US1938620A

US1938620A - Band-pass amplifier

Info

Publication number: US1938620A
Application number: US387836A
Authority: US
Inventors: Rene A Braden
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1929-08-23
Filing date: 1929-08-23
Publication date: 1933-12-12
Anticipated expiration: 1950-12-12

Description

DBC. 12, R Al BRADEN 1,938,620

Filed Aug. 23, 1929 2 Sheets-Sheet l WCM/IVE .5763041 FME-76) INVENTOR RENE A. BRADEN ATTORNEY Dec.

12, 1933. R. A. BRADr-:N

BAND- PASS AMPLIFIER Filed Aug. 23, 19.29

2 sheets-sheet 2` INVENTOR RENE A BRADEN ATTORNEY Patented Dec. 12, 1933 PATENT ori-*ICE BAND-PASS AMPLIFIER Rene A. Braden, New York, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application August 23, 1929. Serial No. 387,836

12 Claims.

My present invention relates to amplifiers, and, more particularly, to band-pass amplifiers employing novel coupling means.

The typical band-pass amplier circuit of present day practice possesses resonance curves, withr various values of coupling between the tuned circuits, which have pronounced, undesirable features. One particularly disadvantagecus characteristic of this type of circuit is that 10. the Width of accepted band is limited by the depression at the center' of the band, which depression becomes deeper as the coupling is increased.

Again, at any frequency, either at resonance l5Y or off resonance, there is a particular value of coupling which gives to the secondary circuit the maximum current that can flow therein. Thus, the optimum coupling is small for the resonance frequency, and becomes larger as the frequency is farther from resonance. The value of coupling and band width at which the center depression begins depends, of course, on the amount of ohmic resistance in the tuned circuits. It is, therefore, customary to insert resistance in theV circuits when a greater band widthr than usual is desired. rIhis has the desired result of increasing the band width. However, the amplification is reduced in proportion to the amount of resistance added.

Now, I have discovered a type of circuit in which the effective value of the coupling varies according to the frequency, being small at the resonance frequency of the tuned circuits, and relatively large for frequencies off resonance. Thus, every frequency within certain limits which impressed in the circuit encounters the proper value of coupling to amplify it properly. All frequencies within these limits are, therefore, amplified equally. Moreover, no resistance need be added to the circuits, and the amplification obtainable from a wide-band ampliiier is thereby, according to my invention, as great as that of a narrow-band ampliiier.

45 Accordingly, it is one of the main objects of my present invention to provide an improved coupling means for band-pass amplifiers by means of which it is possible to construct a bandpass ampliiier passing a wider band than is 50. passed by an amplifier employing the usual, present day circuit, without the use of a large number of tuned circuits in each amplifier stage, and without damping the tuned circuits.

Another important object'of the invention is '35 to provide a method of, and means for, coupling two tuned circuits so that the coupling is relatively small at the resonantl frequency of the circuits, and larger at frequencies, higher and lower than the resonant frequency.

Another object of the invention is to provide a band-pass amplier consisting of two tuned circuits, and means for coupling the two circuits in such a way that the co-eicient of coupling is substantially proportional at any frequency to the difference between that frequency and the resonant frequency of the tuned circuits.

Still other objects of the invention are to iJnprove generally the elciency of band-pass amplifier couplings.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organizationl and method of operation will best be understood by reference to the following description taken in connection with the drawings in which I' have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawings,

Fig. l schematically shows a typical band-pass amplifier circuit employed in present practice,

Fig. 2 shows resonance curves for the circuit in Fig. l' with various values of coupling,

Fig. 3 shows a `circuit embodying my novel 85' form of coupling,

Figs. 4 and 5 show modified forms of the invention,

Fig. 6 graphically shows the coupling coefficient of the circuit in Fig. 5,

Figs. 7 and 8 show'additional modified forms of the invention.

Referring to the accompanying drawings in which like characters of reference indicate the same elements in the different views, Fig. 1 illus- 95 trates an amplifier circuit which is typical of all band-pass circuits which have been used heretofore. The incoming signal energy is impressed across the input terminals of the tube 1, the latter being of the screen grid type. The output circuit of the tube is tuned by the variable capacity 2 andinductance 3.

The input circuit of the screen grid tube'9 is tuned by the inductance 7 and variable capacity 8, the input circuit being coupled to the'output 105 circuit 2, 3 by coupling coils l0, 11, as atM.` The output from the tube 9 is then impressed upon a' succeeding stage and utilized in any well known manner.

In Fig. 2, I have shown resonance curves for the circuit in Fig. l, with various values of coupling M between the two tuned circuits. The abscissa is Frequency, and the ordinate Secrondary current, curve A illustrating the effect of loose coupling, while curve E portrays the result of tight coupling, the curves between designating increasing values of coupling.

The particular feature of this circuit, which it is desired to illustrate by means of the resonant curves of the Fig. 2, is that the width of the accepted band is limited by the depression at the centre of the band, which depression becomes deeper as the value of the coupling M is increased. It will, additionally, benoted that at any frequency, either at resonance or off resonance, there is a particular value of coupling which gives to the secondary circuit the maximum current that can flow therein; and that the optimum coupling is small for the resonance frequency and becomes larger as the frequency is farther from resonance.

The value of coupling and band width at which the depression begins depends upon the amount of resistance in the tuned circuits. It is customary, therefore, to insert resistance in the tuned circuits when a greater band width than usual is desired. This allows the coup to be increased, thus increasing the band width, but the amplification is reduced in proportion to the amount of resistance which is inserted into the tuned circuits.

My invention, as explained heretofore, and illustrated in Figs. 3 to 8 inclusive, involves a circuit in which the effective value of the coupling varies according to the frequency, being small at the resonance frequency of the tunen1I circuits, and relatively large for frequencies ofi resonance, Every frequency within said limits., which is impressed on the circuit, encounters the proper value of coupling to amplify it properly, and all frequencies Within these limits are t. erefore amplified equally. Moreover, no resistance need be added to the circuits, and the ainpiication obtainable from a wide-band amplifier is therefore as great as that of a. narrow-band amplifier.

In Fig. 3, there is shown an amplifier circuit embodying a coupling designed according to my invention. The incoming signal energy is iinpressed across the input terminals of the screen grid tube 1, the output circuit of the tube being shown as iixedly tuned by the capacity 2 and the inductance 3. It is to be understood that a vacuum tube of the triode or tetrode type may be employed in the circuit. The output circuit ofthe tube l is coupled to the tuned input circuit of the screen grid tube 9 by a coupling circuit to be shortly described in detail.

The input circuit of the tube 9 is shown as Iixedly tuned by the capacity 8 and the inductance '7, and the output from the tube 9 impressed upon a succeeding stage and utilised in any well-known fashion. The coupling circuit comprises an inductance coil 4 coupled inductively to the coil 3, and an inductance coil 6 coupled inductively to the inductance '7, the

coils

4 and 6 being connected in series with each other.

A net-work 5 of resistances and reactances is inserted in series with the

coils

4 and 6, the network including an inductance L1 and a resistance R1 in series with each other, and a capacity C in series with a resistance Rz, the latter two elements being in shunt with the series connection of resistance andinductance. It is tc be noted that

coils

6 and 4 are similar to each other, coil 7 is similar to coil 3, and capacity 8 is similar to the capacity 2.

In operation, the net-work 5, consisting of the inductance L1, the capacity C', and the resistances Ri and R2, is tuned to the mid-band frequency or" the band of frequencies which is to be ainplified, the output circuit of tube 1 and the input circuit of tube 9 being maintained xedly tuned to the same mid-band frequency. It is to be observed that the last named two circuits correspend to the primary and secondary circuits of Fig. i, and that they are coupled by the

circuit

4, 5 and 6,

The effective coupling between the circuits 2, 3, and 7, 8 depends upon the coupling between the coils S and 4, and the coupling between the coils 6 and 7, and also on the impedance of the circuit'4, 5, 6, the coupling being diminished as the impedance of this latter circuit is increased. The net-work circuit 5, being resonant to the frequency to which the primary and secondary circuits are tuned, puts a high impedance in the coupling circuit at that frequency, and reduces the coupling to a small value. At irequencies which are higher and lower than the resonant frequency, the impedance of the net work 5, is smaller, and the coupling is greater.

Adjustment of the circuit may be effected by first adjusting the coupling between coils 3 and 4 and between 6 and '7, so that with the net-work 5 short circuited, the two peaks of the resonance curve are iai' enough apart to give the desired band width. Then, the net-work 5 is inserted, and the circuit elements L1, C', Ri and R2 are adjusted so that the impedance at parallel resonance has the value which gives the proper degree oi coupling at the mid-band frequency of the accepted band, and so that the impedance oi net-work 5 changes at the proper rate with increasing or decreasing frequency.

In Fig. 4, I have shown a modification of the circuit shown in Fig. 3. It is to be pointed out at this time that the reference numerals in this figure and the following figures denote the same circuit elements as in Figs. 1 and 3. In Fig. 4, the coupling circuit impedance 5 consists of the coupled inductances L1 and L2 (the inductances being coupled as at M), the condenser C and the resistance R', the condenser, resistance and inductance La being in series with one another.

rihe circuit is otherwise exactly the same as shown in Fig. 3, but is a more convenient circuit than that shown in Fig. 3 from the standpoint of ease of adjustment, it being pointed out that its koperation is exactly the same as in the casc of Fig. 3. The capacities 2, 8 and C' are variable in this modification. In Fig. 5 the coupling circuit impedance 5 consists of two serif-s resonant circuits L1, C1, and L2, C2, the series resonant circuits being connected in shunt. These series resonant circuits are adjusted so that one is resonant at approximately the upper frequency limit of the band, and the other is resonant at the lower frequency limit, while the two in combination are in parallel resonance at the mid-band frequency of the accepted band. In 'this modification, the capacities 2 and 8 are iixed.

In Fig. 6 I have graphically shown the coupling coefficient produced by the coupling circuit in Fig. 5. In this iigure frequencies, as abcissae, are plotted against coupling coeicients, as ordinates, the accepted band being shown as pos- 150 sessing a width denoted between the dotted vertical lines.

Fig. 7 shows an amplifier Vcircuit including fixedly tuned circuits 2, 3 and 7, 8, in which there are provided coupling link circuits,'denoted by 5 and 5", respectively. Circuit 5 is tuned to one frequency limit, and coupling circuit to the other limit of the accepted band.r Link circuit 5 contains an inductance L' and a capacity C' in series, While link circuit 5" con-V tains an inductance L" anda capacity C'I in series. rThe circuit operates in a manner similar to the circuits already described, this particular modification producing low coupling at mid- Each impedance comprises a series resonant ciri cuit tuned to the same frequency as the two tuned circuits 2, 3 and 7, 8, .that is to say, to the mid-band frequency. The impedance 12 is made up of an inductance L in series with a capacity C', while the impedance 13 comprises an inductance L in Yseries with the capacity C".A

At the mid-band frequency, the coupling coils 1G and ll are shunted effectively by resistances R and R", and the effective coupling is reduced to a small value, the amount of reduction being determined by the magnitudes of R and R". At other frequencies above or below the midban i frequency, the series

resonant circuits

12 and 13 have higher impedances, and the effective coupling is increased. The rate at which the coupiing increases as the frequency. departs from the mid-band value, is determined by thev ratio of if: C and L: C".

The coupling systems disclosed herein are particularly applicable to a superheterodyne receiver, or to a transmitter, in which the circuits are left xedly tuned to one frequency. The coupling circuits must be tuned, as well the stage input and output circuits, and there are, therefore, actually three orfour tuned circuits in each stage. Obviously, in the case of a tuned radio frequency receiver, might prove cumbersome. However, in the case of a superheterodyne, or a transmitter for television purposes, the present arrangement might prove to be very desirable because of the extremely wide band which is utilized in a television system.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention by no means limited to the particular organizations shown and described, but that many modifications in the circuit arrangements, as weil as in the apparatus employed, may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. A band-pass amplifier comprising two oscillatory circuits resonant to the same frequency, and a network resonant to said frequency for coupling the two circuits, said network comprising inductive and capacitative reactances of such relative magnitudes; that the coefficient of coupling between said circuits is substantially proportional at any frequency to the difference between-that frequency and the resonant frequency of the tunedv circuits whereby the coupling between the circuits is a minimum at said resonant frequency. 2. A band-pass amplifier stage comprising input and output circuits, two tuned circuits resonant to a desired frequency, a link circuit coupling the tuned circuits, and a parallel resonant circuit inserted in series with the link circuit, tuned to the same frequency as the tuned circuits, said coupling link circuit consisting of inductive and capacitative reactances of such relative magnitudes as to provide minimum coupling between the two tuned circuits at the desired` frequency and maximumcoupling at frequencies 01T resonance.

3. A band-pass amplifier stage comprising two tuned circuits, two link circuits, each independently coupling the tuned circuits, each link circuit tuned by series inductance and capacity to one end of the frequency band which is accepted by the circuits, both said link circuits being in parallel resonance at the mid-band frequency. i

4. A band-pass amplifier comprising two tuned circuits resonant to a mid-band frequency, a link circuit coupling the two tuned circuits, an

Vinduetance coil inseries with the link circuit,

and aseries resonant circuit coupled to the coil in the link circuit and being tuned to said midvband frequency of the accepted frequency band whereby the coupling between the two tuned eircuits is a minimum at said mid-band frequency.

5. A band-pass amplifier circuit comprising auf" pair of tuned circuits resonant to the same frequency, a network resonant to the saidfrequency for coupling said circuits, said coupling network inciuding inductive and capacitative imn pedances ofY such relative magnitudes that the effective value of thek coupling varies according to the frequency, the value of the coupling being small at the resonance frequency of the tuned circuits and relativelyy large for frequenciesfoff resonance. i,

6. In combination, an amplifier having a tuned output circuit, a second amplifier having a tuned input circuit, both said tuned circuits being resonant to a desired mid-band frequency, and a network coupling said tuned circuits, said netg-i` work being resonant to the said frequency and including capacitative and inductive reactances of such relative values that its impedance at mid-band frequency and the rate of change of its impedance for frequencies off the mid-band frequency is such that all frequencies within the band are substantially uniformly amplified.

7. In combination, an amplifier having a tuned output circuit, a second amplifier having a tuned input circuit, both said tuned circuits being resonant to a desired mid-band frequency, and a network coupling said tuned circuits, said network being resonant to the said frequency and including capacitative and inductive reactances of such relative values that its impedance at mid-band frequency and the rate of change of its impedance for frequencies ofi the midband frequency is such that all frequencies withn in the band are substantially uniformly amplifled, said network including a series resonant, path tuned to the upper frequency limit of the band including said mid-band frequency, and a second series resonant path tuned to the lower frequency limit of the said band.

8. In combination, a source of alternating cur,-

rent energy to be amplified, -a tuned-circuit cou-k circuit, a second amplifier having a -tuned -input circuit, both 'said tuned circuits being resonant'V to a desired mid-band frequency; anda network coupling said tuned circuits, said network being resonant to the said frequency and including condensers and inductances of such values that* its impedance at mid-band frequency and the circuits, saidimpedancepath being resonant to rate of change of its impedance for frequencies offl the mid-band frequency is such that allfre quencies within the band are substantially uni` formly amplified.

9. In combination, a source of alternatingcur-v rent energy to be amplified, a tuned circuit coupled thereto, an amplifier having a .tuned output circuit, a second amplifier having a tuned input circuit, both said tuned circuits being resonant to a desired mid-band frequency, and a network coupling said tuned circuits, said network being resonant to the said frequency and including condensers and inductances of such values that its impedance at mid-band frequency and the rate of change of its impedance for frequencies off the mid-band frequency is such that all frequencies within the band are substantially uniformly amplified, said network including a series resonant path tuned to the upper frequency limit of the band including said mid-band frequency, and a second series resonant path tuned to the lower frequency limit of the said band.

10. In combination, a source of alternating current energy of a wide band of frequencies to be amplified, a tuned circuit coupled thereto, an amplifier tube having a tuned input circuit, both said tuned circuits being resonant to the midband frequency cf said wide band of frequencies, and an impedance path coupling 'said tuned circuits, said impedance path being resonant to said mid-band frequency and including capacities and inductances of magnitudes such that the coupling between said tuned circuits is a minimum at said mid-band frequency, and is increasingly greater for frequencies within said band but off the mid-band frequency whereby 'beamplifiei -av tuned circuit coupled thereto, an

lincreasingly greater Vfor Afrequencies within said "allffrequenciesvwithin `the saidvsid'e band are substantially uniformly amplified.

if `1'1'.=In"combination, a source of alternating c'urr'entlenergy of av-wide band of frequencies to amplifier tube having atuned input circuit, both said-tuned circuits being resonant to the midbandvfrequency ofl'said'wide band of frequen- "cies, and an impedance path coupling said tuned said midbandvfrequency and including capacities andinductances of magnitudes such that the 'coupling between said tuned circuits is a minimumatv said mid-band frequency, and is band but offfthe mid-band frequency whereby all frequencies within the` said side band are .substantially uniformly amplified,V said imped- .ance vpath including a series resonant circuit `tuned to the-upper frequency limit of said band 1 and asecond series resonant circuit tuned to the "cessive' screen grid iainplier tubes, means for 100 coupling said source to the input electrodes of the first tube, means for connecting the output electrodes of the secondl tube to a succeeding stage, a coil connected across the output electrodes of the first tube, and a coil connected across the'input electrodes of the second tube, a condenser across each of said coils for resonating the coils to the mid-band frequency of a desired frequency band to be amplified, a coupling network coupled between said coils, said network including a plurality of inductances and condensers having relative magnitudes sufficient to resonate said network to said mid-band frequency and to cause the coupling coefficient between said coils to be a minimum at said midband frequency and to be increasingly greater for frequencies within said desired band but off the mid-band frequency.

RENE A. BRADEN.