US2133795A - Intermediate frequency amplifier - Google Patents

Description

Oct. 18, 1938. L. E. BARTOLJ j 2,133,795

INTERMEDIATE FREQUENCY AMPLIFIER Filed July 28, 1934 17mg 67 ,910 15 17 L48 I/WE/vmB: Zv yE Zia/112271 I c/ 9 j flTTOE/VEK Patented Oct. 18, 1938 UNITED STATEd INTERMEDIATE FREQUENCY AMPLIFIER Loy E. Barton, Collingswood, N. J., assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application July 28, 1934, Serial No. 737,357

12 Claims.

The present invention relates to high frequency radio signal amplifiers, and more particularly, it relates to intermediate frequency amplifiers for superheterodyne radio receiving systems. The invention has for its primary object to provide an improved final or output stage for a high frequency radio signal amplifier such as an intermediate frequency amplifier.

It is a further object of the present invention to provide an output stage for an intermediate frequency amplifier which is adapted to utilize a pentode electric discharge device having two additional diode anodes therein, as a combined intermediate frequency amplifier and detector, and means for providing automatic volume control potentials.

It is a further object of the present invention to provide separate automatic volume control potentials from two different sources, and in conjunction with the last stage of an intermediate frequency amplifier to provide amplified automatic volume control potentials for preceding tubes in the receiving system, and another automatic volume control potential for the last or output stage of the intermediate frequency amplifier, directly.

It is a still further object of the invention to provide a double diode pentode output stage for an intermediate frequency amplifier, wherein the amplifier portion of the pentode device is diodebiased, that is, supplied with biasing potential from the signal rectifier through one of the diodes, and means providing initial negative bias on the control grid of the pentode amplifier, while a positive or zero bias is provided on a diode-anode utilized for detection in a conductively interconnected coupling network.

It is also an object of the present invention to provide, with a single electric discharge device of the multi-electrode type including amplifier and diode rectifier elements, a combined intermediate frequency and automatic volume control amplifier, a diode detector and a diode rectifier AVC system.

In accordance with the invention, a coupling system including the output stage, is providedbetween an intermediate frequency amplifier and an audio frequency amplifier signal output stage, which coupling system comprises a single electric discharge device and circuit means in connection therewith, whereby a signal sufficient to drive the output audio frequency amplifier stage effectively is provided, while, at the same time, audio frequency detection and automatic volume control of the receiving system and of the output stage of the intermediate frequency amplifier is provided for.

In a preferred embodiment of the invention, a combined intermediate frequency amplifier of the pentode type, and detector of the diode rectifier type, is provided as the output or final stage of an intermediate frequency amplifier which may be used to drive directly an output power amplifier stage. Such an arrangement, as the present preferred embodiment of the invention, is shown by way of example in the accompanying drawing in which Figure 1 is a schematic circuit diagram of a radio receiving system embodying the invention; and

Fig. 2 is a similar circuit diagram showing a modification of a coupling means used in the circuit of Fig. 1.

Referring to Fig. 1, 5 is a pentode type electric discharge amplifier device in the final or output stage of an intermediate or high frequency amplifier for a radio receiving system, the signal input circuits of which are indicated by the block diagram 6. In a superheterodyne, this may include the radio frequency amplifier, first detector, oscillator, and any intermediate frequency amplifier stages which precede the output stage.

The amplifier device 5 is provided with a cathode I, a control grid 8, a screen grid 9, a

suppressor grid I0, and an output anode I I, to-

gether with two diode plates or anodes I2 and I3 associated with the cathode I to provide therewith, a pair of diode rectifiers in the same envelope with the pentode amplifier elements.

The intermediate frequency amplifier stage is provided with a tuned input grid circuit I4 connected with the control grid 8 and a tuned output anode circuit I5 connected with the output anode II. The input and output circuits are provided by the tuned windings of suitable input and output coupling transformers I6 and II, respectively, for the stage, the input transformer I6 being utilized to couple the intermediate frequency amplifier stage with the receiver 6.

The output transformer I1 is utilized to couple the output circuit I5 to a diode rectifier circuit I8 which includes the diode anode I2, the oathode I, the tuned secondary I9 of the transformer. I1, and a series circuit impedance in the form of a resistor in two sections 20 and 2 I, connected between the cathode and the low potential side of the input coupling Winding I9, which connection is shown in the present example, between the points A and B in the rectifier circuit. The resistor or impedance 20-4 I is provided with ahigh rent supply terminals 36 and 31.

frequency by-pass condenser 22 for intermediate frequency currents.

Modulated radio signals received in the circuit M at intermediate frequency are applied to the control grid 8 and amplified to a high degree in device 5, and are then applied to the diode rectifier circuit I8 where the signals are demodulated or detected through the medium of the rectifier device |2-|. The audio frequency and direct current components of the rectified signals appear across the series impedance or resistor 20--2|.

The audio frequency or signal component is utilized to drive an output amplifier device 23 through a coupling network connected between the point B which is the high audio frequency potential point of the rectifier circuit, and a common circuit return or ground lead 24. The coupling network in the present example includes a tone-compensated, potentiometer, volume control device 25 in series with which is a coupling condenser 26, a filter resistor 21, and a second filter resistor 28.

The potentiometer device: has a variable volume control contact 29 which is connected to the control grid 30 of the output amplifier device 23. Signals amplified therein are applied to a loud speaker 3| through an output coupling transformer 32.

Any suitable output coupling network may be employed to apply the rectified signal or audio frequency component to the output stage, although an impedance coupling network is at present preferred, including the series potentiometer device and blocking or coupling condenser 26.

Between the point A and the common circuit return lead 24, a self bias resistor 34 providing a source of variable potential is connected in series with a source of substantially fixed potential. The fixed potential source is provided in the present example, by a choke coil or impedance device 35 forming part of a bleeder or voltage supply means between positive and negative direct our- The remainder of the voltage supply means is provided by a resistor section 38, and the common circuit return lead 24 is connected to a terminal D between the impedances 35 and 38. A suitable audio and radio frequency by-pass condenser 39 is provided in parallel with the series connected impedances 34 and 35.

The voltage supply arrangement is such that the potential across the choke coil or impedance 35 is in opposition to the potential established in the self bias resistor 34. Normally, with no signal, in a circuit of the character shown, and with an RCA 2B7 tube utilized as the device 5, about 150 volts may be provided by the impedance device 35 and substantially 190 volts by the resistor 34, the latter having a resistance of about 40,000 ohms.

Initial bias potential for the control grid 8 is derived from the opposing sources 34 and 35,

through a tap connection G between the sections 20 and 2| of the series diode circuit impedances and a potentiometer device connected therefrom to the common circuit return lead 24. The potentiometer device comprises two impedance sections 40 and 4| between which is connected a through a bias potential supply lead 43.

It will be seen that the present connection provides a series circuit in shunt betw the point C and the common return lead 24. The tap 42 is adjustable to provide an initial bias on the control grid 8, that is, between the control grid 8 and the cathode I, of any desired value such as substantially 3 volts negative. The adjustment of the potentiometer is not critical which permits the use of fixed resistors for 40 and 4|. The potential drop, it will be seen, is established through the resistor sections 20, 40, and 4| and the potential supplying the current through the above namedsections is derived from the self bias resistor 34 as the excess potential over that established across the choke coil 35; This, in the present example, is initially substantially 40 volts, that is to say, the point A is 40 volts more positive than the point D. Current through the resistor 48 from the positive supply source also flows through resistors 2|, 40, and 4|, thereby providing potential across said last named resistors and affecting the adjustment of the contact 42.

It will be seen also, that as rectified signal current flows in the rectifier circuit l8 and through the impedance section 20 an increasing negative bias potential with respect to the oathode will be placed upon the control grid 8. This results in a reduction of the anode current and of the potential across the self bias resistor 34. This may continue until the potential across the resistor 34 falls to a value equal to or less than the potential across the impedance 35. Stated in other words, the point A may become equal in potential to or more negative than the point D.

In such case, the fixed potential across the impedance 35 predominates and causes current to flow in an automatic volume control circuit 44 comprising the second diode anode I3 and a series impedance element 45 to which the common circuit return lead 24 is connected at one end 46 and to which automatic volume control leads 4! are connected for supplying variable biasing potentials to the preceding circuits (not shown) in the receiver 6.

The double diode pentode device therefore may supply delayed automatic volume control potentials, the potentials being derived through the second diode anode and a suitable diode circuit impedance device for coupling to the preceding receiver circuits. Since this type of automatic volume control system per se, is not part of the present invention and is described and claimed in my copending application Serial No. 640,946, filed November 3, 1932, and assigned to Radio Corporation of America, further description is believed to be unnecessary.

Suitable values for the resistor sections which i have been found to be satisfactory are as follows: section 20-03 megohm; section 402 megohms; section 4l-5 to 10 megohms. Other suitable values may, however, be used for the various impedance or resistor sections to provide any desired potentials for the operation of the device 5 as a variable grid bias, variable anode current, signal and automatic volume control amplifier. It will further be seen that the device 5 is controlled directly by the AVG potential derived from the impedance 20 so that in effect it becomes a self controlled automatic volume control intermediate frequency amplifier and an amplifier for automatic volume control potentials applied to the preceding circuits of the receiver through a second diode circuit.

Furthermore, the additional biasing potential is derived from the signal current rectified by the diode |2-'| so that the amplifier is of the which are loosely magnetically coupled.

diode-biased type. At the same time there is provided a fixed initial bias which is effective to bias the amplifier when the signal current is zero or in the absence of an appreciable received signal.

It will be seen that the detector or rectifier circuit I8 is connected with the cathode and with the control grid 8 in a conductive network. This circuit arrangement tends to provide a negative potential on the detector diode anode I2 with respect to the cathode because of the initial potential drop in the impedance section 20 of the network. This would normally provide a delay in detection which is not desired in the circuit of the present example. Accordingly, a positive potential with respect to cathode is applied to the diode anode l2 through a variable control impedance 48 connected between the point B on the diode rectifier or detector circuit and a source of positive potential such as the anode circuit [5 for the device 5. The positive connections indicated in the diagram are made to the terminal 36 or any other suitable point of connection on the resistor 38. These connections are omitted in the diagram for the purpose of simplifying the same. In the present example, the resistor 48 may be of a value of 5 to megohms and is reduced in resistance value until the potential at point B is substantially equal to that at point A or is preferably somewhat positive so that the diode anode i2 is at substantially zero potential or positive with respect to the cathode I.

This connection also provides means for reducing the distortion in the diode rectifier when connected to the succeeding stage through impedance coupling means as shown. This is for the reason that an increasing positive potential may be placed upon the detector anode to cause an initial flow of current in the rectifier circuit as described and claimed in my copending application, Serial No. 695,555, filed October 28, 1933, Patent No. 2,078,994, issued May l, 1937, and assigned to Radio Corporation of America.

It will be seen that the detector circuit is arranged to receive the signal output from the pentode amplifier portion of the device 5. With maximum signal input the signal voltage may reach a peak of 150 volts on the diode I2 provided that an efficient coupling means is employed between the pentode output circuit [5 and the detector or rectifier circuit I8. In the present example,,an intermediate frequency coupling transformer having tuned primary and secondary windings is shown. The coupling may, however, be provided by other means such as combined capacity and magnetic means as indicated in Fig. 2 to which attention is now directed.

It will be seen that in the diode rectifier circuit I8 is provided with a tuned inductance 5! while the output anode circuit I5 is provided with a similar inductance 5!, which provide windings The electrical coupling between the circuits i5 and I8 is completed through a direct capacity coupling means or condenser 52, preferably between the high potential ends of the inductances, so that combined magnetic and capacity coupling is provided in such proportions that a relatively strong signal is applied to the detector diode. The preferred receiver arrangement is such that the maximum output swing on the detector diode may be obtained at about 30 to 50% modulation with an input signal of a predetermined value.

Referring again to Fig. 1, it will be seen that the potentiometer or tapped resistance connection 40-4! with the point C to the'common circuit return lead 24 is necessary in order that the control grid 8 may receive an initial potential which, otherwise, in the absence of a signal, would be zero. This is for the reason that with no signal, the rectified signal current in the detector circuit I8 is zero. To cause an initial potential on the grid 8, in the absence of signals, the network 20, 4|], and M is provided across the potential source between points A and D.

From the foregoing description, it will be seen that the coupling network, provided between the cathode, control grid and the diode plates of a double diode pentode device is conductive. The network comprises a pair of series connected impedance elements 2|] and 2| preferably resistors, in the diode rectifier or detector circuit, the anode end or terminal (A) of which is connected to a common circuit return lead, through a self bias resistor and means providing a fixed source of potential, and the diode anode end or terminal (B) of which is connected to said common circuit return lead through an impedance coupling network providing a variable output connection for an audio frequency amplifier stage for the receiving circuit substantially in shunt to the detector output circuit impedance 2D-2l.

Intermediate between the ends of the detector circuit impedance 2l, a tap C is provided between the sections, providing a direct current path from the tap point C to the common circuit return lead 24, which may be the ground or chassis return circuit.

By utilizing a circuit embodying the invention as described in connection with the present example, amplified automatic volume control potentials may be obtained from the output stage of an intermediate frequency amplifier which,

at the same time, provides detection or demodulation of the signal and diode automatic volume control of the intermediate frequency output stage directly, all in conjunction with one electric discharge device of the double diode pentode type, such as an RCA 2137 tube.

Such a system, therefore, provides for the reduction in the number of tubes employed in the receiver without sacrificing the gain and control functions including bias and plate potential V supply from a common source, as is desirable in a receiver for good quality of reception. The present system provides a relatively high value of signal potential from the pentode amplifier section, and is adapted to provide a relatively high signal output to the output stage, while the operation of the automatic volume control is reduced to preferred or desirable limits by suitable tapping-in on the network between the detector diode anode and the cathode. The resistor sections 4i and 43 are then provided to establish a desired initial potential on the control grid of the amplifier and the resistor 48 is with to provide a rectifier, and a control grid, of a conductive coupling impedance circuit network interconnecting said anode, cathode and control grid to supply to said control grid a. portion of the rectified signal as a biasing potential from said rectifier, means for applying an initial negative biasing potential to said control grid through a portion of said network, and means connected in circuit with said anode for preventing the application of said negative potential thereto.

2. In a signal amplifier stage, the combination with an electric discharge amplifier device having a cathode, a diode anode associated therewith to provide a rectifier, and a control grid of a conductive impedance circuit network interconnecting said anode, cathode and control grid to supply to said control grid, a signal controlled biasing potential from said rectifier, means for applying an initial negative biasing potential to said control grid through a portion of said network, and means connected in circuit with said anode for preventing the application of said negative potential thereto, said conductive network including a series resistance element in circuit between the anode and the cathode, a shunt resistance element connected between a tap on said series resistance element and a potential supply point in said network more negative than said cathode, and said grid being connected with a tap on said last named shunt resistance element.

3. In a radio signal receiving system the combination with an electric discharge amplifier device having a cathode, a control grid, and at least two diode anode electrodes associated with the cathode, of a rectifier circuit including a conductive impedance coupling network between said elements, means for maintaining said control grid initially negative and one of said anodes initially positive with respect to said cathode, and means for maintaining one of said anodes more positive than the other of said anodes with respect to said cathode.

4. In a radio signal receiving system, the combination' with an electric discharge amplifier device having a cathode, a control grid, and a diode anode electrode associated with the cathode, of means providing an electrical impedance coupling network conductively coupling said electrodes for the flow of direct current and including a resistance element and a signal input coupling means between said anode and cathode, said control grid being connected with a point intermediate the ends of said resistance element, means for maintaining the anode and cathode at substantially the same positive potential with respect to ground in the absence of signals, and

means in circuit with said grid for maintaining said grid initially negative with respect to said cathode, and means for causing said grid potential to vary in response to and under control of rectified signal currents flowing in said resistance element.

5. In a radio signal receiving system, the combination with an electric discharge amplifier device having a cathode, a control grid, an output anode, and a diode rectifier anode associated with the cathode, of a signal input circuit connected with said control grid, a signal transmission circuit connected between said diode anode, and the cathode, and including an impedance element adjacent to the cathode, said circuit being coupled to the output anode, means providing a connection betweenthe signal input circuit and said impedance element for applying signal variable biasing potential to said control grid from said rectifier circuit, said diode anode, cathode, and control grid being thereby conductively connected together, means including a source of fixed potential and a self bias resistor in series between the cathode and ground, and a potentiometer resistance in circuit therewith in said last named connection between the signal input circuit and said impedance element for impressing an initial negative biasing potential on said control grid, and separate means for impressing an initial positive potential on said diode anode. 6. A radio signal receiving system in accordance with claim 5 further characterized by the fact that the control grid is connected to a tap point on said series impedance element between the terminal ends thereof and an impedance coupling element is connected in circuit with a terminal end of said impedance element more adjacent to the diode anode.

'7. A radio signal receiving circuit in accordance with claim 5 further characterized by the fact that a signal variable source of potential is connected in circuit with said impedance element more adjacent to the cathode, an impedance coupling element is connected in circuit with said first named impedance element more adjacent to the diode anode, and the control grid is connected to an intermediate potential point with respect to said connections on said impedance element.

8. In a radio signal receiving system, the combination with an electric discharge amplifier device having a cathode, a control grid, an output anode, and a diode rectifier anode associated with the cathode, of a signal input circuit connected with said control grid, a second signal input circuitcoupled to the output anode and connected between said diode anode and the cathode, said last named circuit including a potential drop producing impedance element connected adjacent to the cathode, means providing an intermediate tap connection between the signal input circuit and said impedance element for applying signal variable biasing potential to said control grid from said rectifier circuit, said last named means including a portion of a second impedance element connected between said tap and ground for said system, said diode anode, cathode and control grid being thereby conductively connected together, means for impressing an initial negative biasing potential on said control grid and separate means for impressing an initial positive potential on said diode anode.

9. In a radio signal receiving system, the combination with an electric discharge amplifier device having a cathode, a control grid, an output anode, and a detector diode anode associated with the cathode, of a detector circuit connected between the detector anode and the cathode, and

including at least two resistor elements in series adjacent to the cathode, means connected in said detector circuit between the detector diode anode and the resistor elements providing electrical coupling with the output anode, a signal input circuit connected between said control grid and a point intermediate said resistance elements, said control grid, cathode, and detector diode anode being thereby conductively connected together, means providing a common circuit return lead for said system, a self bias resistor and a source of fixed potential in series between the cathode and said return lead, the positive terminal of said fixed source being connected with said return lead, a resistor in circuit with said control grid adjacent to the connection between said series resistance elements, a second resistor in series therewith between the control grid end of said first resistor and the common return lead, and means for impressing a positive potential on said detector diode anode connected with said detector circuit between said series resistance elements and said coupling means.

10. In a radio signal receiving system the combination with an electric discharge amplifier device having a cathode, a control grid, an output anode, and a detector, and a diode anode associated with the cathode, of a detector circuit connected between the detector anode and the cathode and including at least one resistor element connected adjacent to the cathode, means connected in said detector circuit between the detector diode anode and the resistor elements providing electrical coupling with the output anode, a signal input circuit connected between said control grid and a point on said resistance element, said control grid, cathode, and detector diode anode being thereby conductively connected together, a common circuit return lead, a self bias resistor and a source of fixed potential in series between the cathode and said return lead, the positive terminal of said fixed source being connected with said return lead, and a resistor in circuit with said control grid, said resistor being connected between said point of connection for said input circuit with the resistance element and said return lead.

11. In an intermediate frequency amplifier a final amplifier stage comprising an electric discharge device having a cathode, a control grid, a screen grid, a suppressor grid, an output anode and two separate diode anodes associated with the cathode, a self-bias resistor in circuit with the cathode, a filter choke coil between the selfbias resistor and the control grid providing a potential in opposition to that provided by the self-bias resistor, and a diode rectifier associated with the cathode and interconnected by conductive connection with the control grid, and means in said conductive connection for preventing the application of a biasing potential to the control grid from being applied to the diode anode.

12. In a superheterodyne receiver a combined intermediate frequency amplifier and detector stage comprising an electric discharge amplifier device having a signal grid, a cathode and an auxiliary anode adjacent to the cathode providing a diode rectifier, means forimpressing intermediate frequency signals on said diode rectifier, a diode output impedance in circuit with said rectifier having an intermediate tap thereon, means for applying a positive potential to said tap including a series connected potentiometer resistor connected at one end with said tap, and means providing a variable tap connection for said signal grid on said potentiometer resistor.

LOY E. BARTON.

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