US2637786A - Bridge amplifier circuit - Google Patents

Description

y 5, 1953 R. w; BORDEWIECK 2,637,786

BRIDGE AMPLIFIER CIRCUIT Filed June 22, 1950 IN VEN TOR;

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Patented May 1 953 BRIDGE AMPLIFIERUI RCUIT Robert W. Bordcwieck, Southboro, Mass., assignorto MoorelElectronicLaboratories lncorporated,. Worcester, Mass a; corporation of; Massach'ursetts Application June 22, 1950,,ScrialNor-1fi9fl2di 2 Claims. 1

This invention relates to improvements in a bridge amplifier circuit, wherein a Very low-impedance balanced-output is provided fora very high gain bridgeamplifien- This makes possible the use of a simple bridge amplifier with the advantages of high stability and constant gain as wellas absence of diflicultie's caused bysh'iftin tube characteristics, vibration, thermal effects, etc.

These objects are attained by methodsdescribed in the accompanying drawing in which the figure shows a schematic electric diagram of one form of the invention.

Referring to the figure, numeral is a source of filament supply voltage and high voltage D. C. for the bridge amplifier and low impedance balanced output circuit. Numerals I2 and I4 are terminals connected to the heaters X, X of the cathode follower tubes 10 and 80. Numerals l6 and I8 are terminals connected to the heaters Y, Y of the bridge amplifier tubes 40 and 50. High voltage terminal 22 is connected by lines 24 and 25 to plates 16 and 86 of cathode follower tubes 10 and 80 respectively, and through ballast resistor 2B and line 28 to voltage regulator tube 34 and thence through lines 36, 38, and 90 to a second voltage regulator tube 92, and thence to the low voltage return by line 98.

The source of regulated voltage at point 29 is connected by line 30 to the plate load resistors 41 and 51 respectively of bridge amplifier tubes 40 and 50 respectively; and the source of regulated voltage at point 29 is connected by line 30 through screen dropping resistors 45 and 55 to the screens 46 and 56 of bridge amplifier tube 49 and 50 respectively. Suppressors 48 and 58 are connected to the cathodes 42 and 52 respectively and thence by common line 4| to cathode resistor 60 and to a balancing network consisting of potentiometer GI and resistors 62 and 53 connected in parallel with potentiometer 6| and thence through lines 94, 96, and 98 to the low voltage return point 20.

The grid 54 of bridge tube 50 is connected by grid resistor 64 to the junction point between resistors 62 and 63. The grid 44 of tube 40 is connected through grid resistor W0 and line I03 to the'slider I02 of potentiometer 6|. The high side of the input to the amplifier is directly connected from terminal I04 to grid 44 and the low side of the input connects from terminal I00 to the slider I02 which may be used as a ground reference by strapping together points I08A and I09 by connecting link I01, or these may be left floating by removal of link I01.

The source of regulated voltage at point 29 is also connected through a bleeder biasing resistor 2 32 .to: the-iunctionpoint ofresistors 0 l.., and B2,.toprovide a-rsmallamount of fixed bias used in; conjunction with the self-bias developed across resistor 6.0 and the biasing network; consisting of resistors iii, 62 and 63.- Plate 49.is connectedby line ,49A to-. grid84 of cathode :follower tube 80. Plate 59isconnected by line 59A to grid 14 of cathode, follower-tube .10.. CathodelZ of cathode follower'tube. 10 .is connected to point 10A which will hereinafter be called output terminal A, and. also through cathode load resistor "II and line 38 to the voltage regulator tube 92, and thence to the B minus return through line 98. Cathode 82 is connected to terminal A which will hereinafter be called output terminal B and through cathode resistor 8| to voltage regulator tube 92.

An electro-mechanical device I I0 is shown, this device representing any of the various types of indicators, recorders, relays, solenoids, etc, which can be used to advantage in the output of the circuit of the figure. Since it is the basic circuit which is here presented and not a specific application, it is not desired to be limited to the use of a relay, recorder, or indicator, but it is to be understood that the present circuit can be used to advantage with any electrical ouput device.

The operation of this circuit is as follows:

With no signal impressed on input I04, the slider I02 is moved up or down to vary the initial bias on the grid 44 of tube 40 either above or below that of grid 54, so as to effect a perfect balance between the voltages at the plates 49 and 59 of the bridge amplifier tubes, or if it is desired to initially offset the balance at the plates of these tubes to a predetermined amount of either polarity, this may be done. The voltages at the plates 49 and 59 are directly coupled to the grids 84 and 14 respectively of cathode follower tubes 80 and 10 which in themselves would provide an appreciably reduced impedance. That is, several megohms could be used for plate load resistors 41 and 51 and cathode resistors 7i and BI could be made in the order of 10,000 ohms. However, by the novel addition of a voltage regulator tube between the cathode follower resistors 8| and H and the B minus return, it is possible to use resistors on the order of several hundred ohms in the cathodes of the cathode follower tubes. By this development it is possible to produce an extremely low output impedance from the output cathode follower stage and still maintain all of the advantages that a balanced bridge amplifier circuit can possess plus additional advantages arising from the much higher plate load resistors which can be used if the bridge amplifier tube does not have to drive the load directly.

Having thus described my invention and the advantages thereof, I do not wish to be limited to the details herein disclosed otherwise than as set forth in the claims, but What I claim is:

1. A bridge amplifier comprising a source of electrical energy, a pair of amplifier tubes each having at least a plate, a cathode, and a control grid, a pair of cathode follower output tubes each having at least a plate, a cathode, and a control grid, and a voltage regulator tube, the said amplifier tube plates being connected through separate plate resistors to the positive terminal of said source of electrical energy, the said amplifier tube cathodes being connected together through a common self-biasing network to the negative terminal of said source of electrical energy, the cathode follower tube plates being connected to the positive terminal of said source of electrical energy, the said cathode follower tube control grids being connected separately to the plates of the said amplifier tubes, the cathode follower tube cathodes being connected through separate cathode resistors to the anode of the said voltage regulator tube, and the cathode of the said voltage regulator tube being connected to the negative terminal of the said source of electrical energy, the amplifier output being taken between the cathodes of the said cathode follower tubes, the said voltage regulator tube and the said cathode resistors being effective in combination to provide the amplifier with very low impedance output.

2. The circuit of the preceding claim wherein the said self-biasing network comprises two resistance branches, the first said branch being tapped and the tap connected to the control grid of one of said amplifier tubes, and the second said branch comprising a potentiometer having its slider connected through the amplifier input circuit to the control grid of the other said amplifier tube, said network comprising means whereby the bias of one of said control grids may be adjusted selectively above, equal to, or below the bias of the other said control grid.

ROBERT W. BORDEWIECK.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,286,894 Browne et a1 June 16, 1942 2,490,167 Storm Dec. 6, 1949 OTHER REFERENCES Text Book-Vacuum Tube Amplifiers-Valley, Jr., and Wallman-Radiation Laboratory Series-1948 edition-McGraw-Hill Book Co., publisher, page 476, p. 487.

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