US2212337A - Electron discharge device circuit - Google Patents

Description

Aug.. 20, 1940. s. T. BREWER 2,212,337

I ELECTRON DISCHARGE DEVICE CIRCUIT I Filed Jan. 27, 1959 ATTOR/V Patented Aug. 20, 1940 PATENT OFFICE ELECTRON DISCHARGE DEVICE CIRCUIT y Sherman T. Brewer, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 27, 1939, Serial No. 253,123

3 Claims.

This invention relates to electron discharge device circuits, and, more particularly, to circuits utilizing negative feedback.

It is known to use electron discharge devices orA tubes connected in balanced or push-pull arrangement to-obtain a circuit from whose output even order harmonics are substantially eliminated. If it is desired to control or vary the output of such an arrangement, it is necessary, if the even harmonic balance is to be maintained, that the gains of the tubes be equal to each other for all settings of the output control.` Ii two ganged-potentiometers are used as an output cond trol, the necessity of maintaining a balance places a very severe tracking requirement on the potentiometer. It would be preferable, therefore, for the output control to consist of a single variable impedance element which varies the gains of the tubes simultaneously.

An object of the invention is to vary the output of a balanced or push-pull circuit and yet maintain the harmonic balance of the output.

A feature of the invention comprises varying the output of a balanced or push-pull circuit by varying the amount of local degenerative feedback in said circuit.

Another feature of the invention comprises utilizing local degenerative feedback for each tube of a balanced or push-pull circuit and simultaneously varying the amount of degenerative feedback for each tube without affecting the balanced 'character'of the output of the circuit.

Afurther feature comprises regulating the gain of an ampliiier comprising an odd number of electron discharge devices in tandem, in which' impedances are included in the common paths of lthe cathode-control grid and cathode-anode circuits of the initial and the Final devices, by varying an impedance connected between the cathode ends of the cathode impedances.

In one embodiment in accordance with the invention, a pair oi electron discharge devices or tubes are connected in balanced or push-pull arrangement, the control grid-cathode and cathodeanode circuits or each device having an impedance, preferably, a resistance, therein .providing a predetermined biasing potential for the control grid and a preassigned degenerative feedback of signal. rThe output of the balanced cir cuit is controlled or regulated by simultaneously changing the gains of the tubes, the control being exercised through a variable impedance, preferably, a variable resistance, connected in shunt of the series-connected cathode resistances and located'in the alternating current path only of the output circuit of the balanced circuit. The variable resistance may be of the type in which a slide or contact is adjusted in position to vary the amount of resistance effectively connected or removed from the circuit with which it is associated; or, it may be of the type comprising a material having a non-linear current-resistance characteristic, Whose resistance varies with the amount of current flowing therethrough, for ex'- ample, silver sulphide.

In another embodiment, in accordance with the invention, a plurality of electron discharge devices or tubes are connected in tandem, the initial and the iinal ldevice each having impedance means common to their control grid-cathode and cathode-anode circuits, and having additional impedance means connected between the cathode tion which follows, taken in conjunction with l the appended drawing, wherein: Fig. 1 shows a single-stage balanced or pushpull circuit embodying the invention;

Fig. 2 shows a circuit arrangement embodying theV invention for regulating the gain of an amplifer in a transmission line; and

Fig. 3v shows a multistage amplifier having an odd number of stages embodying the gain control arrangement of the invention.

Fig. 1 shows a pair of electron discharge devices or tubes lil, I0 connected in balanced or pushpull arrangement in a wave translating circuit that may comprise a single stage of an amplifier, an oscillator or a detector. Each device comprises a cathode I2, I2', which may be of the indirectly heated type; an anode I3, I3; and an input control grid I4, I4. Each control grid is connected to a signal input terminal I5, and each anode is connected to an output terminal I6 and, through an output resistance Il, I'I, to the positive terminal of a suitable anode power supply. A resistor i8, I8 is connected in the cathodecontrol grid and cathode-anode circuits of each tube. A variable resistor I9 is connected in shunt of the resistors I8, I8 connected in series. The resistor I9 may be of the type including a slide or a contact, adjustment of which varies the amount of resistance effectively in the circuit, or it may be of the type in which the resistance varies with the amount of current flowing therethrough, i. e.,

it may be one, or of a material, having a nonlinear current-resistance characteristic.

The resistors I8, I8 provide a predetermined biasing potential for the control grids and, when signal is applied to the control grids, a local degenerative feedback from the output to the input of the tubes. The resistor I9 enables the gain of the push-pull circuit to be varied by variation in the amount of degenerative feedback, Without affecting the even harmonic balance obtainable with a push-pull arrangement of a pair of electron discharge devices.

With the arrangement described, if feedback in the tubes is set equal at one value of output b-y selecting appropriate values of resistance for the resistors i8, I8', feedback in the tubes Will track automatically at all o-ther values as the resistance of the element I9 is varied. It will be noted that there is no direct current potential across the resistor I9. This reduces contact noise inasmuch as with zero signal on the grids, changes in the resistance of element I9 produce no output. The direct current operating potentials of the tubes are constant and are independent of the setting of the feedback control. Although the latter may be adjusted so that the feedback of the signal has been reduced to zero, there will be feedback to suppress any unbalance in even order harmonics which might be generated.

Inasmuch as alternating current only ows through resistor I9, it is possible to use a resistor of a material having a non-linear currentresistance characteristic that could not be used advantageously if direct current were to ow through it, for example, silver sulphide. With this material, the gain of the push-pull stage may be caused to vary automatically as. the load thereon varies from a preassigned normal value.

The gain control arrangement described with reference to Fig. l may be applied to vary the gain in a negative feedback amplier containing an odd number of stages, for example, the threestage amplier shown in schematic in Fig. 3. The impedances Z4 and Z5 represent interstag'e networks, and the impedances Z1, Z2 and Z3 comprise the beta circuit of a negative feedback ampliier of the type described in H. S. Black Patent 2,102.6'71, issued December 21, 1937. 'Ihe networks Z1 and Z2 are designed to provide the desired phase shift and gain around the feedback loop, and Z3 is a variable lmpedance, preferably a variable resistance. for changing the gain of the amplifier. Although separate biasing resistors 20 and 29 are shown, the networks Z1l and Z2 could be designed to include the necessary resistance to provide the desired bias on the grid of the respective stage. Assuming equal bias on thefirst and the final stages of the amplier, there will be no direct current potential across Z3. Variation in Z3 will Vary the amount of feedback around the amplier and vary its gain, without disturbing the direct current operating conditions.

Fig. 2 shows how the invention may be applied to a circuit arrangement for regulating the gain of an amplifier in a transmission line transmitting, for example, carrier frequency currents; The line 40 has therein an amplifier 4|, shown schematically, Which may have three stages and be of the negative feedback type disclosed in H. S. Black Patent 2,102,671, vissued. December 2.1, shuntv impedance pad 42 is shown connected in the beta circuit 43 of the amplifier 4l. One element of the impedance pad comprises a resistance element 44 having a high negative temperature coeiiicient of resistance..

The resistance element is controlled by a heater element 45. An increase of current ow in the heater element decreases the resistance of element 14 for decreasing the amount of feedback through the beta circuit 43 to increase the gain of the ampliiier. A decrease in the current flow through the heater element produces an opposite action whereby the gain of the amplifier is decreased.

A regulator circuit 45 is provided for controlling the heater element 65 to maintain the output from the amplifier substantially constant.

The transmission line transmits not only the carrier currents but also a pilot current used for controlling the operation of the regulator circuit, the pilot current being affected in the same manner as the carrier frequency currents by changes in the amplier output. -The regulator circuit comprises a. line dl connected to the line lf3 on the output side of the amplifier ill; a lter d8 for passing the pilot frequency only; a control circuit il@ embodying the gain control arrangement of Fig. 1*; a detector circuit 58; an amplifier 5l; and a line 52 for connecting the output of the amplifier 5I to the heater element 45.

The control circuit 49 comprises an input transformer 53 the ends of Whose secondary winding are connected through condensers to the control grids of the electron discharge devices or tubes 54, 54 connected in push-pull arrangement. The anodes are connected to the ends of the primary winding of an output transformer 55. The grid-cathode and cathode-anode circuits of each tube contain a biasing and degenerative feedback resistor 56, 56', a variable resistance element 51 being connected in shunt of said cathode resistors connected in series. The element 5'! is of a material having a nonlinear current-resistance characteristic, and preferably of silver sulphide, a materialhaving a high negative temperature coeflicient of resistance.

For normal output of the line amplifier the control circuit is adjusted so that the current ow inthe heater element is of a value such that the amplifier has the desired normal gain. Should the gain of the line amplier vary for any reason, the change in its output will be reflected in the control circuit of the regulator through the Variation in the pilot frequency input thereto. If it is assumed that the output of the line amplifier decreases, the pilot frequency input to the control circuit decreases, the output of the control circuit is less, the current flowing in the heater element is less, and the latter has a decreased eiTect on the resistance element in the pad in the beta circuit of the amplifier, causing the resistance element to increase in resistance and the amount of feedback to decrease whereby the gain of the amplier is increased.l

prising a control grid, a cathode and an anode, an anode circuit for each device, means comprising an individual unbypassed impedance in each anode circuit to control the potential difference between each grid and its associated cathode, and means comprising a variable impedance connected in shunt of said impedances, said shunt impedance being of a material having a non-linear current-resistance character- 10 istie.

2. The combination as claimed in thepreceddevice, and a variable resistor shunted across said resistors, said variable resistance being of a material having a non-linear current-resistance characteristic.

' SHERMAN T. BREWER.

Images