US1998482A - Circuit scheme adapted to preclude spurious couplings in radio frequency amplifiers - Google Patents

Description

Apmfi 23, E. ZEPLER CIRCUIT SCHEME ADAPTED TO PRECLUDE SPURIQUS COUPLINGS IN RADIO FREQUENCY AMPLIFIERS Filed Sept. 27, 1955 INVENTOR EN CH Z E PLEFR ATTORNEY Patented Apr. 23, 1935 UNITED" STATES I CIRCUIT SCHEME ADAPTED TO PRECLUDE SPURIOUS COUPLINGS IN RADIO 'FRE- QUENCY AMPLIFIERS Erich Zepler, Berlin, Germanmassignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application September 27, 1933, Serial No. 691,129 In Germany September 20, 1932 2 Claims. 5 (Cl. 179171) The present invention relates to amplifier circuits and more particularly to means for preventing undesired coupling between the various stages of the amplifier. An object of the present 5 invention is to, so to speak, separate, as regards radio frequencies, the cathodes or heater wires of the various tubes.

In the drawing:-

Fig. 1 illustrates, in diagrammatic form, a circuit arrangement which will be referred to hereinafter for purposes of explaining the operation of the invention, and

Fig. 2 is a circuit diagram of certain portions of an amplifier, incorporating the preferred embodiment of the invention.

In the case of receivers having a very high radio frequency amplification factor the optimum mode of construction is to dispose each tuned circuit with the tube pertaining thereto within a distinct metallic box and to join these boxes with one another single-pole-fashion. But such a form of construction is costly for constructional reasons, and moreover it is unstable in a mechanical respect. Instead of using individual boxes a more recommendable plan is to employ a joint casing furnished with partitions. But such a design is impossible because the shortcircuit currents set up by the coils inside the casing would enter into mutual coupling relations. It is customary to accommodate the rotary condensers inside such a metallic casing. While such a construction still satisfies generally present day requirements, it is attended with difficulties whenever D. C. separation of the easing in reference to the cathode is demanded.

Fig. 1 illustrates a pair of thermionic tubes, provided respectively with tuned circuits l and 2. The common cathode circuit of the two tubes is united to the shielding casing by means of a condenser Ck.

The couplings then arising can be seen from Fig. 1. The grid points of the tuning circuits I and 2 (especially the stators of the rotary condensers) present an appreciable capacitance against the casing. The ensuing capacitive casing currents have their return by way of the grounding condenser Clc'of the casing, in other words, they thus occasion capacitive coupling.

Now, according to the present invention the cathodes of the tubes to be balanced out or neutralized are separated from one another as regards radio frequencies while each one thereof is separately united with the casing. (Fig. 2.) Where short waves are concerned, the impedance residing in the connection between casing and cathode will suflice to create disturbing or spurious couplings; hence, the invention will become of added value whenever no D. C. separation of the casing from the cathodes is required.

Fig. 2 also illustrates a pair of thermionic tubes, provided respectively with tuned circuits I and 2. The cathode end of the tuned circuit I is connected to the end of the shielding casing through the condenser Ck while the cathode side of the tuned circuit 2 is connected to the casing through a separate condenser Ck. The cathode circuits of the two tubes are united by means of the two impedances shown. It will be seen from the above that in a system such as shown in Fig. 1, the single condenser connection between the cathodes of the two tubes and the shield causes high frequency currents in one of the circuits to be transferred to the other circuit through the common cathode connection. Thus, in a system such as shown in Fig. 1, a high frequency loop circuit is formed between the grid of the first tube and the shield due to the inherent capacity between the tube circuit and the shield. The remaining portion of the loop circuit comprises the condenser Ck and the cathode of the tube. The same thing is true of the second tube. Hence, there result two loop circuits having a common portion Ck, which means that there will be undesired coupling between the two loop circuits. In accordance with the present invention, each cathode is connected to the shield through an individual condenser and the common cathode connection is provided with chokes. Thus, the high frequencies are, in effect, isolated in each circuit, since each circuit forms a complete loop initself and the chokes act to prevent the transference of energy from one to the other through the common cathode circuit.

I claim:

1. In a circuit arrangement of the kind described, at least two electronic tubes each thereof being provided with input and output circuits,

a connection common to the cathodes of said and the capacity connection between the shielding means and the cathode, a radio frequency choke in said common cathode connection whereby high frequency currents are prevented from being transferred from one device to the other thereof through the common cathode connection.

2. In a cascaded radio frequency amplifier a pair of electron discharge tubes each thereof being provided with tuned input circuit, a shielding means for said amplifier, a single cathode circuit for both said tubes, a connection including a condenser between the cathode of one of said tubes and the shielding means, a connection including a condenser between the cathode of the other tube and the shielding means and means including a radio frequency choke coil in the single cathode circuit between the cathodes of the two tubes for preventing the transference of .high frequency energy from one of the tuned circuits to the other thereof through the single cathode circuit.

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