US2481870A - Wide band amplifier - Google Patents

Description

W 7 L Q Sept. 13, 1949. J. L. POTTER WIDE BAND AMPLIFIER Filed Dec. 14, 1945 tuiilit 3; .m hkmm 055m Svu Elm h vv U Y A i w N mil BEBE? Rm xalkbn .V/V I n n. Quinn Hm mwwmw H H w H I- U1 M mhw m 1 m H mm m Li a fi o Nm "H... mm on Patented Sept. 13, 1949 WIDE BAND AMPLIFIER James L. Potter, Highland Park, N. J., assignor vto L. S. Brach Mfg. Corp., Newark, N. J.

Application December 14, 1945, Serial No. 634,966

8 Claims (01. 250-20) This invention relates to a wide band amplifier which is particularly adapted for use for amplifying all desired signals received by an antenna system from .5 to 20 megacycles. The system to be hereindescribed is especially useful in the demonstration rooms of stores for the purpose of operating loop-type receivers, the object being to raise the signals received by the store antenna to a satisfactory level for demonstrating the radio sets sold by the store. While the system herein disclosed was developed for the aforesaid purposes it is also highly useful in amplifying video signals in television systems as well as radar equipment and also for apartment house radio distribution. Hence, the system to be described must operate as heretofore mentioned over a very wide band.

At the present time a great many radio demonstrating rooms are located in steel-frame buildings which act as shields against the reception of radio waves so that it is necessary to use antennas located outside the building. A great many radio receiving sets as manufactured at the present time are built with self-contained loop antennas which make it necessary to disconnect the loop antenna and connect the set to the demonstrating antenna in the store. This may not always be possible with some of the present day sets especially the midget loop sets, hence it is one of the objects of my present invention to provide a system which will permit operation of the loop type sets by re-radiatlng the signal in the store demonstrating room and the booster amplifier to be described increases the signal so that the loop-type sets can operate satisfactorily above the local man-made static noises generally in the demonstration room, which noises are inherent in a busy business district. The system which I have evolved covers the regular broadcast band as well as the short wave band so that all wave-type receivers can be operated or demonstrated. Furthermore, an additional amplifier is added to cover the frequency modulated band (F-M) from 88 to 106 megacycles, for the demonstration of (F-M) receivers. My improved type of signal booster amplifier is illustrated in the attached drawing, wherein:

Figure 1 illustrates diagrammatically a complete operative system which I have found to work entirely satisfactorily.

Figure 2 illustrates diagrammatically the application of my improved system to an apartment house distribution system.

In the drawing the am litude modulated (A-M) antenna I carries a twisted pair high frequency cable 2 which extends from a (F-M) dipole antenna 3 to a coupling unit 4 that in turn is connected to a twin conductor cable I carried in a tubular shield 6. It is essential that the shield 6 be grounded at I to the building near the base of the antenna I, which is supported by an insulator 8 and connected by conductor 9 to the center of a coupling unit 4. The cable 5 can be any length up to 400 or 500 feet or greater, If necessary, to run from the building antenna I to the demonstration room. The cable 5 is connected through a coupling unit I II having windings I l and I! the latter of which is connected to (F-M) amplifier l3 which supplies the (F-M) radio receivers, or if the (F-M) amplifier I I has a self-contained antenna the signal may be directly radiated. The center of the winding I I is connected by conductor N to an input attenuator I5 made up of a plurality of resistances Ii to 24 inclusive. This attenuator is used for adjusting the amplifier output to a satisfactory level to prevent overloading the output stage and causing excessive cross-modulation. The attenuator I5 is connected to a wave trap 25 which may be made up in a satisfactory manner, an impedance coil 26 and condenser 21 being located for the purpose of attenuating strong signals of local stations to prevent over-loading of the amplifier. A multiplicity of wave traps may be used if there are several local stations which cause interference due to a strong signal. The wave trap or traps is connected by a conductor 28 to an (A-M) amplifier broadly referred to by the number 29. This amplifier comprises a tube 30 which is a conventional pentode (such as the 6AC7) Since the functions of the control, screen, and suppressor grids in the pentode tube 30 are old and well known further description of the operation of this tube does not seem to be necessary; however, it is to be noted that the plate 3| of tube 30 is connected directly to the grid 32 of tube 33 as well as to a resistance 34 which in turn is connected to an impedance 35 and a resistance 36 to a power supply 31. Thus a cathode-follower circuit is established which permits a wider band amplifier for a given gain. The maximum frequency of an amplifier is generally defined as For a given amplifier gain at mid-frequency R0 (34) is fixed in value, hence the lower Co is made 3 the greater f becomes. The value of Co is reduced by using the direct coupled cathode-follower by a factor of /2 to /3 of a conventional circuit hence making the frequency range 2 to 3 times greater for a given gain. This is due to the fact that the input capacitance of cathode-follower stage is equal to C'gp-i-C k(1A) where C is grid to plate capacitance of the follower and C is the grid-cathode capacitance of the follower and A is the gain of follower and is near unity in this case. The stray capacitance Co is further reduced by eliminating the coupling condenser at this point. The cathode-follower has a very low output impedance being approximately equal where Gm is the trans-conductance of the tube (33) in mhos.

Since this impedance can be made extremely low, the coupling condenser 38 is placed in its output circuit and connected to the grid of stage 39 without appreciably afiecting the high frequency gain of the amplifier. The inductances and are used to compensate the high frequency response of the amplifier. The resistance 4| corresponding to the resistance 42. is connected across the amplifier stage 39. If necessary, one or more amplifier stages 43 may be utilized. The stages 39 and 43 are for all practical purposes identical with the stage 29.

Extending from the last amplifier stage is a radiating conductor 44 connected to ground I through a resistance 45. The radiating wire 44 is long enough to cover the length of the demonstration room or rooms and while the energy radiated therefrom is a small fraction of a watt, if the loop receivers 46 are located within 20 feet of the wire 44, I have found that at least normal signal strength will be received. In Figure 2 I have shown how the (A-M) amplifier may have its output conductor 44 run through a shielding tube 41 in a store or apartment house and loops 48 taken out through junction boxes 49 for the purpose of energizing loop receivers similar to 46. From the preceding it will be seen that the (F-M) dipole antenna 3 picks up (F-M) signals which are transmitted over the cable 2 and to 5 through the coupler it through the (F-M) amplifier 13. Also it is seen that the cable 5 transmits both the (F-M) and (A-M) signals which are utilized as heretofore described. Some of the advantages which are derived from the system may be enumerated as follows:

(1) The (A-M) and (F-M) signals may be fed over a single twin conductor cable.

(2) A unique method of separating the amplifier stages with a cathode-follower to permit greater gain per stage for a given band width.

(3) The use of a cathode-follower directly coupled to the amplifier tube whereby the gain of the cathode-follower stage is nearly unity when operated under the stated conditions.

(4) Higher gain is obtained per stage which means fewer stages are needed thus simplifying the frequency compensation problem.

(5) Since two types of compensation per stage are used the possibility of reducing phase distortion in video amplifiers at high frequencies is increased.

(6) The use of the cathode-follower materially reduces the value of the stray capacitance between stages and its effect.

(7) The low output impedance of cathode-fol.-

4 lowers permits the coupling condenser to be placed in its output and drive a rather high input capacitance tube without appreciably affecting the usable high frequency response of the amplifier.

(8) Since the stray capacitance is reduced, the output voltage is increased in proportion and also gives less distortion for a given output.

(9) The system described provides a method for radiating energy to loop radio sets in store rooms in steel frame buildings.

(10) The system further provides a method of preventing interference between radio sets due to local oscillator interaction such as is likely to occur when sets are connected directly to a common antenna system.

(11) The system provides a means for boosting an incoming signal so as to overcome local noises in a store where the system may be used.

(12) An amplifier system for overcoming losses in long transmission lines at (F-M) frequency.

(13) The system provides a method for distributing radio-frequency energy in apartment houses for the operation of all-wave receivers.

These and other advantages will be discerned by one skilled in the general radio field.

Having thus described my invention,

What I claim is:

1. A radio message receiving system including an output conductor and a twin conductor shielded cable with means for passing thereto from suitable antennas b'oth frequency modulated and amplitude modulated signals, means for separating the two types of signals and means for specially amplifying the A-M signals, said last mentioned means including at least one amplifier stage having a pentode tube and a follower tube, the plate of the pentode tube being connected to the grid of the follower tube as well as to a source of power, the cathode of the follower tube being connected to said output conductor which acts to radiate energy to receiving sets placed in pick up position to the output conductor.

2. A radio message receiving system as set forth in claim 1 further characterized in that the separating means includes an inductive coupling unit having one winding connected across the ends of said twin conductor cable and another winding connected to a frequency modulated amplifier, the center of said one winding having a connection to the control grid of said pentode tube as well as to the ground, through a suitable impedance in the form of a resistance.

3. A radio message receiving system as set forth in claim 1 further characterized in that an attenuator and wave trap means are provided to receive the amplitude modulated signals before they go to a first stage amplifier, the attenuator being composed of a plurality of series connected resistors with tap offs at the end of each of the resistors, through resistors to ground with adjustable means for varying the characteristics of the attenuator.

4. A radio message receiving system including two antennas, one being a frequency modulated antenna and connected to a twisted pair cable while the other is a tubular member of signal conducting material within which the said cable extends, a coupling unit connected across the inner ends of the cable twisted pair while the center of the coupling unit is connected to said tubular member, a twin conductor cable carried in a grounded shield and connected at one end to said coupling unit, another coupler unit which acts to separate the frequency modulated and amplitude modulated signals connected to the other end of lower tube, the control grid of the pentode tube being connected to the junction of said trap and resistance while its plate is connected directly to the grid of said follower tube as well as to a source of power through certain impedances, and an output conductor connected to the cathode of said follower tube, said conductor acting to 'radiate signal energy to radio receiving sets in near by relation to said conductor.

5. Means for radiating energy to radio sets in a room of a building having a steel frame, said means including an amplitude modulated antenna, a twin-conductor cable carried in a shield, a coupling unit for connecting the cable to said antenna, the shield being grounded to the building frame preferably near the base of the antenna, a coupler unit connected to said cable at its opposite end, an amplifier stage having an amplifier tube being connected to said coupler unit, a follower tube having its grid connected directly to the plate of the amplifier tube as well as to a source of power, along with the plate of the amplifier tube and a single radiating conductor connected to the cathode of said follower tube and extending in proximity to radio receiving sets.

6. Means for distributing radio energy to apartments in an apartment building, comprising a duplex antenna system composed of a dipole antenna connected to a twisted pair cable for picking up frequency modulated signals and an amplitude modulated antenna for picking up amplitude modulated signals, the said cable being positioned within the amplitude modulated antenna, a coupling unit connected to both antennas, a twin conductor cable connected at one end to said coupling unit and extending therefrom in a grounded shield, a coupler unit connected to the opposite end of said co-axial cable for separating the amplitude modulated and frequency modulated signals, amplifying means having at least one stage including an amplifier tube and as follower tube, the control grid of the amplifier tube being connected to said coupler unit while its plate is connected directly to the grid of the follower tube as well as to a source of power, and a single radiating conductor connected to the cathode of said follower tube and extending through the apartment building with outlets to the various apartments.

7. A radio message receiving system as set forth in claim 1 further characterized in that said amplitude modulated antenna is hollow and carries a twisted pair cable there within which is used to pass the frequency modulated signals from the frequency modulated antenna, a coupling unit to receive said frequency signals, said coupling unit having its center connected to the base portion of the amplitude modulated antenna while the ends of the coupling unit are connected to the conductors of said twin conductor cable having a shield grounded near the base of the amplitude modulated antenna.

8. A radio message receiving system as set forth in claim 1 further characterized in that an adjustable attenuator and wave trap means as described are provided to receive from said separating means and pass the amplitude modulated signals before they go to a first stage amplifier.

JAMES L. PO'I'I'ER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,853,181 Shumard Apr. 12, 1932 1,971,235 Rettemeyer Aug. 21, 1934 2,120,823 White June 14, 1938 2,365,453 Byrne Dec. 19, 1944 FOREIGN PATENTS Number Country Date 462,536 Great Britain Mar. 8, 1937

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