US1997665A - Amplifier - Google Patents

Description

April 16, 1935. H. A. WHEELER AMPLIFIER Filed March 10, 1933 INVENTOR- 4 HAROLD A.WHEELER BY 7 m, 10m,

MMU M. ATTORN EYS .fication and claims taken in connection with Patented Apr. 16, 1935 r UNITED STATES AMPLIFIER Harold A. Wheeler, Great Neck, N. Y., assignmto Hazeltine Corporation Application March 10, 193:, Serial No. 880,208

14 Claims. (01479-171) This invention relates to amplifiers for use in sound reproduction, and more particularly to an amplifier of the push-pull type. I

The invention contemplates the provision of a pentode for supplying the input to a push-pull amplifier.

In the operation of the ordinary push-pull amplifier it is'necessary to provide speciallyconstructed transformers both for the input andoutput of the push-pull connected tubes. This adds considerably to the cost of a radio receiver or other amplifier system and increases the space required for the amplifier units.

It is the object of the present invention to overcome the above-noted objections and to provide an input coupling system for supplying the input of a push-pull vacuum tube amplifier system without the necessity of using a special or extra. transformer.

It is another object of this invention to give improved audio-frequency amplification.

These and further objects of this invention will become apparent from the following specithe accompanyingdrawings.

In accomplishing the objects of this invention, the signal to be amplified and supplied to the push-pull amplifier is impressed upon thethird or outer grid of a pentode type vacuum tube. The potentials upon the various electrodes are so arranged that the currents in the anode circuit and in the circuit of the'second or middle grid will vary inversely as the potential of the third grid is altered. The anode circuit is coupled to the input of one of the push-pull amphfier tubes and the circuit of the second grid is coupled to the input of the other tube. Thus, inversely-varying voltages are impressed upon the inputs of the two push-pull amplifier tubes as the voltage of the signal input upon the third grid of the pentode is varied.

In the accompanying drawing,

Fig. 1 is a circuit diagram showing a preferred embodiment of this invention;

Fig. 2 is a diagram showing the secondgrid-current/third-grid-voltage and anode-current/third-grid-voltage, characteristic curves of a pentode coupling tube.

In Fig. l the output of an audio-frequency amplifier tube I0 is coupled to the pentode'coupling tube l2 by means of the coupling condenser H to the signal input circuit connected between the input electrodes of the pentode. The signal input circuit, which as shown comprises gridleak resistor l3, has one terminal connected to the input or outer grid electrode and has the second terminal connected to the cathode through the biasing resistor H. The input circuit is therefore connected between the outer or third-grid electrode and cathode of the pentode. The resistor It provides a negative bias potential to the third grid relative to the cathode. The anode circuit of the pentode l2 includes an impedance, as for instance resistor l5, and at least a portion of a high potential source or battery IS. The middle grid or second-grid electrode circuit includes a second impedance, as for instance the resistor I1, and the entire. high po tential source or battery Hi. The impedances' I5 and I! in series with each output electrode are for producing a substantial output voltage variation corresponding to the variations of output currents. A bias upon the inner or first-grid electrode of the pentode I2 is provided by a portion of the battery l6. By this means the first grid is maintained at a fixed voltage relative to the second terminal of the grid-leak resistor l3 which comprises the input circuit. The vacuum tubes 22 and i9 are connected for push-pull amplification, with both their inputs and outputs connected in phase opposition. The variations in voltage across resistor 15, produced by fluctuations in the anode current through the resistor, are impressed upon the input of the pushpull amplifier tube 22 through the connection including coupling condenser l8, and the fluctuations in voltage across resistor I1, produced by variations in the second-grid current through the resistor, are impressed upon the input of the push-pull amplifier tube l9 through the connection including coupling condenser 20; A by-pass condenser 2| is provided connected between the battery ends of resistors I5 and II respectively. The inputs of the vacuum tubes 22 and 19 include grid- leak resistors 23 and 24 respectively.

Although various voltages for the several electrodes of the pentode may be found suitable for operatlonin accordance with the present invention, potentials suitable for the operation of a type 5'? pentode have been found to be as follows, each voltage being given with reference to round:

Suitable battery and bias voltages for producing In order to produceequal and opposite voltage changes upon the inputs of the push-pull tubes 22 and II, the proportions of the various resistors are quite critical. For the conditions given above, the following values have been found suitable:

Ohms Resistor l4 30,000 Resistor l5 200,000 Resistor i1 250,000

with the values of voltages and resistors given as above, the variation oi the third-grid voltage due to the signal impressed thereon results in inverse variations of the anode and second-grid currents, as shown more particularly in Fig. 2, which attention is now invited.

In this figure, the abscissas represent the signal voltage impressed upon the third grid, the voltages noted being relative to ground. In curve A the ordinates represent the corresponding anode current, and in curve B the ordinates give the corresponding values of second-grid current. The dotted curve C represents the total of the secondgrid and anode currents, which total, it is to be noted, varies but slightly as the third-grid voltage is varied.

In operation, as the voltage of the third grid of the pentode l2 changes postively, the anode current will increase, negatively changing the voltage impressed upon the gride of the tube 22. At the same time, the second-grid current will decrease, positively changlng the voltage impressed upon the grid of the amplifier tube l9. Since the second-grid voltage has a considerable effect on the total cathode current, and this voltage varies in operation, there is a tendency for the total cathode current to vary considerably, and thereby make it impossible to secure nearly equal inverse variations of current to the plate and the second grid. In the circuit shown, excessive variation of the total cathode current is prevented by the cooperation of the first grid and the cathode resistor ll, which latter is not bypassed at audio frequencies. The first and second grids may be made of uniformly fine mesh to aid in securing this result.

This cathode-current-regulating circuit operates as follows: The cathode resistance is given a relatively high value, developing a positive voltage on the cathode. The first grid is given a somewhat lower and unvarying pofltive voltage I60, which nearly counterbalances the effect of the cathode resistor on the average cathode current. This has the effect of a small negative bias on the first grid'relative to cathode. When the thirdgrid voltage varies in the positive direction, the second-grid voltage varies in the same direction to a greater degree, or in other words, the second grid voltage variation has the same instantaneous polarity as the signal voltage. This increases the total cathode current and increases the positive voltage on the cathode. In other words, the voltage pply and the cathode resistance are proportioned to produce cathode voltage fluctuations in the presence of voltage variations acres the input circuit. Since the first-grid voltage remains constant, its negative bias relative to cathode is effectively made greater, providing a corresponding grid-cathode voltage variation in a direction to at least; partly suppress the total cathode current variations, and thereby limiting to a small value the increase of total cathode current. There are thus provided means for maintaining the total output current nearly constant, its variations being substantially less than either output current variation, whereby the latter are made nearly equal.

In experiments using the above circuit constants, the total-current' variation has been reduced to 20 percent of the plate-current variation or 25 percent of the screen-current variation. In other words the second-grid-current variation has been made 80 percent of the platecurrent variation. This inequality is compensated by a reciprocal inequality of resistors I1 'and ii.

tioned that average plate or outer output electrode.

current should be very nearly half the average second-grid or inner output electrode current,'or

one-third the total cathode current. The average voltage, and particularly, minimized distortion in the amplified output corresponding to strong input signals.

In the same experiments, the inverse output voltage variations at the plate and at the second grid were each about five times the signal voltage applied to the thirdgrid. when the signal did not exceed 10 volts peak on the third grid, there was practically no distortion, and the output voltage varlations were equally amplified replicas of the signal voltage. The plate and second grid may be called respectively the outer and inner output electrodes as distinguished from the third grid which is the input electrode. The voltage variations at the plate and the second grid have respectively opposite and like instantaneous polarity relative to the signal voltage on the input electrode.

It is to be noted that no secondary emission effects are utilized in accomplishing the objects of the present invention. This is, of course, due to the fact that the voltage of the third grid remains at all times negative relative to the cathode, thereby preventing secondary emission from either second grid or plate.

It is obvious that a vacuum tube having a larger number of electrodes may be similarly connected by disregarding some of the electrodes, the general arrangement and values of voltages and resistances, however, being equivalent to those disclosed above.

' What is claimed is:

1. An input coupling system for a push-pull vacuum tube amplifier comprising in combination a vacuum tube having cathode, anode, and three grid electrodes, an input circuit connected between one'of said grid electrodes and said cathode, a resistor connected in the circuit of each of another of said grid electrodes and said anode, and means for impressing the changes in voltage across said resistors upon the input electrodes of the vacuum tubes comprising the push-pull amplifier.

2. A push-pull amplifier which comprises a vacuum tube having cathode. anode, and three grid electrodes, an input circuit, including a source of negativepotentialforoneofsaidgridelectrodu,

connected between said electrode and cathode, total output current nearly constant, whereby an impedance connected in the circuit of each of a second of said grid electrodes and said anode, two vacuum tubes connected for push-pull amphfication, and connections for impressing changes of voltage across said impedances upon the input electrodes of said push-pull connected vacuum tubes.

3. A push-pull amplifier which comprises in combination a vacuum tube having cathode, anode, and three grid electrodes, an input cir-' cuit, including a source of negative potential for one of said grid electrodes, connected between said electrode and cathode, circuits for a second of said grid electrodes and said anode, each circuit including a resistor and a source of high potential, said resistors being so proportioned that the said second grid electrode and anode currents will vary inversely as the voltage of the firstmentioned grid electrode is varied by the signal being amplified, and two vacuum tubes having their inputs connected across said resistors respectively and their outputs connected in opposition, whereby sa'id two tubes will act as a, push-pull amplifier of the signal impressed upon the input of said first-mentioned vacuumtube;

4. In an input coupling system for a pushpull vacuum tube amplifier, the combination of a coupling tube having a cathode, anode, and at least two grid electrodes, a signal input circuit connected between one of said grid electrodes and said cathode, output circuits for each of a second of said grid electrodes and said anode, each circuit including a resistor and a high-p0 tential source, the potential of said last-mentioned grid electrode being higher than that of the anode, whereby the currents to the anode and said last-mentioned grid electrode vary inversely as the voltage of said first-mentioned grid electrode is varied by the signal being amplifled, and connections from said resistors, to the inputs respectively of the. vacuum tubes comprising the push-pull amplifier, whereby said tubes are exited out of-phase as the signal is impressed upon' the input of said coupling tube.

5. In an amplifier circuit, the combination of a vacuum tube having two output electrodesami an input electrode located therebetween, said vacuum tube having the characteristic that a signal applied to its input electrode produces inverse variations of the output current to its output electrodes, and means for maintaining the total output current nearly constant, whereby said variations are made nearly equal.

6. In an amplifier circuit, the combination of a vacuum tube having two output electrodes and an input electrode located therebetween, said vacuum tube having the characteristic that a signal applied to its input electrode produces inverse variations of the output current to its output electrodes, and means for maintaining the total output current nearly constant, said means being proportioned to make the variation of the total output current substantially less than either individual output current variation.

'7. In an amplifier circuit, the combination of a vacuum tube having two output electrodes and an input electrode located therebetween, said vacuum tube having the characteristic that a signal applied to its input electrode produces inverse variations of the output current to its output electrodes, an impedance in series with each output electrode for producing a substantial output voltage variation corresponding to said current variation, and means for maintaining the.

said variations are made nearly equal.

8. In an amplifier circuit, the combination of a vacuum tube having two output electrodes and an input electrode located therebetween, said vacuum tube having the characteristic that a signal applied to its input electrode produces inverse variations of the output current to its output electrodes, an impedance in series with each cutput electrode for producinga substantial output voltage variation corresponding to said current variation, and means for maintaining the total output current nearly constant, said means being proportioned to make said variations nearly equal and greater than the signal voltage.

9. In an amplifier circuit, the combination of a vacuum tube having two output electrodes and an input electrode located therebetween, said vacuum tube having the characteristic that a signal applied to its input electrode produces inverse variations of the output current to its output electrodes, an impedance in series with each output electrode for producing a substantial output voltage variation corresponding to said current variation, and means for maintaining the total output current nearly constant, said means being proportioned tomake said output voltage variations equally amplified replicas of the signal voltage.

10. In an amplifier circuit, the combination of a vacuum tube having a cathode, an inner output electrode, an input electrode, and an outer output electrode, all situated in the order named, an outputimpedance and a voltage source connected in series with said inner output electrode, and a second impedance connected in series with said cathode, said impedances being proportioned to produce a voltage variation at the inner output electrode which is an amplified replica of a signal voltage applied to the input electrode, said variation having the same instantaneous polarity as the signal voltage. 11. In an amplifier circuit, the combination of a vacuum tube having a cathode, an inner output electrode, an input electrode, and an outer output electrode, all situated in the order named, an outwhereby disortion in the amplified output corresponding to strong input signals is minimized.

12. A vacuum-tube circuit comprising a vacuum tube having situated in the order named a cathode, a grid, an output electrode, an input electrode and another output electrode; an input circuit having one terminal connected to said input electrode and a second terminal connected through a resistance to said'cathode; and means for maintaining said grid at a fixed voltage relative to said second terminal, said means and said resistance being proportioned to produce cathode voltage fluctuations in the presence of voltage variations across said input circuit, and thereby to maintain the cathode current nearly constant.

13. A vacuum-tube circuit comprising a vacuum tube having situated in the order named a cathode, a grid, an output electrode, an input f 4 1 r electrode and another output electrode; an input circuit having one terminal connected to said input electrode and a second terminal connected through a resistance to said cathode: and means for maintaining said grid at a fixed positive voltage relative to said second terminal but less positive than said cathode, said means and said resistance being proportioned to produce cathode voltage fluctuations in the presence oi! voltage variations across said input circuit, and thereby to maintain the cathode current nearly constant.

14. A vacuum-tube circuit comprising a vaccamed by said inner output electrode voltage 1( variations.

HAROLD A. WHEELER.

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