US2179265A - Direct current amplifier - Google Patents

Description

Novf 7, 1939. D. G c. LUCK DIRECT CURRENT AMPLIFIER Filed Aug. 14, 1937 2 Sheets-Sheet l Nov. 7, 1939. D. G c. LUCK DIRECT CURRENT AMPLIFIER Filed Aug. 14, 1957 2 Sheets-Sheet 2 (Ittomeg Patented Nov. 7, 1939 UNITED STATES DIRECT CURRENT AMPLIFIER David G. C. Luck, Haddon Heights, N. l, assignor to Radio Corporation of America, a corporation of Delaware Application August 14, 1937, Serial No. 159,025

3 Claims.

The present invention relates to a direct current amplifier embodying a plurality of electric discharge amplifier tubes responsive to an applied direct current variation at the input circuit to provide a corresponding change in the output voltage or current.

It is an object of the present invention to pro vide an amplifier to which an alternating potential may be applied and controlled by a direct J Voltage of relatively low value whereby, in effect, amplification of the direct control voltage by an alternating current amplifier is permitted, thereby avoiding the instability common to direct current amplifiers.

It is also an object of the present invention to provide an electric discharge tube amplifier responsive to relatively small variations in a direct voltage applied thereto to control an alternating current in phase and magnitude, whereby the amplifier may be utilized for the control of the operation of alternating current motors and the like.

Other objects and advantages of the invention will be apparent from the following description when considered in connection with the accompanying drawings in which Figure 1 is a schematic circuit diagram of an amplifier embodying the invention, and

Figures 2 and 3 are similar schematic circuit diagrams of modifications of the circuit of Fig. 1.

Referring to Fig. 1, a detector stage 5, utilized as a source of controlling potential, is provided with input leads 6 and l connected respectively with the control grid 8 and the grounded cathode 9 of the tube 5. The output anode I ii is connected through a series output resistor l I with a positive anode potential supply lead l2. The anode is also connected to ground through a suitable filter capacitor 13. Signals other than direct current variations are taken from the anode through a series resistor 44 and output coupling capacitor it for any suitable utilization device (not shown).

The detector stage 5 is primarily utilized in controlling a main direct current amplifier included within the dash line rectangle l6 and comprising at the input end a pair of pentode electric discharge amplifier devices ll and it, each having a cathode I9, a control grid 20a or 20b, a screen grid 2 l, a suppressor grid 22 and an output anode 23.

The control grid 20a is connected to the control potential source and is biased to a degree controlled by the resistor 39. The control grid 20b is grounded. The output anodes are con nected in push-pull or balanced relation to each other and coupled to a pair of balanced or pushpull amplifier tubes 25 and 26.

On the input side, the control grid Ella of the tube H is connected through a series isolating resistor 2'! with a variable source of negative biasing potential comprising the movable contact 23 of a resistor element 253 connected across a battery or other suitable direct current potential source 3! interposed in the output circuit of the detector 5. An input terminal 38 is connected with the ground lead l, with the cathode 9 of the detector and with the cathodes it of the amplifier devices ll and it through a self bias resistor 39. The control grid 20b of the amplifier I8 is also connected to the ground lead l through a lead 40 and the control grid Ziia of the amplifier tube H is grounded through a by-pass capacitor M.

It will, therefore, be seen that both control grids of the balanced amplifiers ill and it may receive the same biasing potential, the tube 58 receiving the bias directly from ground, the tube ll receiving the bias through the input circuit 3836-3l-292l. The connection for the grid 20a of the amplifier tube ll is directly through conductive connection with the anode it of the stage 5 and is, therefore, subjected to the anode potential. This connection is provided to permit adjustment of the value of the unidirectional control across leads 3i33i3 to a value for which zero output is desired, This biasing arrangement permits the biasing potential on the two tubes to be equalized by adjusting the biasing potential on the tube ll with respect to the fixed biasing potential on the tube 18.

The suppressor grids 22 are connected to the cathodes Hi. The output anodes 23 are coupled to the tubes 25 and 26 by a balanced or push-pull coupling means admitting of relatively high gain. In the present example, this coupling means is of the impedance or resistance coupling type comprising anode coupling resistors connected in balanced relation to the output anodes 23 and having a common anode potential supply lead t3 which receives positive anode operating potential from a positive supply lead id through a series voltage reducing resistor 45. A suitable by-pass capacitor 46 is provided for the lead it? to ground 7. The grids of the tubes 25 and 26, indicated at 41 and 38, respectively, are coupled to the output resistors 42 through coupling capacitors 49, which are connected to grid resistors 5E3 having a common return lead 5! to ground and to the cathodes 52 through a self bias resistor 53. The tubes 25 receive anode potential from the anode supply 44 through a series resistor 54, a common supply lead 55 and the push-pull or balanced primary 56 of a coupling transformer 571 to which primary winding the anodes 58 are connected.

The transformer 51 is provided with a pushpull or balanced secondary winding 59 to which the grids ti of a pair of power amplifier tubes ti! and 62 are connected in balanced or push-pull relation. The tubes 6! and 52 are also provided with a balanced or push-pull output transformer $3 having a balanced primary winding 64 connected with the anodes 65 and with the positive potential supply lead 44 through a common lead 66. Each half of the primary winding is provided with a spark gap indicated at 61 for preventing excessive potential surges across the primary winding. The input secondary winding is connected at the center tap 68 directly with the cathodes 89 and the ground lead 1, the latter connection being indicated by the lead 10, this connection being made through capacitors 68a and 88b for the purpose of suppressing interference from a motor '33 which is controlled through the circuits previously described.

The secondary H of the output transformer 63 is provided with output leads [2 connected with the armature of the alternating current motor is which is provided with a field winding l4 connected through supply leads I5 and a phase controlling capacitor 16 and resistor 11 with an alternating current supply source indicated by the leads E8. The supply leads T8 are isolated from the motor leads 15 by an isolating transformer 79 which may step up or step down the alternating voltage as may be required for the motor winding 14, the proper phase being determined by the capacitor 16 and resistor 'l'l.

Alternating current energy from the secondary winding of the isolating transformer is applied to the motor element 13 to operate it in either direction. Thus, alternating current energy is applied to the balanced control stage of the amplifier comprising the tubes I1 and I8 on the screen grids 2i through a supply lead 8! which is connected to the alternating current transformer T9 to derive a suitable potential therefrom with respect to the cathode return or ground lead 1 which is extended to connect with one side of the alternating current secondary, as indicated at 3!. A predetermined potential is derived with respect to the terminal 8|, for the lead St, by a suitable tap 82 on a potentiometer resistor 83 connected across the secondary of the transformer 19, thereby applying the alternating potential existing between the tap 82 and the terminal 8! across a portion of the resistor 83, between the ground lead i or cathode return and the screen grids 2!. This potential is so chosen that when applied through the successive amplifier stages it results in a normal operating voltage at the output leads 12 for the full operation of the motor.

The alternating current is thus introduced or coupled electronically to the push-pull or balanced amplifier channel at the input or control stage, the input or control grids of which are normally balanced to provide zero output. The control grids 28 of the control stage are normally biased to a region of considerable curvature of the tube characteristics. The direct control voltage from the control potentiometer 28Z9 is applied to the one control grid. For one control voltage value corresponding to the fixed bias provided on the control grid 26b of the tube 18, the tubes are thus balanced and no output occurs.

If the control potential is now varied in a positive or negative direction, the potential on the control grid of the tube ll will increase or decrease with respect to the voltage on the control grid of the static tube l8, thereby causing the gain of the tube H to decrease with increased negative bias and to increase with decreased negative bias with respect to the gain of the static tube l8 and thereby causing the alternating potential to be amplified in the amplifier stage 25-26 and in the power output stage iii-62 and to be applied through the leads 72 to the motor element i3. Departure from the balanced condition of the control stage thus produces a control potential which is utilized to produce an output voltage corresponding to the direction of departure from balance as one half or the other of the balanced amplifier channel is caused to have the higher gain. With this arrangement, the alternating current motor 13-44 may be caused to operate in either direction with increasing power or speed as the contact 28 of the control potentiometer is adjusted in either direction from the condition of balance or as the control potential causes the condition of balance to be varied in either direction on the control amplifier ll.

Sudden or abrupt changes in potential producing a transient voltage wave of any appreciable magnitude are reduced by the filter 2l-4l. However, as a precaution against surges, the power amplifier is provided with the flash-over gap across each half of the output transformer primary as hereinbefore described.

The amplifier output stage must be capable of transmitting the required amount of power for operation of the motor. The preceding stage or stages represented by the voltage amplifier stage 2?; 2%; must be capable of providing sufficient gain for operating the power output stage and the input or control stage must be adapted to provide a push-pull or balanced output with a parallel connected input circuit for the control grids whereby the input amplifiers or control stage may be brought to a condition of static balance so that the control potential may be effective to control the operation of the motor as previously indicated. The control source driving the balanced input stage must, of course, provide a sufficiently wide range of control potential to increase the biasing potential on the control tube H to provide full gain through onehalf of the amplifier channel and to decrease the biasing potential suificiently to provide full gain through the opposite half of the amplifier channel whereby the motor is caused to operate with predetermined power rating and speed in either direction.

The system of Fig. 2 is similar to that of Fig. l in some respects, but diifers therefrom in that the alternating voltage is applied to a pair of control grids instead of the screen grids as in Fig. 1. Zero output of the control tubes H--l8 is established by means of a bias potential source 90 across which is connected a resistor 91 for biasing the control grid of the device l l and a resistor 92 for biasing the control grid of the device l8. With these connections, the potentiometer Qi is adjusted for operation. on the most curved part of the tube characteristic with the normal value of direct current input, and the potentiometer 92 is adjusted to produce zero alternating current output for normal direct current input. The

operation of this system is substantially the same as that of Fig. 1.

The system of Fig. 3 differs from that of Fig. l in that the alternating current is introduced into the plate circuit of the control tubes 93 and 94. This system, like those previously described operates to control the alternating current output in response to variation in the unidirectional input or control voltage.

From the foregoing description, it will be seen that through the medium of a balanced direct current control stage having the input grids connected in parallel and the output anodes connected in push-pull relation, a relatively low alternating current potential may be introduced electronically into the control stage on the screen grids of a pair of pentodes constituting such stage and that relatively small variations in direct controlling potential may be caused to provide effective control of the alternating current output of the amplifier channel, both in magnitude and in phase, thereby to provide an alternating current control potential adapted to operate alternating current apparatus such as motors and the like at controllable levels and in either direction.

I claim as my invention:

1. A direct current controlled alternating current amplifier including a pair of electron discharge devices provided with push-pull connected output circuits and with parallel connected pairs of input circuits, means for applying alternating potential to one of said pairs of input circuits, means for applying a unidirectional control potential to one of the other pair of said input circuits, and means for adjusting said unidirectional control potential to a predetermined value at which no alternating current is produced in said output circuits whereby variations of said control potential from said value varies the alternating current delivered from said output circuits.

2. In the operation of a direct current controlled alternating current amplifier including a pair of electron discharge devices provided with push-pull connected output circuits and with pairs of parallel connected input circuits, the method which comprises applying alternating potential to one of said input circuit pairs, and applying to one input circuit of said other pair a unidirectional control potential and adjusting said unidirectional control potential to a normal value at which no alternating current is produced in said output circuit, whereby variations of said control potential from said value in opposite directions reverses the phase of the alter nating current delivered from said output circuit.

3. The combination of a pair of electron discharge devices provided with push-pull connected output circuits and with a plurality of input grid circuits, means including an alternating current connection to two of said grid circuits in parallel for producing alternating current in said output circuits, means for applying bias potentials to another of said input circuits, and means for applying to one of said input circuits a unidirectional control potential whereby the alternating current delivered from said output circuits may be controlled as to phase and level.

DAVID G. C. LUCK.

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