US2120794A - Balanced amplifier circuit - Google Patents

Description

June 14, 1938. zlLLGER 2,120,794

BALANCED AMPLIFIER CIRCUIT 7 Filed July 26, 1953 2 Sheets-Sheet l llllt llll II" I INVENTbR /4/?/V0 Z/ZZGf/F BY fl/M 2W ATTORNEY June 14, 1938 A Z R BALANCED AMPLIFIER CIRCUIT Filed July 26, 1933 2 Sheets-Sheet 2 A? IIIIIIIIIIIII R R Y m5 m m m V T NM T A Z 0 Patented June 14, 1938 UNITED STATES PATENT OFFICE BALANCED AIVIPLIFIER CIRCUIT Arno Zillger, Narbcrth,

Television Corporation, Wilmington,

to National D'el., a

Pa, assignor 13 Claims.

This invention relates to amplifiers and particularly to those of the push-pull type which are intended to be used with extremely high frequenores.

One of the objects of the invention is to pro vide a means for balancing the input circuit for a push-pull amplifier so as to permit the distortionless passage of high frequencies.

Another object of the invention is to provide the same capacitance on each side of the input circuit of a push-pull amplifier.

Still another object of the invention is to provide a means to go from a single stage of ampli fication to push-pull amplification without introducing an unbalanced condition in the push-pull amplifier.

Another object of the invention is to provide a balanced output circuit for a thermionic amplifying tube.

Other objects of the invention and objects relating particularly to a method of connecting the various elements of the circuit will be apparent as the description thereof proceeds.

The invention has been illustrated in the accompanying drawings in which? Fig. 1 is a circuit diagram of an amplifier with battery plate supply, showing my improvedmeans for balancing the circuit.

Fig. 2 is a circuit diagram of a system in which the plate current is supplied by a power unit.

Fig. 3 is a diagram of a portion of a circuit using resistance coupled amplification in the push-pull amplifier; and

Fig. 4 is a circuit diagram showing the means for balancing applied to an arrangement employing plate rectification.

The use of push-pull amplifiers is usually recommended where high quality distortionless reproduction is required and excellent results are obtained by the usual type of push-pull amplifier when used with frequencies in the audible range. The use of extremely high frequencies in the neighborhood of 280,000 cycles, for instance, presents difficulties with such an amplifier which are not noticed particularly at the low audible frequencies. The input to a push-pull amplifier is ordinarily from a single tube, and as theplatecathode impedance of that tube is different from the impedance of the circuit between the grounded cathode and the low side of the amplifier input, there will be an unbalanced condition in the input of the amplifier which will cause attenuation of the higher frequencies, thus greatly interfering with the desirability of this circuit for this purpose. This defect I have overcome by placing a balancing resistance in the input circuit so as to equalize both ends of the input coupling with respect to ground.

In Fig. 1 is shown a receiver with the usual radio frequency transformer H], the primary H of which has one end connected to the aerial l2 and the other end to the ground l3. The secondary 14 of the transformer has one end connected through a grid leak and condenser I5 to the grid I6 of a tube l1, while the other end is connected to the cathode l8 of the tube. A variable condenser I9 is shunted across the secondary l4 in accordance with usual practice.

The plate 20 of the tube is connected to one end of the primary 2| of a push-pull transformer 22, while the cathode I8 is also connected to ground and to the negative side of a B battery 23 following the usual custom. Between the point of desired plate potential on the B battery, however, and the other end of the primary 2|, I provide a resistance 24, having such a value that together. with the internal resistance of the battery the total resistance equals the plate to cathode resistance of the tube. By this means the input circuit for the transformer 22 is balanced with the result that higher frequencies in the system are not interfered with.

The transformer 22 has the usual two part secondary 25, one end of which is connected to the grid 26 in a tube 21, while the other end of the secondary is connected to the grid 28 of 29. The cathodes 30 and 3| of these tubes are connected together and to the positive side of a C battery 32, the negative side of which is connected to the midpoint of the secondary 25 of the transformer. This puts a bias on the grids of the tubes 21 and 29. The plates 33 and 34 of the tubes 21 and 29 respectively are connected to the ends of the primary 35 of another transformer 36, the midpoint of this primary being given a positive potential on the B battery as indicated.

Several stages of amplification may be used as indicated by the dotted lines 31 and 38, the system terminating in a pairof tubes 39 and 40 both sides being connected to the grids 4| and 42 respectively. The plates 43 and 44 of these tubes are connected respectively to the ends of the primary 45 of the output transformer 46, the midpoint 41 thereof being connected to the B battery as indicated. The secondary 48 of the output transformer 46 is connected in series with the desired translating device, such as a neon lamp 49 which may be used for television reception. It will be understood that the transformers the tube 22, 3B, and 46, are especially designed to transmit cycles in 'the neighborhood of 200,000 per second.

In Fig. 2 a circuit is shown which is similar to that shown in Fig. 1 except that the plate current for the tube is furnished by a power supply .unit. In order to accomplish this a transformer having its primary 5| connected to the usual alternating current supply has a small secondary 52 the ends of which are connected to the oathode or filament 53 of the rectifier tube 54. The anodes 55 and 56 are connected to the ends of the main secondary 51 of the transformer 50 and the midpoint 58 of this secondary is connected by means of a wire 59 to the low side of a voltage dividing resistance 60. The midpoint of the secondary 52 is connected by means of a wire 5| to a choke coil 62. this coil is connected in turn by means of a wire 63 to a second choke coil 64 the other end of which is connected by means of a wire 65 to the high potential end of the voltage divider 60. Condensers 66, 61 and 68 are connected respectively.

between the wires 6! and 59, 63 and 59, and 65 and 59. This power unit described represents a standard form now in general use.

The balancing resistance 24 which is connected to the end of the primary 2| of the transformer 22 is then connected by means of a wire 59 to a point 10 on the voltage divider 50 which will give the proper potential, as for instance 180 volts. The value of the resistance 24 will then be such that when added to the resistance H of the Volt- 7 age divider between the point 70 and the negative end, the combined resistances will equal the plate-cathode resistance of the tube ll. If desired in this instance the grids 26 and 28 of the tubes 21 and 29, respectively, may be biased by means of a resistor 12 as indicated.

The portion H of the voltage divider is a fixed resistance and hence the resistor 24 may be given a fixed value and when once found need never be changed. However, in Fig. 1 where a battery is used the internal resistance of the battery may vary from 24 ohms when the battery is new to 2400 ohms after the battery has been in service for some time. Hence it may bedesirable where batteries are used to provide a variable resistance 24 so that this change of resistance in the battery may be compensated for. However, the resistance of the battery is so small as compared to the plate-cathode resistance of the tube that in many cases it need not be considered at all.

In one instance a number 56 tube was used in the input circuit which had a plate-cathode resistance of approximately 30,000 ohms. The resistance of the transformer primary was 200 ohms. Six 45 volt B batteries were used having an internal resistance of 4 ohms each, making the resistance of the battery 24 ohms. I-Ience the balancing resistance was given a value of 29,976 ohms which gave a complete balance to the input circuit and thus prevented distortion from the output of the amplifier.

While the invention has been described with a transformer coupled amplifier it will be evident that a resistance coupled amplifier may be improved in the same manner.

In Fig. 3 the tube I1 is shown with its plate 20 connected to a resistance 13 which forms the first resistance in a resistance coupled push-pull amplifier, condensers 14 and being used between the ends of the resistance 13 and the grids of the succeeding tubes (not shown). 7 A balancing resistance 76 is shown connected between the end of the resistance l3 and the potential supply The opposite end of on the battery '11. This resistance is given a Value in accordance with the description above which together with the internal resistance of the battery will equal the plate-cathode resistance of the tube II. This use of the invention with resistance coupled amplification is also of course adaptable to any source of plate potential, as for instance, the power supply unit described in connection with Fig. 2.

The foregoing description refers to grid leak and condenser rectification. However, the invention is also adaptable to plate rectification and Fig. 4 illustrates one manner for so adapting it. In this figure the tube 18 has its grid 19 connected directly to one end of the radio frequency transformer secondary 80. The other end of the secondary is connected to ground and a condenser 8| is shunted across the coil.

The cathode 82 is connected to ground through a resistance 83 usually in the neighborhood of 150,000 ohms. The plate or anode 84 of the tube may be connected, for instance, to one end of the primary 85 of the input push-pull transformer 86. The low end of this primary is then connected through the balancing resistance 81 to the B battery 88, the negative side of which is connected to ground, as shown.

The value of the resistance 81 will be greater than the resistances 24 or 16 because already in the circuit with the plate resistance of the tube is the large resistance 83. The resistance 81, therefore, plus the internal resistance of the bat-- tery should be made to equal these two resistances.

While reference has been made to grid-leak and plate rectification, it will be evident to those skilled in the art that the invention may be used with a push-pull amplifier receiving its input out of any other tube of a cascade amplifier.

Also while the cathode has been shown and described as being grounded it will be understood that a filamentary cathode with one side grounded or a filament provided with a center tap to ground may be used in the same manner, and it is intended that these constructions be considered as equivalent.

It will be evident from the above that I provided a means to balance the input of a push-pull amplifier so as to eliminate the attenuation of high frequencies where the amplifier is designed to transmit such. The invention is particularly 0 desirable in connection with television apparatus where high frequencies are usually necessary to obtain good detail in the image produced. While the invention has been described with certain specific types of amplifiers it will be evident that many changes may be made without departing from the spirit of the invention and I do not, therefore, desire to limit myself to what has been shown and described except as such limitations occur in the appended claims.

What I desire to claim is:

1. An input circuit fora push-pull amplifier comprising a thermionic tube having a cathode and an anode, a device operatively connected with the amplifier having a predetermined resistance connected across said anode and cathode, and an impedance between said device and said cathode and having such a value as to create substantially the same impedance in said circuit from said cathode to each end of said device.

2. An input circuit for an amplifier comprising a thermionic tube having a cathode and an anode, an electrical device connected between said cathode and anode and operatively connected to said amplifier, a ground connection on said cathode, and means inserted between said ground connection and the nearest end of said device for introducing an impedance in that side of the circuit effectively corresponding to the plate-cathode impedance of the tube.

3. An input circuit for an amplifier comprising a thermionic tube having a cathode and an anode, an element connected between said anode and cathode and operatively connected to said amplifier, a ground connection on said cathode, means to give said anode a high potential with respect to said cathode, and a resistance between said element and said cathode equal to the plate-cathode resistance of the tube less the resistance of said potential supplying means.

4. An input circuit for a push-pull amplifier comprising a thermionic tube having a cathode and an anode, an element operatively connected. to said amplifier having one end connected to said anode, means to connect the other end to the positive pole of a B battery, means to conneot the negative side of said B battery to ground and to said cathode, and a resistance substantially equal to the plate-cathode resistance of the tube between said element and said ground connection.

5. In a push-pull transformer coupled amplifier a transformer, a thermionic tube, a source of potential, means to connect one end of the primary of said transformer to the anode of said thermionic tube, means to connect the other end of said primary to said source of potential, means to connect the other end of said source of potential to ground, means to connect said cathode to ground, and a resistance having a value approximately equal to the plate-cathode resistance of the tube inserted between said transformer primary and said ground connection.

6. A transformer coupled push-pull amplifier comprising a transformer, a thermionic tube having an anode and a cathode, a source of high potential, means to connect one end of the primary to the anode in said. tube, a means to connect the other end of said primary to said high potential source, means to connect the other end of said source to ground, means to connect said cathode to ground, and a variable resistance inserted between said primary and said ground connection and having a maximum value equal to the plate-cathode resistance of said tube.

'7. In a resistance coupled push-pull amplifier a resistance, a thermionic tube, means to connect one end of said resistance to the anode of said tube, means to connect the other end of said resistance to a potential supply, means to connect the negative side of said potential supply to ground, means to connect the cathode of said tube to ground, and a balancing resistance in the circuit between said main resistance and said source of potential and having a value corresponding to the plate-cathode resistance of said tube.

8. An input circuit for a pushpull amplifier comprising a thermionic tube having a cathode and an anode, a device operatively connected with the amplifier having a predetermined resistance connected across said anode and cathode, means to ground a point on said circuit, and means to create substantially the same resistance in said circuit from said grounded point to each end of said device.

9. An input circuit for a push-pull amplifier comprising a thermionic tube having a cathode andan anode, a device constituting a load for said tube having one end connected to said anode, a source of potential having its negative side connected to said cathode, and a compensating device connected between the positive side of said source of potential and the other end of said load device and having an impedance such that with the impedance of said source it will substantially equal the cathode-anode impedance of said tube.

10. An input circuit for a push-pull amplifier comprising a thermionic tube having a cathode and an anode, a load device, a source of potential, a compensating device, and means to connect said load device, said source of potential, and said compensating device in series between said anode and cathode, said compensating device having an impedance such that the impedance from the cathode to each end of said load device is substantially equal.

11. An input circuit for a push-pull amplifier comprising a thermionic tube having a cathode and an anode, a load circuit including an energy transferring device connected between said cathode and anode, said load circuit being so arranged that there is substantially the same impedance from the electrical midpoint of said energy transferring device in either direction to said cathode.

12. In an amplifier a thermionic tube having an anode and a cathode, a load circuit connected between said anode and cathode and including an energy transferring device, and means to make the impedance of the continuous portion of said circuit between the electrical midpoint of the energy transferring device and the cathode substantially equal to the cathode-anode impedance of said tube.

13. An amplifier comprising a thermionic tube having a cathode and an anode, and a load circuit including an energy transferring device connected between said cathode and anode, the impedance of said load circuit from the electrical midpoint of said energy transferring device in either direction to said cathode being substantially the same.

ARNO ZILLGER.

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