US2008996A - Radio amplifier - Google Patents

Description

C. H. SUYDAM July 23, 1935.

RADIO AMPLIFIER Filed July 27, 1932 FIG.

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IIIIMMMIIL-J I-Ilflllg INVENTOR CLINTON H. SUYDM ,ewza? ATTORN Y @ma Vm Patented July 23, 1935 UNITED STATESl PATENT OFFICE RADIO AMPLIFIER Application July 27, 1932, Serial No. 624,942

2 Claims.

The present invention relates to vacuum tube devices and more particularly to vacuum tube amplier circuits. The invention, while not limited to such use, is especially applicable to am- 5 pliflers which are included in radio transmitters.

The general object of the invention is to connect a vacuum tube for use as an amplifier so that the input and output circuits are coupled together within the tube to a much lesser extent than with present amplifier connections.

Another object of the invention is to procure from a three-electrode tube of usual type, used as an amplifier, results comparable with those procured with a four-electrode (screen grid) tube as usually connected.

A further object of the invention is to provide circuits for a three-electrode vacuum tube amplifer which prevent the setting-up of oscillations and this without using additional neutralizing or balancing circuits.

Ordinarily, vacuum tube amplifiers comprise an input circuit across the filament and grid electrodes of a vacuum tube, and an output circuit across the filament and plate electrodes. The grid is then used as a control electrode and serves to vary the space current in the tube in accordance with potential variations in the input circuit. Since vacuum tubes have inherent capacitances between electrodes, there is a tendency for energy to be fed back from the output to the input circuit, this feed back taking place, when usual connections are employed, through the capacitance existing between the plate and grid and in such phase relationship as to produce oscillations. The phenomenon described above materially limits the usefulness of the device as an amplifier.

Accordingto the present invention the input circuit is connected across the filament and grid electrodes, and the output circuit is connected across the plate and grid electrodes. Due to the physical arrangement of the electrodes in tubes in common use, the capacitance between the filament and plate is a small fraction of that existing between the grid and plate, and consequently a tube connected in accordance with the present invention has a much smaller capacitance between lts input and output circuits. As a result of this smaller capacitance, the energy feed back and, therefore, the tendency of the amplifier to oscillate is greatly decreased.

The invention is described in greater detail in connection with the accompanying drawing and is set forth more particularly in the appended claims.

In the drawing,

Fig.,l is a. usual vacuum tube amplifier circuit shown for purposes of comparison only;

Fig. 2 is a preferred form of my invention wherein a vacuum tube amplifier, connected in 5 accordance with the invention, is coupled to an oscillation generator of conventional design;

Fig. 3 is another form of my invention in which two vacuum tubes perform the function of amplifying an input signal; and

Fig. 4 is similar to Fig. 3 except that the lilament current is supplied from an alternating current power source.

Referring now to Fig. 1, which, as stated above, shows a usual amplifier circuit, the two vacuum 15 tubes I0 and II are, respectively, oscillator and amplifier. In these tubes the elements I2, I5; I3, I 6; I4, Il are, respectively, cathode, grid and anode electrodes. Signals are generated by the tube I0 and its associated circuits in a well 20 known manner and cause potential variations to be impressed on grid I3 at a high frequency rate through the variable coupling capacitance I8. 'I'he grid I3, located between the cathode or electron emitting electrode I2 and the anode I4, 25 controls the flow of electronic current from cathode I2 to anode I4. The current flowing in the output circuit I9 is, therefore, controlled in accordance with the input voltage. With this arrangement the inter-electrode capacitance which 30 couples the electrodes is that between grid I 3 and plate or anode I4. In the common type of tube in present use, this capacitance is relatively large and, as a result, the coupling between input and output circuits may be sufliciently great to pro- 35 duce oscillations. It is obvious, from Fig. 1 and the above description, that no neutralizing or balancing circuits are employed.

Referring now to Fig. 2, which shows a preferred form of my invention, the output of tube 40 I0, which is an oscillation generator identical with that of Fig. 1, is coupled to the input circuit of tube II, which circuit includes cathode I2 and grounded grid I3. The filament or cathode I2 is heated by a filament supply source between 45 which and the filament are interposed the radio frequency choke coils 20 and 2I. These coils are provided to maintain a high input impedance to tube II. Condensers 22 and 23, shown dotted, may be provided to permit tuning the filament 50 supply leads to act as rejection filters to the radio frequency, if necessary.

The electron stream flowing from cathode I2 to anode I4 is now controlled by the radio frequency potentials applied directly to the cathode 65 itself, and the output is, of course, dependent upon such control. When the circuit is connected as here shown. the inter-electrode capacitance which couples the input and output circuits is that between the filament I2 and plate I4. This is much smaller than that between the grid and plate (as in Fig. 1) and, consequently, the tendency to feed back energy is greatly reduced.

Fig. 3 is quite similar to Fig. 2 but here the output circuit of an oscillator or other signalling circuit is indicated at 30 and is coupled to the ampliier by a transformer having primary coil 3I and secondary 32. The amplifier comprises tubes 33 and 34 having cathode, grid and plate electrodes, respectively designated 35, 36; 31, 38; and 39, 40. The input circuit to the first tube 33 is connected across the cathode and grid electrodes 35 and 3l in the conventional manner. The output electrode or anode 39 of tube 33 is connected to the filament 36 of tube 34 and, since the filament 36 is isolated, the space current supplied by the battery B' must necessarily pass through the vacuous spaces of tubes 33 and 34 in series. The radio frequency output circuit comprises parallel reactances 42 and 43. Reactance 43 is the primary winding of the output transformer, the secondary 45 of which is connected to the load. By-pass condenser 44 and choke coil 49 prevent radio frequency currents from flowing in the power source. The isolation of filament 36 is accomplished, as before, by the insertion of radio frequency choke coils, here designated 41 and 48.

With these connections the grid 38 between filament 36 and plate 40 of tube 34 functions in a manner similar to that of the screening electrode in a conventional four-element screened grid tube and prevents energy feed back from the output circuit 43 to the input circuit 32. Grid 31 of tube 33 acts as a control electrode influencing the electron stream in accordance with variations in the input circuit 32.

Fig. 4 is essentially the same as Fig. 3, the

only diierences bting that the source of heating current for the tube filaments is here shown as alternating current which is supplied from the mains through transformers I and 52; and the fact that grid or screening electrode 38 is connected to ground through blocking condenser 50. Condenser 50 permits the maintenance of grid 38 at a constant positive direct current p0- tential supplied from a source B through a filter choke 53. This direct current potential, applied to the screening electrode, reduces the tube impedance and is a well known device needing no further description. It is to be noted that a direct current potential may be applied to the grid I3 of Fig. 2 in the same manner.

The foregoing description of the various circuits shows that by this invention the power output of a given tube may be considerably increased. since the input potential may be increased without causing oscillation and this without employing neutralizing circuits.

The above is given for purposes of description only and Is in no way to limit the scope of the Y electron tube having anode, cathode and grid` electrodes. means for heating said cathode, an input circuit connected between said cathode and grid to vary the potential of said cathode at a high frequency rate, and radio frequency choke coils between said cathode and said heating means to maintain a high input'impedance to said tube.

CLINTON H. SUYDAM.

DISCLAIMER 2,008,996.-Olz`nton H. Sag/dam, East Orange, N. J. RADIO AMPLIFIER. Patent dated July.23, 1935. Disclaimer filed February 23, 1938, by the assignee, International Standard Electric Corporation. t

Hereby enters this disclaimer to claims 1 and 2 of said Letters Patent, except als these claims are limited to a non-oscillating amplifier as set forth in the description of said Letters Patent.

[Ocal Gazette lllarch 22, 1.938.]

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