one of the electron collecting electrodes.

- systems or modifications for carrying my inven- 3. In an alternating current operated A. C. or

D. C. voltage amplifier, a pair of electron discharge tubes each having an anode, a cathode and a grid electrode, a source of alternating current including a transformer having a pair of line conductors connected thereto, a direct connection from one of said conductors to the anode of one of said tubes,v a connection including a resistance from the other line conductor to the anode of the other tube, means for adjustably connecting the cathodes to said resistance, a

source of input potentials connected to one of the grid electrodes and to a mid-tap of the transformer secondary, and an output circuit connected between the first mentioned line conductor and the anode of said other tube.

4. The invention defined in claim 3 wherein a second pair of tubes similarly connected as the first pair are coupled in cascade thereto by means of a resistance-capacity filter.

5. In an alternating current operated direct current amplifier, a pair of electron discharge tubes each having at least an anode, a cathode and a grid electrode, a source of alternating current including a transformer having a pair of line conductors connected thereto, a direct connection between one of the anodes and one of said line conductors, a capacitive connection from the other anode to the same line conductor, a connection between each of the cathodes and the other line conductor through a resistance-condenser combination, a source of input potentials connected tothe grid electrode of one of said tubes and the mid-tap of the transformer secondary, a resistance connected between the anodes of said tubes, a variable connection from said last mentioned resistance to the grid electrode of the other tube, and an output circuit" having terminals each connected to one of 'the anodes.

6. An A. C. operated D. C. amplifier system comprising an electron discharge tube comprising a cathode, an anode and at least three interposed grid electrodes, an input source of D. C. potential adapted to be connected to two of said grid electrodes, an output circuit connected between the remaining grid electrode and the anode, an A. C. power supply source connected between said remaining grid electrode .and the cathode,

and a load resistor connected between the cathode and the anode.

'7. An A. C. operated D. C. amplifier system comprising an electron discharge tube comprising a cathode, an anode and at least four interposed grid electrodes, an input source of D. C. potential adapted to be connected to the first and third grid electrodes, .an output circuit having a pair of terminals one connected 'to the second and fourth grid electrodes and the other connected to the anode, an A. C. power supply source connected between the latter grid electrodes and the cathode, and a load resistor connected be-,

tween the cathode and the anode.

' FRANCIS H. SHEPARD, JR.

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