Amplifier circuit

April 25, 1939.

L. M CRAFT

AMPLIFIER CIRCUIT

Filed Dec. 26, 1935

2,155,404

Sheets-Sheet 1

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April 25, 1939.

L. M CRAFT

AMPLIFIER CIRCUIT

Filed Dec. 26, 1935

2,155,404

Sheets-Sheet 2'

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Patented Apr. 25, 1939

2,155,404

UNITED STATES PATENT OFFICE

2,155,404

AMPLIFIER CIRCUIT

Liva Morgan Craft, Cedar Rapids, Iowa, assignor, by mesne assignments, to Collins Radio Company, a corporation of Iowa

Application December 26, 1935, Serial No. 56,266 .

Claim. (Cl. 179—171)

My invention relates broadly to electrical amplifier circuits and more particularly to an arrangement of amplifier circuit for eliminating or reducing retroactive currents due to capacity cou5 pling between electrodes of the electron tubes.

One of the objects of my invention is to provide means for overcoming or neutralizing the effect of capacity coupling between electron tube electrodes to the end that a greater amplification .10 of input power may be obtained without resulting instability of operation.

It is a well known fact that in the three element vacuum tube, such coupling cannot be avoided due to the mutual capacity of the ele15 merits of the structure. According to my invention, I induce a voltage in the output circuit of the tube due to mutual induction between the input and output circuits in such a manner as to compensate for the current which flows be-^i! tween the grid and plate due to this mutual capacity.

Another object of my invention, therefore, is to provide means for inducing a voltage in the plate circuit of a three-electrode amplifier tube ²⁵ to produce a current in opposition to the current passed by the inter-electrode grid-plate capacity in the amplifier tube, whereby the effect of the grid-plate capacity coupling is neutralized.

A further object of my invention is to provide -³⁰ means for inducing a voltage in the plate circuit of an amplifier tube from a current in the grid circuit by mutual induction, the resulting current in the plate circuit being effective to neutralize the current due to the grid-plate capacity ³⁵ coupling in the amplifier tube.

Still another object of my invention is to provide inductances in the grid and plate circuits of a three electrode amplifier tube coupled in such a manner that voltage induced in the plate in40 ductance is connected with the plate electrode in the same polarity as the signal voltage impressed upon the grid electrode, that is, in a counterregenerative sense; the current resulting from th induced voltage being effective to neutralize 45 the current due to the grid-plate capacity coupling in the amplifier tube.

A still further object of my invention is to provide means for varying the mutual inductance of inductances connected in the plate and grid cir50 cults of an electron tube as set forth in the preceding paragraph, the mutual inductance being varied simultaneously with the tuning of the plate or the grid circuit, whereby the neutralization is complete at all frequencies received.

My invention will be more fully understood from the specification hereinafter following by reference to the accompanying drawings, in which:

Figure 1 is a schematic circuit diagram of a three element vacuum tube connected as an am- 5: plifier with mutual inductance between input and output circuits in accordance with my invention; , Fig. 2 is a simplified diagram of the impedance elements of the alternating current network illustrated in Fig. 1; Fig. 2a is a theoretical graph employed in. connection with Fig. 2 to illustrate the theory of operation of the system of my invention; Fig. 3 is a circuit diagram of a modified form of my invention; Fig. 4 is a circuit diagram of a further modified form of my invention; and W Fig. 5 illustrates one form of mechanism which may be employed in connection with the system shown in Fig. 3 or Fig. 4, for varying the inductive relation of a pair of inductance coils.

The circuit arrangement of my invention may 20 be applied to amplifiers having various types of input circuits, as, for example, an untuned transformer secondary, or condenser feed to grid with means for supplying proper bias. I may use a tuned circuit Ci—Li connected to the input of 25 the amplifier as shown in Fig. 1 with advantage. Inductance L2 and condenser C2 form the output circuit, with mutual inductance M between input and output, with a coefficient of coupling substantially less than unity and in a positive sense. :30 By coupling in a positive sense, I mean that species of coupling between the two coils which, if acting alone, would cause the plate p and grid g to be at the same polarities with respect to the cathode c of the amplifier electron tube T. The 35 grid-plate inter-electrode capacity is represented at Cgp in Figs. 1 and 2.

If a current Ii is assumed flowing in Li, then referring to Fig. 2, Ei is equal to IiwLi and

Ε₂=-/ιωΜ, ⁴⁰ where Ei and E2 are the voltages acting around the circuit in a clockwise direction. Assuming temporarily that Ca is replaced by a short circuit, the current through the short circuit due to the 45 voltage Ei will be

Ic_m)=EwCgp=Iia²LlCgr>

which leads Ei 90° in phase. Also, the current through the short circuit due to the voltage Ea will be _r _ E₂ _IiM ^Li~uL₂~ L₂ which lags E₂ 90° in phase. The voltages Ei and E2 are 180° out of phase due to the fact that Ea 55

2,156,404 is generated by mutual induction from Ei. According to my invention, however, the terminals of the Inductance L2 are so connected in circuit, in relation to the voltage Ei, that, as set forth 5 above, the same polarity is applied to the grid and plate electrodes. Referring to the assumed short circuit across C2, the polarities of the applied voltages Ei and E2, are, therefore, alike; but because of the fact that the current Ic_OT leads 10 Ei by 90° and the current Il₂ lags E2 by 90°, the currents produced by the two voltages are in opposition.

The current in the short circuit, then, is Ic_m—II., and will be zero when Ic„=Il,, or ¹⁶ M—a²CgpLiL₂.

ω=2ττ times the frequency of Ii.

The above description can perhaps be better understood by reference to Fig. 2α which indicates theoretically the phase relations of the volt²⁰ ages and currents considered with respect to the assumed short circuit, the curve representing each being clearly designated on the drawings, in connection with Fig. 2α. Line x—x gives, at its^;iritersection with the several curves, theoretical in²⁶ stantaneous values of the various currents and voltages, and indicates that the- currents lc_op- and Il₂, as represented, are equal· arid flowing in opposite directions. The resultant current, or ₃₀ that which would flow in the short circuit assumed, is, therefore, zero.

As there will be no' current in the short circuit, it may be replaced by any value of C2 and no voltage will appear across it and no circulat_3g ing current flow in L2—C2. This neutralization is complete at only one frequency but gives a workable amplifier over a two to one frequency range.

In order to increase the effectiveness of the neutralizing system of my invention over a wider frequency range, an arrangement such as that shown in Fig. 3 may be employed. The circuit illustrated diagrammatically in Fig. 3 is substantially identical with that shown in Fig. 1 and _4g comprises an input circuit including inductance Li and condenser Ci connected with the grid g, and an output circuit including inductance L2 and condenser C2 connected with the plate p, of the amplifier electron tube T which also includes ¢0 the cathode c. The inductances Li and L: are coupled, as in the circuit shown in Fig. 1, with a mutual· inductance of M, which, however, in the system illustrated in Fig. 3, is variable and controlled simultaneously with the tuning of the out55 put circuit through the action of a cam element e. From the equation M==a²C_ffPLiL2, where u=2vf, it is clear that if the mutual inductance M is varied in proper proportions as the frequency f is varied, neutralization is effected over the en5® tire frequency range covered. Also, according to my invention, the mutual inductance may be varied simultaneously with the tuning of the input instead of the output circuit.

JU In- Fig, 4, I indicate a further modification of the system of my invention wherein portions only of the inductance in the input and output circuits are coupled. Inductances li and Is are coupled, with mutual inductance M, and the in70 ductance relation thereof is variable in connection with the variation of the tuning of the input circuit (or the output circuit) through the cam element e'. The tuning in the modified circuit of Fig. 4, Is effected by varying both the induc75 tance and the capacitance of the circuit: Li arid

Ci in the input circuit; and L2 and C2 in the output circuit.

Fig. 5 illustrates a mechanism adaptable to the systems shown in Figs. 3 and 4 for varying the inductive relation of a pair of coils in connec- 5 tion with a tuning element such as a variable condenser. The mechanism shown in Fig. 5 comprises a pair of inductance coils ii and is arranged in variable inductive relation, the coil ii being movable with respect to the coil is. The coll ii 10 is mounted on a support d, connected with a bar a which is slidably mounted in supporting guides bi and bs. At the lower end of the support d is a pin or roller r engaging the eccentric slot e in the rotatable eleriient m. A variable tuning 15 condenser C has its shaft s connected through a coupling k and a flexible drive shaft f to the rotatable element m, so that as the tuning condenser is varied the inductive relation of the coils ii and is is varied in accordance with the 20 contour of the eccentric slot or cam slot e. It is noted from the equation M=a²CgpLiLs that M must vary in proportion to the square of the frequency; and the contour of the eccentric slot e is devised to provide for such relative variation 25 of the mutual inductance. The slot e shown in Fig. 5 is merely illustrative of the position and function of the slot in the mechanism, and is not an accurate trace of the particular form of slot adaptable to the system of my invention. 30

The circuit arrangement of my invention has numerous advantages among which are the inexpensive assembly possible by use of the circuit and the simplicity of the circuit. As it is ordinarily necessary to shield the input and output 35 circuits, this is. eliminated by using this coupling in a predetermined manner. The circuit of my invention, also has the advantage that there is no tapping of the coils required, nor are neutralizing condensers necessary. Also, there is less loss in the circuits due to lack of shielding and due to the fact that coils having opposite ends at high radio frequency potentials from cathode and ground have larger dielectric losses than single ended coils. 45

My invention extends to all arrangements in which there is mutual inductance between the input circuit and the output circuit in such a manner as to secure the advantages already stated and explained. The invention is not limited to the specific circuit details illustrated in the accompanying drawings, as mutual induction may occur only between portions of Li and L2, as suggested in Fig. 4.

The circuit arrangement of my invention is ₅₅particularly adapted for eliminating or substantially reducing electrode capacity effects in transmitting amplifiers- without the employment of auxiliary neutralizing circuits which add cost and complication to the equipment. go

While I have described my invention in one of its preferred embodiments, I desire that it be understood that modifications can be made and that no limitations upon my invention are intended except as may be imposed by the scope of 05 the appended claim.