US1759631A - Means for amplifying electrical currents of audible frequencies - Google Patents

Description

May 20, 1930. w. VAN B. ROBERTS 1,759,631

MEANS FOR AMPLIFYING ELECTRICAL CURRENTS OF AUDIBLE FREQUENCIES Filed Feb. 2a, 19 6 l l l l l l l INVENTOR WALTER VAN B. ROBERTS ORNEY Patented May 20, 1930 UNITED STATES PATENT ounce WALTER VAN B. ROBERTS, OF BRONXVILLE, NEW YORK, .ASSIGNOIE. TC RADIO CORPORA- TION OF AMERICA, A CORPORATION OF DELAWARE MEANS FOR AMPLIFYING ELECTRICAL CURRENTS OF AUDIBLE FREQUENCIES Application filed February 26, 1926. Serial No. 90,771.

It is well known in the art that in an audio frequency amplifier a gain in quality is secured if the grids of the amplifying tubes are kept at a potential more negative than that of the negative end of the filament. The present invention relates to a novel means for obtaining this negative grid potential. Other purposes and objects may be observed in connection with the annexed specification, claims and drawings in which Fig. 1 shows a two stage amplifier, and

Fig. 2 shows a multi-stage amplifier.

A condenser (1) of lar e capacity (I have found that one microfara is a suitable value) is connected between the common lead (3) to the negative ends of the filaments and the common grid return wire (4) from the secondaries of the transformers. With this arrangement a negative bias of the grids is produced b the signals themselves.

The theory of the operation of this device is as follows -The audio frequency currents to be amplified are caused to flow through the primary winding 5 of the first transformer. The successive positive halves of the alternations of potential of the grid end of the secondary winding 6 of the last transformer draw electrons to the grid 9 until condenser 1 is so charged that both transformer secondaries 2 and 6 and both grids 8 and 9 are at a negative potential with respect to the negative filament lead 3 and this negative potential is approximately e ual to the peak voltage of the secondary 6 of t e last transformer.

If no leakage of charge occurs the grids remain at this negative potential unless stronger signals come along and still further increase the negative potential. In actual practice a permanent negative bias independent of subsequent signals is not secured because no insulation is perfect and even in the best evacuated tubes there are always some positive ions which are attracted to the grids and gradually neutralize the charge on the condenser. Hence, the negative grid potential tends more or less slowly to diminish toward equality with the peak volta e applied. The important fact however is that the negative grid potential never falls appreciably below the peak applied voltage at an time. The action of a grid bias battery di ers from the self biasing action described above in that the peak applied voltage may rise far above the voltage of a biasing battery and remain so for any lengthof time.

It is sometimes desirable to provide a discharge path for the condenser supplementary to that afforded by insulation imperfection and positive ions in the tube, so that if a strong crash of static or other voltage of much greater than normal intensity causes the potential of the grids to assume too great a negative value, the excess charge will leak off in a reasonable length of time. I have found that a resistance of one or two million ohms or more is suitable for this purpose and that it may be connected between grid and negative filament terminal, or in parallel with the condenser as shown in Fig. 1 where the connections to the resistance 10 are shown in dotted lines.

I have explained the invention as applied to a two stage audio frequency amplifier where it serves to maintain equality of bias on the two grids. The scope of the invention is not limited-to this case however, as it is equally applicable to any amplifier of more or less than two stages. Furthermore, a multistage amplifier may be considered to be a cascaded combination of amplifiers each of fewer stages, and each of these may have a self biasin device. The biasing potentials developed by such successive devices will be increasingly great because the signal is increasingly strong in successive sections of the amplifier.

The above described method of self biasing may be advanta eously combined with the battery method 0 grid biasing, by properly inserting a grid battery in series with the self biasing device, so that the grid potential never falls appreciably below the peak value of the impressed voltage nor below the battery voltage.

Fig. 2 shows a multistage amplifier emresistances 10 and 12 that 'may be chosen to have any suitable value.

A convenient arrangement in practice, especially when the grid potential of all the tubes in the amplifier is derived from a single device, is to provide binding posts 13 and 14 between which the condenser can be connected as shown in Fig. 1. These same posts are then available for connecting to a bias battery when desired. Or, between them a bias battery and condenser provided with a suitable high resistance discharge path may be connected in series.

Having thus described my invention, I do 15 not wish to be limited by the exact construction shown but only to the extent of the following claims: 7

1. In combination, a series of amplifiers, each having anode, cathode and control elecg0 trodes, input and output circuits, the cathodes being inductively connected, the control'electrodes being connected to their respective input circuits, the anodes being connected to their respective output circuits, and another connection from the input circuits to, the cathodes including a bias condenser and a bias battery for maintaining the said control electrodes at a biasing potential which is substantially equal to the peak value of the signal ac voltage applied to the control electrodes, and

above the battery potential.

2. In combination in an audio amplifier having anode, cathode and control electrodes, a series connection between the control electrode and the cathode including a bias condenser and a source of constant bias potential; for maintaining the bias potential of the control electrode at least as great as the peak value of signal voltage applied to said control electrode at all times, and above the potential of said source of constant bias potential.

WALTER VAN B. ROBERTS.

Images