US1943986A - Resistance coupled amplifier - Google Patents

Description

Jan. 16, 1934. A. NYMAN 1,943,986

RESISTANCE COUPLED AMPLIFIER Filed Rab. 12, 1951 2 Sheets-Sheet 1 IN VENT 0R3.

A TTORNEY Jan. 16 1934.

Filed Feb. 12, 1931 2' Sheets-Sheet 2 I INVENTOR.

r filexamler IV 0700 E I N i a ATTORNEY .Patented Jan. 16,1934

UN 'rso STATES RESISTANCE COUITLED AMPLIFIER Alexander Nyman, Dobbs Ferry, N. Y., assignor to Radio Patents Corporation, New York, N. Y., a corporation of New York Application February 12, 1931. Serial No. 515,154

11 Claims. (Cl. 179-171) This invention relates to an improvement in resistance coupled amplifiers oi the type wherein a number of thermionic tubes are connected in cascade by means of resistors connected in the cat'liiode circuit, in preference to the anode circui The particular improvement refers to circuits using screen grid tubes for amplifying purposes and utilizing thereby their high amplification constant and a power tube, or a tube with large power output for the last stage of amplification, which can then be directly utilized for supplying an acoustical device or any other device that may be operated by said amplifier.

With amplifiers of high amplification constant, one of the chief sources of difllculty is the presence of unfiltered pulsations of potential in the power supply. By proper connection, I have been able to overcome the efiects of such pulsatlOIlS;

An object of my invention is to provide means for minimizing the effects of pulsations of sup- .ply current. 1

A further object of my invention is to provide circuit means for operating high amplification screen grid tubes in resistance type amplifiersf Another object of my invention is to provide novel circuit means or combining amplifier valves having high amplification constant'with amplifier valves having high power output in a unitary eflicient system.

Still a further object of my invention is to provide means for by-passing sections of coupling resistors to secure the most effective operation of the system.

I have illustrated my invention by reference to drawings, of which Figure 1 represents an embodiment of one form of connection in which one high amplification valve and one power valve are utilized.

Figure 2 shows'a connection utilizing two high amplification screen grid tubes and one power tube.

Figure 3 shows a connection utilizing two high amplification screen grid tubes and one power tube connected to an input circuit of an antenna and radio frequency filter, and a loud speaker output.

Referring to Figure 1, I have shown two tubes of which 1 is a screen grid tube having a high amplification. constant, and 2 is apower tube or a tube with relatively large plate current suitable for supplying su'fllcient energy for operation of a loud speaker or some other device.

I have illustrated at point 3 a positive terminal which may be connected to a suitable source of direct current potential, and at 4 the corresponding negative terminal. This source may be rectified alternating current from source P, rectified by rectifier R and filtered to a certain degree by filter F, but retaining, nevertheless a certain amount of pulsation which the connections of my system are intended to eliminate. In this way, the degree of filtration can be reduced with a consequent saving in expense of the filter.

The plate 5 of tube 1 is connected directly to the positive supply circuit, 'while the filament 6 is connected through resistors R2 and R1 to the negative supply lead. I have shown a tap connection 7 on resistor R2, which, together with the connection to grid 8, forms the input circuit for impulses to be amplified, and will apply the "necessary small bias to the control grid 8 of tube 1.

I have also shown a potentiometer R4 with a tap connection G to a screen grid 9, which pro- 'vides the necessary. potential for securing the maximum amplification from tube 1, and a tap connection H through a small condenser C2 to the filament 6. The object of this connection through condenser C2 is to counterbalance the effect of hum pulsations that may be introduced into the tube circuit by the grid biasing connection G.

Moreover, I supply a condenser C1 which is a by-pass on resistor 2, and a connection at point 10 between R2 and R1 to the grid 11 of tube 2. Thus, the resistor R1 forms a coupling element. The filament 12' of tube 2 is connected through a resistor R3 to the negative main, and the resistor R3 is chosen of such a value that the grid 11 of the .tube 2 is normally at a potential below the potential of filament 12, and of such a value as is necessary for the grid bias to secure the proper operation of the power tube.

Thus, in a particular example, if the power tube operating voltage is 250 volts with a grid bias of --50, and for the best operating condition,

the tube 1' requires 110 volts, then the resistor R1 should also have about 110 volts to secure the proper resistance coupling eifect. Therefore, the potential of the grid 11 would be volts above the negative supply lead, and the resistor R3 must have a drop of 150 volts to secure a grid potential of tube 2 of 50 volts below the filament. The supply voltage in this case should be equal to 400 volts. The resistor R2 must be designed to consume. the excess voltage of 200 volts.

It is quite evident that without the by-pass condenser C1, the changes in plate current of tube 1 would cause variations of potential to be distributed over the resistors R2 and R1 in proportion to their values and therefore at point 10 the variation of potential would be only about Thus, the full changes of potential of the filament 6 are applied to grid 11 of the power tube.

I have shown the plate 13 of the tube 2 connected to an output circuit which may contain a loud speaker or some other device and the return lead therefrom connected at 3 to the positive supply lead. A by-pass condenser C3 serves to return the variations in plate current which feed the loud speaker to the filament 12 of this tube.

Considering the efiect of pulsation of potential in the supply circuit, it will be apparent that a rise in potential of the positive lead will cause an increase in the potential of the filament 6 of tube 1. At the same time, there is also an increase in the potential of filament 12 of tube 2. Since, as has been shown, the filament 6 is connected through condenser Cl and resistor R2 to point 10 and hence to grid 11 of tube 2, if the rises of potential of point 10 and filament 12 can be made equal, then the efiect of the pulsation will not be amplified.

To secure such an equal increase, it will be necessary to make the ratio of impedance of the circuit from point 3 to filament 12 in relation to the impedance of the circuit from filament 12 to the point 4 on the negative lead, equal to the ratio of impedances for tube 1 between the plate 5 connected to the positive lead and the point of connection 10 to the impedance of resistor R1. If the tubes had the same characteristics, this relation would automatically be achieved, but since one of the tubes is a power tube and is, moreover, by-passed by condenser C3 and contains a loud speaker in its circuit, and the other tube is a high amplification tube with a condenser C1 and resistance R2 in series, I supply an additional condenser Cd in parallel with resistor R3 and adjust this 'iondenser to such a value as to secure the above proportionality of impedance.

Such an adjustment can be carried out in practice, after complete assembly of the circuit by listening on the loud speaker and varying the value of C4, and possibly also C3, until the bum in the loud speaker caused by the pulsations is reduced to a minimum.

Referring to Figure 2, the connections will be seen to be very similar to those of Figure 1, except that I have shown two high amplification tubes- 1 and 3-connected incascade from the point 14 between resistors R6 and R5 in series with tube 3, to the grid 11 of tube 1. The resistor R6 serves to produce a grid bias in the input circuit and may be by-passed by a small condenser C5.

As can be seen from the diagram, the voltages are so arranged that with a power supply of 650 volts, the power tube utilizes 400 volts on the plate with volts negative on the grid; while the remaining 250 volts are consumed in resistor R3. This will have a value of resistance R1 in series with tube 1 for coupling purposes, with a potential drop of 180 volts which is equal to the potential drop in tube 1, and the remainder of the voltage of 290 is consumed in resistor R2, .which is by-passed by condenser C1 so as to utilize efiectively the changes in potential of the filament 6 of tube 1.

With regard to tube 3, it will consume 180 volts, and allowing, say 2 volts, for the grid bias, the remaining 468 volts are consumed in resistor R5, thus permitting very effective resistance coupling from point 14 to grid 11 of tube 1.

In accordance with the same principle for compensation of hum, as was discussed in connection with Figure 1, I supply a condenser C6, which by-passes' a section of resistor R5. The 'value of this condenser and the amount of resistance which it by-passes are chosen so as to bring the hum potential of point 14; to exactly the same figure as the hum potential of the filament 6 of tube 1. Thus, no amplification of hum will occur in tube 1.

It is quite evident, that the hum potential of filament 6' is leading in phase with respect to the hum potential in the positive lead at 3, on account of the capacity of C1. By the insertion of capacity C6 shunting a section of resistor R5, the value of the hum potential at 14 can be adjusted to have the same magnitude and the same phase lead as that on the cathode 6.

I have also supplied the condensers CBand Cd, which secure a similar'balance of potential on the connection point 10 to the grid 11 of tube 2 and on the cathode 12 of the same tube, thus preventing the amplification of hum in this stage.

Referring to Figure 3, I have shown a complete circuit with connection from an antenna 16 through a tuning circuit comprising an inductance coil 17 and a variable condenser 18 to ground 19 and a circuit coupled to the inductance 17 consisting of coil 20 and a variable condenser 21. The variable condensers 18 and 21 may have a common adjusting means, as indicated at 21' and the input of the amplifier is shown connected directly across the variable con= denser 21. I

It will be seen that the connection of the am= plifier tubes is similar to that of Figure 1, err-- cept that potentiometer Rd is omitted and instead the potential of screen grid is supplied through resistors R6 and R6, of such a value as to give the necessary screen grid potential. By this arrangement, the efiect of pulsations in the supply leads on the screen grid is minimized, since both the plate circuit and the screen grid are connected to the same leads.

I have illustrated this circuit with two high amplification tubes 22 and 23, each one of the type known as indirectly heated; that is, comprising heater filaments 24 and 25 and separate cathode elements 26 and 2'7. The resistors R5 and R6 are connected in series with .the cathode 26 to the negative supply lead, the resistor R6 serving to provide the necessary grid bias to the grid 28 of tube 22 and also a connection to the central point of the filament supply transformer 29 for this tube. The same point may be utilized for connecting to the grid of the second amplifier tube. A by-pass condenser may 130 be supplied across the resistor R6, but in general, since the grid bias of this tube is very low, of the order of 1 or 2 volts, such a by-pass condenser will not be necessary.

The connections of tube 23 are, in general, 135 the same as tube 22, except that I have supplied resistors R1 and R2, as inFigure 1, and have tapped R2 at a point 30, for connecting to the central point of the filament supply transformer 31 for this tube. The power amplifier tube 2 140 is connected, as in Figure 1, with the exception that the filament 12 is shown to be supplied from a transformer 32, the central point of which 33 is connected to resistor R3 and the by-pass condensers C3 and C4. The plate supply circuit is illustrated in this figure as consisting of a. transformer 34, supplied by alternating current mains and delivering power to a rectifier tube 35, which may be of the full wave type. The rectified power is filtered through a condenser C5 9 and a choke coil 36 and may, in addition, in-' clude the leads 37 to the field coil of an electrodynamic loud speaker, if such is used.

It will be seen that the compensation of hum effects between the tubes 22 and 23 is secured by the relation of the impedances of these tubes to coupling resistors. In general, the proportion of impedances on both sides of cathode 26 is the same as in both sides of the cathode 27, and therefore the change of this cathode potential, due to pulsations of the supply potential, is identical, "but on account of the by-pass condenser Cl, there may be a slight unbalance in the same, what I wish to-claim as new, is as follows: y

1. In a resistance coupled amplifier-comprising two or more cascade connected thermionic valves, each with cathode, anode and at least one grid, a circuit including in series acoupling element, said cathode and said anode, a direct coupling connection from said coupling element to the grid of the succeeding valve, means including impedance elements for adjusting the impedance on either side of said cathode 'tohave the same proportion in each of the valve circuits and thereby compensate for pulsations in supply potential.

2. In a multi-stage cascade amplifier for electrical impulses with a common power supply comprising valve elements, each including one input electrode, one output electrode, one electrode common to input and output circuit, and

. coupling means in series with said common electrode. a method for compensating for pulsations in said power supply consisting of utilizing said common electrode and said coupling means of a preceding stage for coupling said impulses to the input of the subsequent stage and balancing the impedances oneither side of said common electrode relative to said power supply leads so that their ratio is substantially equal in all stages.

3. In a cascade amplifier of several stages for discharge valves, output and input electrodes of said valves, coupling elements between stages of said amplifier,.means to minimize the amplification of disturbing influences such as pulsations of line potential, comprising impedance elements shunting a part of at least one of said coupling elements to secure substantially, equal amplitude and phase of the disturbing potential from the coupling element to one 0! the input electrodes of the posterior stage, and the cooperative input electrode of the same. stage.

cathode and said tap valves; 9. direct coupling connection from the cathode of one valve to the grid of a succeeding valve; and a further impedance diflerent from and associated with said first impedance for applying maximum signal potential variations to the grid of the succeeding valve.

5. In a cascade amplifier comprising a plurality of amplifying valves having cathode, grid, and plate electrodes; a common plate potential supply source, said valves being connected to said source substantially in parallel; resistance coupling means in the cathode leads of said valves; 9. direct coupling connection from said" means to the grid of the succeeding valve; and means including impedance devices associated with said coupling resistance for applying maximum signal potential variations to the grid of the succeeding valve. v

6. In a cascade amplifier as described in claim 5 in which said last means is comprised of an electrical condenser connected in shunt relationship with said coupling resistance.

7. In a cascade amplifier comprising a plurality oiamplifying valves having cathode, grid and plate electrode; a common plate potential supply source, said valves being connected to said source substantially in parallel; impedance coupling means connected in the cathode leads of said valve; a direct conductive coupling connection from a tap point of said coupling means to the grid of the succeeding valve; and further means including reactance devices associated with said coupling impedance for applying maximum signal potential variationsto the grid of the 8110'! ceeding valve.

8. In a cascade amplifier as described in claim '7, in which said last means is comprised by an electrical condenser connected in parallel to the point of said coupling impedance.

9. In a cascade amplifier comprising a plurality of amplifying valves having cathode, grid, and

plate electrodes; a common plate .potential supply source, said valves being connected to said source substantially in parallel; resistance coupling element connected in the cathode lead of said valves; a direct coupling connection from a tap point of said resistance element to, the grid of the succeeding valve; and further means including reactance devices associated with said 1215 coupling resistance for imparting maximum signal potential variations to the grid of the suc ceeding valve.

10. In a cascade amplifier as described in claim 9, said last means is comprised of an electrical 180 condenser .connected between the cathode and said tap point of said coupling resistance.

11. In a cascade amplifier comprising a plurality of amplifying valves having cathode, plate, and control electrode; a common plate potential supply source, said valves being connected to said source substantially in parallel; coupling .elements connected in the cathode leads of said valves; a direct coupling connection from a point of said coupling elements to the grid of the succeeding valve; and means including reactance devices connected to said coupling elementsfor producing maximum signal potential variations at said coupling point.

e NYMAN.

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