US2289091A - Thermionic tube amplifier - Google Patents

Description

Jufiy 7, 1942. EL 2,89,

THERMIONIC TUBE AMPLIFIER Filed Jan. 4, 1.941

INVENTOR D. 7'. BELL 5V A T TOR/V5 V Patented July v7, 1942 THERMIONIC TUBE ADIPLIFIER- Delamar '1. Bell, Chatham, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application January 4, 1941, Serial No. 373,086

12 Claims.

This invention relates to multistage thermionic tube amplifiers and more particularly to interstage networks 'for coupling together two stages in such amplifiers.

The principal objects of the invention are to. reduce the cost and size of interstagenetworks and to increase the range of transmission characteristics obtainable.

A multistage thermionic tube amplifier requires one or more interstage networks to couple the 'output or plate circuit of one tube to the input or gridcircuit of the next succeeding tube. The chief function of suchnetworks is to provide the'required transmission characteristic for the amplifier. It is also important that the networks be economical as to both cost and space requirements.

'I he interstage network of the present invention isic'apable of providing a wide variety of desirable transmission characteristics in a comparatively simple structure which is reasonable in cost and small in size. The network includes two inductively coupled inductors connected in either the series aiding or the series opposing relationship in a shunt impedancebranchbetween the two tubes to be connected. An impedance branch, usually including a capacitor and in some cases a resistor, may be connected in shunt with one or with each of the inductors. Another capacitor may be connected in shunt with both 30 of the inductors. .The required capacitance may g be partly or wholly furnished by the stray capacitances associated with the component elements and the wiring of the amplifier.

The two coupled inductances may be furnished by two separate inductors. To save cost and space, however, a single tapped winding or two windings on the same core may be used. Furthermore, a change in the coefficient of coupling between the inductors, or in the sense ofthe connection, efiects'a' change in the values of the componentimpedance elements constituting the network and in practice, these factors are so chosen that themost advantageous values are secured. This is another important factor in reducing the size and cost. In some caseaby the use of coupled inductors, there may be provided transmission characteristics which are otherwiseunderstood from the following detailed descrip-, tion and by reference to the accompanying drawing the single figure of which is a schematic circult of. a multistage thermionic tube amplifier comprising two stages coupled by an interstage network in accordance with the invention- The amplifier has a pair of input terminals I,

2 upon which are impressed the signals to be amplified and a second pair of terminals 3, 4

to which is connected a load impedance 5 which may include the input circuit of a succeeding stage. The amplifier includes a thermionic tube 6 having a cathode I, a grid 8 and a plate 9 and a second tube I0 having a cathode H, a grid l2 l5 and a plate l3. The plate voltage, supplied by the,battery I5, is impressed on ,thetwo tubes in series. It-is necessary, of course, that the load 5 have a direct current path for the plate current of the tube In. The amplifier may be grounded, if desired, as indicated at the point 20.

The tubes 6 and III are coupled by an interstage network comprising the shunt impedance branch connected between the point IS, in an \electrical path between the plate 9 and the grid '25 I2, and the point ll, in'an electrical path between the two cathodes l and II. The network includes the series connected inductors L1 and L2 which, in the example shown, are in a path common to the output circuit of the tube 6 and the 5 value of which may be found from the expression the wiring, and likewise the capacitance of thecapacitor C: may be partially or entirely supplied unattainable. This is due to the fact that the 'inherentstray admittances associated with the circuit may be successfully incorporated'into the interstage network, whereas without using inductive coupling these admittances would be tdo. large for inclusion.

The nature of the invention will be more fully where K is the coefiicient of the coupling. The inductor L1 is shunted by a capacitor C1 and the .inductor L2 is shunted byan impedance branch Z which includes a capacitor C2 and-a resistor R2 connected in parallel. I

The capacitance of the capacitor 01 may be furnished in whole or in part by the stray capacitance associated with the inductor Li, including by the stray capacitances associated" with the inductor L2, the impedance branch Z and the. wiring. The capacitor C3 has a comparatively large capacitance and its function is to block the fiowof direct current while freely passing all/ frequencies" within the band of interest. In' an+ alyzing the performance of the'interstage network it may, therefore, be considered that the points I! and III are common. Thecapacitance of the capacitor C4 includes the stray capacitance oi the remainder of the network connected between the points l8 and H, the capacitance between the high side wiring and the ground point 20, the capacitance between the cathode I and the plate 9 of the tube 8, the capacitance aaeaoei 1100 ohms, which is the proper value to provide the required grid bias for the tube when the The resistor B may be made variable. as indicated by the arrow, to allow an adjustment of this voltage. In some cases, if the direct current resistances of the inductors L1 and L: are given the proper values, the resistor R may be omitted. It is also obvious that the resistor R will be Just as effective for voltage control it placed in series with the inductoraas, for example, it connected between the points I! and 2|.

The inductors L1 and La may be connected in either the aiding or the opposing relationship and the coeiiicient of coupling may have any value up to and including unity. If the series aiding connection is used, as K is increased in value the number of turns of wire required for the inductors is reduced and as a result the size and cost are decreased and better coil constants are secured. Furthermore, the required impedance of the branches shunting the inductors is decreased. On the other hand, if the series opposing connection is employed, the required battery I5 is 150 volts; and therefore the resistance R may be omitted.

What is claimed is:

1. In a multistage amplifier comprising a pluralit of thermionic tubes each having a cathode, a grid and a plate, an interstage network tor coupling two of said tubes comprising an electrical path between the plate of one of said two tubes and the grid of the other of said two tubes. a second electrical path between the cathodes of said two tubes and a third electrical path connected between a point in said first-mentioned path and a point in said second path, said third path including two inductively coupled inductors connected in the series opposing relationship and an impedance branch connected in shunt with one of said inductors.

2. In a multistage amplifier comprising a plurality of thermionic tubes each having a cathode, a grid and a plate, an interstage network for coupling two. of said tubes comprising an electrical path between the plate of one of said impedances of the shunt branches are increased.

In practice the sense 01' the connection and the value of K are chosen to provide a network which is the most satisfactory as to cost of component elements and size.

It is to be understood, oi course. that the impedance branches shunting the inductors Li and La, represented in the figure respectively by C1 and Z, may have any degree of complexity. The

number and type .of the component elements in' each of the branches, and theirdisposition, depend only upon the transmission characteristic to be provided by the network. The configurations shown are. to be considered only as an illustrative example.

The values of the component elements required for a typical interstage network designed in accordance with the invention will now be presented. The network has the circuit shown in the figure and is capable of providing a sub-' The inductors L1 and 1e are connected series two tubes and the grid of the other of said two tubes, a second electrical path between the cathodes of said two tubes and a third electrical path connected between a point in said firstmentioned path and a point in said second path,

ode, a grid and a plate, an interstage network for coupling two of said tubes comprising an electricalpath between the plate or one of said two tubes and the grid 01' the other of said two tubes, a second electrical path between the cathodes of said two tubes and a third electrical path connected between apoint in said iirstmentioned path and a pointv in said second path,

aiding and the coeiiicient of coupling K is 0.73.

The value given for C4 is the averagepver the frequency range of 10 to 72 kilocycles. The sum 01' said third path including two inductively coupled inductors connected in series, an impedance branch including the parallel combination of a capacitance and a resistance connected in shunt with one of said inductors and a second capacitance connected in shunt with both of said inductors. I

' 6. A network in accordance with claim 2 in which said inductors are connected in the series aiding relationship.

7. A network in accordance with claim 2 in which said inductors are connected in the series opposing relationship.

8. A'network in accordance with claim 2 which includes a third capacitance connected in shunt with both of said inductors, and in which said inductors are connected in the series opposing relationship.

10. A network in accordance with claim 5 in which said inductors are connected in series aiding relationship.

11. A network in accordance with claim 5 in which said inductors are connected in the series opposing relationship.

12. In a multistage amplifier comprising a plurality of thermionic tubes, each having a cathode, a grid and a plate, an interstage network for coupling two of said tubes comprising two between the cathode and the grid of other of said tubes, an impedance branch including a capacitance connected in shunt with one of said inductors, a second capacitance connected in shunt with the other of said inductors, a third capacitance connected in shunt with both of said inductors and a resistance connected in said inductively coupled inductors connected in series 10 input circuit in series with said inductors.

in an electrical path which is common both, to

DELAMAR T. BELL.

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