US1986331A - Admittance neutralizing amplifier - Google Patents

Description

Jan l 1935. P. T. FARNswoRTH 1,986,331

' ADHITTANCE uEU''RAIIZING AMPLIFIER Filed waren 11, 1929 INVENTR ATTORNEY Patented Jan. 1, 1935 `UNITED STATES PATENT oFFicE n 1,986,331 l ADMITTANCE NEUTRALIZDWG AMPLIFIER. Philo T. Farnsworth, San Francisco, C alif., as-

signor, by mesne assignments, to Television Laboratories, Inc., San Francisco,J Calif., a corporation of California .Application March 11, 1929, Serial No. 346,078 29 Claims. (Cl. 179-171) My invention relates to amplifiers, and its broad purpose is to provide means for neutralizing admittances, and particularly the susceptive component thereof. ,y i r An object of my invention is to provide an amplifier having a flat amplification character-- istic over an extremely wide frequency band.

Another object of my invention is-to' provide a means of neutralizing the/input admittance of an amplifier.

Still another object of my invention is toA provide a cascade amplifier having a single tuning element and of extreme selectivity.

A further object of my invention is to provide anamplier in which the effect of inter-electrode capacities is minimized, and in which such capacity eifects as do obtain are not cumulative. My invention possesses other objects and valuable features, some of which will be "set-forth in the following description. of my invention which is illustrated in the drawing forming part n of the specification. It is to' be understood that I do.v not limit myself to the showing made by the said description and drawing, as I may adopt varying forms of my invention within the'scope of the claims. Referring to the drawing:

Figure l is a circuit' diagram of my invention as embodied in a radio receiver.

Figure 2 is a curve showing the variation of current with potential in the delta circuit of a vacuum tube as used in myvinvention.

Figure 3 is a diagrammatic representation of the equivalent circuits in a single amplifier stage'. In working with extremely high frequencies, one of the principal difficulties encountered is the inherent capacity of the parts of the devices used. Great care is taken in manufacture to make these capacities small, but it is impossible to eliminate them entirely. In vacuum tube amplifiers, the effects of such capacities are particularly troublesome. Such amplifiers to be effective must work into impedances of several thousand ohms. The inherent capacities between the tube elements are shunted across the impedances, thus greatly reducing the' effective impedance into which the tube Works and reducing its amplication. This effect is further complicated by the fact'that there is added to the actual or geometric capacityl of the tube a virtual capacity which is equal to the product of `the grid plate capacity of the tube and its eiective `amplification constant. In'cascade amplifiers, the capacity effects are cumulative, and where an amplifier having n stages used, the

loss in amplification athigh frequencies is equal `to the loss in a single stage carried to the 11.th

power.

By means of my invention the effects just men' tioned are minimized. The effect of input ca- 5 pacity may'be completely overcome. Nov effective capacity is added due to the 'amplification constant of the tube, and the effect of the output capacity may be made negligibly small. Moreover, it is possible by my invention not only to l0 neutralize capacity effects, but also the effects of admittances having any phase or frequency characteristics.

- to the ratio of the potential of the control terminal to the difference of potential between the control and output terminals, the neutralization of the .input admittance will be complete at all frequencies. These amplifying devices may be connected 30 in cascade, the output admittance of each device being neutralized by the succeeding device, and by connecting a variable or frequency selective impedance in the output circuit of the last device so that this device will amplify over only a narrow 35Y band of frequencies, an extremely selective ampli#- er may be constructed, since the input and intermediate circuit admittances will be neutralized only at-the frequencies at which the final device amplies. 40

In the embodiment of my invention which I have chosen for detailed description, the reference character 6 indicates the amplifying device, which in this instance is a vacuum tube of thefour ele- 45 ment, or so-called shield grid type. The tube has a filament '1 which serves as a cathode and is heated by a battery 8. The other electrodes of the tube comprise the control element or grid 9, the anode 11 and a delta electrode l2, so called 5o because it is used to emit secondary electrons or delta rays. It is to be noted that the connection and function of the anode and delta electrodes may be Vinterchanged without producing other than quantative changes in the result, 'that is, 55

'either the shield grid or the the anode.

A potential source 13 supplies the energy for the space current within the tube. The voltage required from this source depends upon the particular tube used. With commercial types of tube, it may be from 200 to 300 volts, but special tubes may be designed, departing radically from this value. The negative terminal of the source 13 is connected to the cathode. The most positive point on the source connects with the anode 11 and the delta electrode is connected to a tap 14, having a potential intermediate between the anode and the cathode. An impedance 16, preferably resistive, is connected between the delta electrode and the tap.

If the tap 14 be moved, varying the potential of the delta electrode, the current flowing through the resistance 16 will vary as shown by the curve of Figure 2. .As the potential of the delta electrode is increased above that of the cathode, the current first increases parabolically as in the ordinary audion tube and as shown in the portion 21 of the curve. A point 22 is soon'reached, however, where further increase in voltage causes a decrease in current, which becomes zero at the point 23 and negative in the portion of the curve 24. At the point 26, the negative current no longer increases, but falls off to the portion 27 and plate may be used as becomes zero when the delta electrode reaches the same potential as the anode.

In the practice of my invention, the tube may be operated with the delta electrode at such a potential as to operate on either of portions 24 or 26 of the curve. When operated on the portion 24, the delta circuit of the tube has a negative resistance and by making this resistance substantially equal to the positive resistance of the circuit element 16, extremely large amplication may be obtained. When operated at this portion of the curve, however, the tube is apt to be unstable,

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and I prefer instead to use the portion 26 of the curve.

When operated at this point of the curve, small variations in delta potential cause no change in current, and the delta resistance of the tube is therefore innite. Physically this means that the current through the delta circuit depends only on the number of electrons striking thesecondary emitter 12, and I have found that in practice this current may be two to three times that owing between the cathode and anode in ordinarycominercial types of tubes.

The current between cathode and anode is controllable by means of the grid 9, as in the ordinary audion. 'I'he delta current changes pro,- portionally to the anode current, and there is of course a voltage drop across the impedance 16 which is proportional to the current owing therein. The change in current in the delta circuit for a unit change in grid potential is the mutual conductance of the tube and is designated ,by

amd. 'I'he effective amplification-p of the tube is equal to the product of amd and the external.'

grid 9, and if the impedance 16 be'made practically pure resistance, lthe delta potential will be exactly in phase with the grid potential. This is the essential feature o f the amplifying devicetol be used with my invention, and any device having this characteristic, of which there are several, may be used.

The input circuit for the tube 6 is shown as comprising an antenna 31 and ground 32 connected to the two terminals of the condenser 33. The grid 9 is biased by a grid leak 34 and grid or C battery 36, which are of about the same values as would be used were the tube used in'a conventional circuit. The grid and delta electrodes are joined by a neutralizing network which in this instance is typified by the condenser 37.

The functioning ofthe neutralizing network depends upon the principle that the effective admittance of a circuit depends upon the current flowing into it when a potential is applied; if no current flows, the admittance is zero; if current does flow the admittance is positive or negative, susceptive or conductive, depending upon the phase of the current owing with respect to the applied potential.

In Figure 3 the potential applied to a tube circuitis represented as derived from the generator 40, which may be any potential source whatever, and as of magnitude e. Applied to the filament terminal F and grid terminal G of the tube, this potential causes a potential E=pe between the terminal F and delta terminal D. The potential E isfrepresented as derived from the drop across the resistance R of the external or output circuit of the tube due to the current from a fictitious generator 41. The internal resistance of the tube is represented by Rd. This method of representwhichare in series and are effectively shunted across the output resistance R, it being assumed that the series impedance is so large as-to" have no material effect on either the magnitude or phase of the output current when so'shunted. It is also assumed that Z and Z' are similar networks, 'differing in magnitude but not in phase or frequency characteristics.

Neglecting for the moment the potential e and considering only the potential E, it will be seen that thispote'ntial divides across the impedances Z and Z' in proportion to their magnitude, the point G assuming a potential intermediate between that of D and F, and by a proper choice of Z the potential of G may be made equal to e. This will be true when In terms of the effective amplification constant p of the tube, the condition obtains when 'i' It will beseen-that when the above equation holds, no current will ow from the generator 40, since its terminals are at the same vpotential as F and G, and hence the admittance y of the tube input circuit is completely neutralized.- That the potential E is .derived indirectly from the potential e does not affect the case. It will further be seen that any component of the input admittance can be neutralized, for we may consider the admittance y as representing the neutralized component, andreplace the generator 40 by a network equivalent to the unneutralized portion of the admittance. As is well known, any impedance network may be represented, at a given frequency, by a plurality of parallel branches. Therefore we may represent an impedance of 5000 ohms as two 10,000 ohm branches in parallel. We may neutralize the admittance of one of these branches, leaving the effective input impedance to the tube-10,000, although actually a single 5000 ohm resistor is used.

The method is perfectly general. We may 4neutralize al1 of the susceptance componenti.

leaving a pure conductive input admittance; we may neutralize any portion of either susceptance, conductance, or both; or we may overneutralize. In the latter case, if the over-neutralized component be conductive, we have regeneration; if

it be susceptive, we obtain an effective susceptance which is 180 out of phase with the neutralized susceptance.

In general, it is preferable to give the neutralizing network the same physical form as the neutralized admittance, parallel networks being utilized to neutralize admittances in parallel, and, series networks to neutralize admittances in series.

It must be kept in mind that the internal capacity between the grid and the delta electrode is a portion ofthe neutralizing network. If the input capacitance be very small it may be necessary to add to it in order to prevent over-neutralization, giving the input circuit an inductive admittance. This inductive admittance differs from that due to an inductive shunt, however, in that it increases with frequency, that is, it is a negative capacitance, and mathematically must be treated as such.

Referring again to Figure 1, the admittance to be neutralized is that of the condenser 33 which.L may be of the order of 1000 mmf. The mutual conductance of the commercial type o'f tube is about 200 micromhos. If the impedance 16 is' made a resistor of 1A megohm value, the effective amplification of the tube will be 50. The condenser 3'7 having a capacity of 20.4 mmf. will therefore satisfy the equation of neutralization given above. It is to be-noted that there is thus included in the output of the tube 6, in parallel with the impedance'l, the capacity of the condensers 33 and 3'1 in series. This capacitance isi also paralleledby the inherent capacitance between the delta electrode 12 and the anode 11.

Coupled to the outputcircuit of the tube 6 by means of the condenser 51 is the grid 52 of a sec.

ond tube 53, of the same general typeqas the tube 6, and having an anode 54 and delta electrode 56.

A grid leakl 57 and C battery 58 are provided for biasing the Vgrid lof this tube.

Connecting the grid 52 and delta electrode 56 is a neutralizing network comprising a condenser 59 in serieswith a condenser 61 and resistor 62 in parallel. .'Ihecondensers 51- and 59 are block ing condensers merely, and are made so large as to have a negligible impedance at the frequencies for which the amplifier is designed.

The delta electrode 56 issupplied with direct potential through a resistor 63 which connects to a tap on the source 13 in the 'same manner as the impedance 16. This lresistor is so chosen as to give the desired amplification constant to the tube 53, which, for the sake of stability and ease of operation, will usually be made lowerthan'that of the tube 6.` The eective value of the condenser 61 is chosen with respect to the amplification of the tube 53 so as to neutralize the output capacitance of the tube 6. The conductance of the resistor 62 neutralizes 'a portion of the conductance of the impedance 16 and grid leak 57 in parallel. By neutralizing this conductance, it is possible to make the resistance of the element 16 fairly low in actual value and still obtain from it the high amplification corresponding with high resistance when the tube 53 is functioning.

Assuming that the output impedance of the tube 53 is 25,000 ohms so that the tube has an effective amplification constant of 5, the condenser 61 will require a capacity of 4 mmf.` to neutralize the 20 mmf. effective in the output circuit of the tube 6, with perhaps an additional mmf. required to neutralize the capacity between the delta electrode 12 and the anode 11. The amplifier as thus far described, using commercial tubes'and without any tuning elements, will give about four times the amplification of a similar amplifier, unneutralized, at a frequency of 1,000,0004 cycles, and with unit vo'tage impressed upon the input. With unit current in the input circuit, the ratio in favor of the neutralized amplifier is much greater, being of the order of 10,000 to 1. By providing a tuned circuit in theoutput of the tube 53, this disparity in amplification maybe made to provide great selectivity in the amplifier.

In the present case, it is contemplated that the tube 53 be used as a detector as well as for neutralizing the capacity in its input circuit. In order to accomplish this,.the C battery v58 is made of such a value as to give the so-called plate detection, the principles of which are well understood in the art. The output circuit of the tube therefore carries u currents of both radio and audio frequencies.

A tuned circuit which comprises a variable condenser 64 paralleled by an inductor 66 is connected in parallel with the resistor 63. This circuit is designed to resonate oven` the band of frequencies to which it. is desired to have the apparatus respond. In order that the tuned circurrent, a condenser` 67 is connected in series therewith. This condenser is chosen to have an impedance which is high in comparison with the impedance 63 at the highest audio frequency to be used, but which-is low for the radio frequencies used:Y Its value is not critical, but in the present instance, 1000 mmf. would be suitable.

At frequencies adjacent its resonant frequency,-

the tuned circuit has an impedance greatly in excess of the resistor V63, which therefore determines or limits the outputimpedance of the tube. Inthis frequency range, the tube is therefore feeding into its calculatedoutput impedance and is effective to neutralize its input admittance.-

is allows the tube 6 also to feed into its proper input admittance and the entire circuit is effective for amplification. It is understood,`of

course, that the capacity between the anode 54- and delta electrode 56, and the effective capacity of the neutralizing circuit are effective in part amplification of the tube 6 and rendering itineffective to neutralize the admittance of the con denser 33./'The effect is similarv to that of the Ordinary type of cascade amplification with resonant circuits in the antenna and inter-tube stages, but the mechanism is entirely different."

The audio frequency component in-the output of tube 53 is passed on to a power tube which is coupled thereto through a large condenser68. This tube has a grid 69 biased through a resistor 71 by means of the C battery '72 and the customary loudspeaker or translating device 73 is included in its output circuit. Many possible modifications of the circuit just described will be apparent to those skilled in th art. AThe tube 53 may be used as an amplifier merely, instead of the combination amplifier and detector. Inthis case, the condenser l67 would be omitted. A detector tube would replace the power tube 70 with the audio frequency-'amplifier following. Another possible modification is to provide a separate amplifier tube which does not lead into any following circuit, but whose sole purpose is to neutralize the output admittance of the preceding tube.

One of the features to be'noted in the described device is the dual function of the resistor 63. This serves not only to supply direct potential to the delta electrode 56 but also to limit the impedance of the output circuit. If this is not done, the impedance o'f the tuned circuit may rise greatly above the calculated value, over-neutralizing the admittance of the input circuit and possibly causing' self-oscillation in the amplifier.

The neutralizing circuit has the described effect only when the phase angle of the output impedances of the tubes are4 small. One of the functions of the resistor 63 is to keep this phase angle at a low value for the frequencies to which the reaction of the tuned circuit is capacitive untill the tuned circuit reactance has fallen so low that the tube practically ceases to amplify, and thus serves further to prevent oscillation of. the tube.

It is to be noted -that the .conditions for self- `oscillation of a tube, used as in the amplifier of my` invention, thru its internal capacities differ from those for a similar condition in the ordinary audion amplifier, in that the reactance of the output circuit must be capacitive instead of fnductive. As in the ordinary amplifier this output reactance must be high.

WhileI have described my amplifier as used with standard commercial tubes, it willbe obvious 'that it may be made even more effective with tubes especially designed for this particular application. For example, the standard tubes have a capacity between anode and delta electrodes .of about 20 mmf. It is quite possible to make such tubes with a capacity as low as 3 or 4 mmf., and

a correspondingly greater effectiveness of neutralization without tuned circuits may be obtained therewith. Moreover, such tubes may be made with the mutual conductance greatly in excess of the 200 micromhos of the commercial tube. One such tube has a mutual conductance of approximately 4000 micromhos. Such a tube will require but 1250-ohms in its output circuit to give an amplification constant of 5. This low value of resistance is decreased only about 61/% at one million cycles with a shunt capacity of' 30 mmf., While a similar shunt capacity across the 25,000 ohms resistance 63 decreases its effective value about 60%. The value of' the special tube is therefore obvious and such tubes are described in my copending-application, Ser. No. 552,339, filed July 22, 1931.

In order that my invention may be of value, it is not necessary that high voltage amplification be used. With tubes of high mutual conductance it may be advantageous to make the effective voltage amplification of the tube approach 1. The neutralizing admittance y' then approaches infinity, and the external resistance r becomes relatively very small. 'I'he advantage in this case is that the input susceptance i's effectively shunted across this small resistance instead of being shunted across the high resistance of the input circuit. In this manner the impedance of such an essentially high impedance device as a photoelectric cell may be balanced against the impedance of a transmission line over a wide band of frequencies and with a great gain in the effectiveness of the phato cell. l

Oneof the important uses of my device is in connection with photo cells even where it is not necessary to match their impedance against that of a transmission line. To beeffective, such a cell entire band of frequencies which the cell is required to pass.

To sum up, the amplifier of my invention`permits the neutralization of the whole or any part of the admittance of its input circuit by means of' a neutralizing network connecting the delta or -output electrode, with the control electrode, the ratio of the neutralizing admittance to the neutralized component being equal to the ratio of the potential of the control electrode to the differ- ,Y ence of potential between the output and control electrodes.

The amplifiers may be connected in cascade, each amplifying device being used to neutralize `the effective output admittance of the preceding device. The effective output admittance introduced by the neutralimng network vis inversely lproportional to the amplification of the system times the input admittance, and by the proper choice of tubes, the decrease in amplification due `to the output admittance component may be made negligibly small. Highly selective amplifiers may be made by including a. tun'ed circuit in the output oi the final tube in cascade, so that the neutralizing effect occurs only at frequencies approaching the resonant frequency of this output circuit. i

Byaproper choice of tubes and neutralizing net` work, amplifiers may be made having substantially uniform amplification characteristics over a frequency band of from zero o several million ycles, or over `any portion `of such a band. By the use of this amplifier, it is also possible to increase the effectiveness of photo-electric cells and similar apparatus and tomatch impedances -between apparatus units and transmission lines to get the maximum possible effectiveness. t The principle of my invention is not limited to linear amplifiers but may also be used in modu transmitting or receiving purposes. Althoughl poses here set forth.

I claim:

1. -An amplifier comprising an amplifying device having a control terminal and an output terminal, the potential of said output terminal-varying in the same sense as the potential of said control terminal, an input circuit connected to said control terminal and having admittance, and a circuit connecting said output and control terminals and having admittance in phase with a component of said input circuit admittance for neutralizingl said component.

2. An amplifier comprising an amplifying de-` vice having a control terminal and an output terminal, the potential of said output terminal varying in the same sense as the potential of said control terminal, an input circuit connected to said control terminal and having admittance, and a circuit connecting said output and control terminals and having admittance in phase with a component of said input circuit admittance, the ratio .of said admittance to said component being substantially proportional to the ratio of the potential of the control terminal to the difference of potential between the control and output ter- 3. An amplifier comprising an amplifying device having a control terminal and an output terminal, the potential of said output terminal varying in the same sense as the potential of said control terminal, an vinput network connected tovsaid control terminal, and a `second network con# necting said output and control terminals and having an admittance'l which varies with fre'- quency in proportion to the variation of an admittance component of the input network.

4. An amplifier comprising anamplifying de*- vi'ce having a control terminal and an output ter'- minal, the potential of said output terminal varying in thesame sense as the potential of said control terminal, an input circuit connected to said control terminal and having admittance, an output circuit comprising substantially pure conductance, and a circuit connecting said output and control terminals and having admittance in 5. An amplifier comprising an amplifying del vice having a control terminal and an output terminal, the potential of said output terminal varying in the same sense as the potential of said control terminal, an input circuit connected to said control terminal and having susceptance, and a neutralizing circuit having susceptance in phase with said input circuit susceptance and connecting said output and control terminals for neutral- Vizing the susceptance of said input circuit.

5 control terminal, an input circuit connected to said control terminal and having susceptance,

and a neutralizing circuit having susceptance in phase withsaid input ycircuit susceptance, the susceptance ratio of said neutralizing circuit and said input circuit being substantially proportional to the ratio of the potential of the control terminal to the difference of potential between said` output and control terminals.

7. An amplifier comprising an amplifying device having a control terminal and an output terminal, the potential of said output terminal varying in the same sense as the potential of said control terminal, an input circuit connected to said control terminal and having capacitance, and a capacitive neutralizing circuit connecting said output and control terminals.

8. An amplifier comprising an amplifying device having a control terminal and an output.

terminal, the potential of said output terminal varying in the same sense as the potential of said control terminal, an input circuit connected tosaid control terminal and having capacitance,

'and a capacitive neutralizing circuit connecting said output and control terminals, the capacitance ratio 8f said neutralizing and control circuits being substantially proportional to the ratio of the potential of the control terminal to the difference ofpotential between said output and control terminals. 9. In an amplifier, a tube having a cathode, an anode, a. control electrode and a delta electrode; an input circuit having admittance and connected to the cathode and control electrodes, an output circuit connected to the cathode and delta electrodes,l and a circuit connected between the delta and control electrodes for jneutralizing a portion of the admittance of said input circuit. 10. `In an amplifier, a tube having a cathode,

an anode, a control electrode and a delta elec- Vtrode; an input circuit having admittance and connected to the cathode and control electrodes,

an output circuit connected to the cathode and delta electrodes, and a circuit connected between the delta and control electrodes and having ad-y lll. An amplifier comprising an input circuit having an admittance, an amplifying device connected to said circuit and adapted to deliver an output voltage in phase with its input and varying with the impedance of its output circuit, an output circuit for said device comprising a variable impedance, and an admittance neutralizing circuit connecting said input and output. circuits.

l2. An `arnpliiier comprising an input circuit having an admittance, an amplifying device connected to said circuit and adapted to deliver an output voltage in phase with its input and varying with the impedance of its output circuit, an output circuit forsaid device comprising a tuned impedance, and an admittance neutralizing circuit connecting said input and output circuits.

13. An amplifier comprising an input circuit having an admittance, an amplifying device connected to said circuit and adapted to deliver an voutput voltage in phase with its input and varying with the impedance of its output circuit, an output circuit for said device comprising a tuned impedance, means -for limiting the impedance of said output circuit, and an admittance neuceptance, an amplifying device connected to said circuit whose output potential varies in the same sense as its input potential, means for varying the effective amplification of said device, and means for neutralizing a portion of the susceptance of said input circuit, said portion varying with the effective amplification of said f device.

@18. An amplifier' comprising a capacitive input circuit, an amplifying device connected to said circuit whose output potential varies in the same sense as its input circuit, means for varying the effective amplification of said device, and a capacitive circuit from the output of said device to supply charging current for the capacitance of the input circuit.

1'7. An amplifier comprising an input circuit having admittance, an amplifying device connected'to said circuit and whose output potential varies in the same sense as its input potential, an output circuit for said device comprising a tuned impedance for varying the impedance frequency characteristic of said circuit, and a network con-A nected to said output and input circuits for neutralizing a portion of the admittance of said input circuit at frequencies adjacent the resonant frequency of said tuned impedance.

18. An amplifier comprising an input circuit.

having .admittance,- an amplifying device connected to said circuit and whose output potential varies in the same sense as its input potential, an output circuit for said device comprising a tuned impedance for varying the impedance frequency characteristic of said circuit, a network connected to said output and input circuits for neutralizing a portion of the admittance of said input circuit at frequencies adjacent the resonant frequency of said tuned impedance, and means for limiting the impedance of said output circuit.

19. A cascade amplifier comprising a plurality of amplifying devices whose output potentials vary in phase with their input potentials, coupling means connecting said devices, an input circuit for said amplifier, circuits for each of said devices for neutralizing a portion of the input admittancel thereto, said portion varying with the impedance of the output circuit of the device, and an output circuit for said amplifier having a variable impedance therein.

20. A cascade amplifier comprising a'plurality of amplifying devices whose output potentials vary in phase with their input potentials, coupling means connecting said devices, an input circuit forsaid amplifier, circuits for each of said devices for neutralizing a portion of the input admittance thereto, said portion varying with the impedance of the output circuit of the de-l vice, and an voutput circuit for said amplifier having a tuned impedance therein.

21. The method of operating an electrical discharge device having a control electrode and a delta electrode adapted to give secondary emission of electrons, which comprises establishing said delta electrode at a potential such that small changes in potential cause no appreciable change in the conductive current thereto, and varying said current by varying the potential of the control electrode.

22. In combination with a vacuum tube having an effective amplification which varies as an inverse function of its output admittance, an output circuit having an admittance greater than that required to give the desired amplification, and means for neutralizing a portion of said admittance. 23. In comb'mation with a vacuum tube having an effective amplification which varies in inverse proportion to its output admittance, an output circuit having an admittance greater than that required to give the desired amplification, and means for neutralizing a portion of said'admittance.

24. In a cascade amplifier, a plurality of vacuum tubes, a coupling network between said tubes having finite susceptance and comprising fixed elements only, and means for neutralizing said susceptance.

25. In a cascade amplifier, a plurality of vacuum tubes, fixed circuit elements having finite susceptance coupling said tubes, and means including a succeeding tube for neutralizing the susceptance of each of said elements.

26. A selective amplifier comprising a nonselective -fixed circuit of finite susceptance, means for impressing the frequencies to be separated across said circuit, and means for neutralizing the susceptance of said circuit at the selected frequency. l

27. In a cascade amplifier, a plurality of vacuum tubes, coupling between said tubes comprising fixed circuit elements only and of finite susceptance at frequencies adjacent the frequencies selected, and means for neutralizing the. susceptance of said circuit at the selected frequency.

28. A selective cascade amplifier comprising a plurality of amplifying tubes, non-selective coupling means between certain of said tubes, a selective output circuit for one of said tubes, and means for aecting the amplifying characteristics of each of said tubes by the operation of said selective circuit, whereby' selective amplication is obtained from the non-selectively coupled tubes.

29. A selective amplifier comprising a nonselective circuit, a selective circuit, an amplifier tube interposed between said circuits, and means including said tube for varying the frequency response characteristics of said non-selective circuit in accordance with the frequency response characteristics of said selective circuit.

PHILo T. FARNsWoRTH.

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