US2085407A - Amplifying device - Google Patents

Description

AMPLIFYING DEVICE Filed Dec. 51, 1954 INVENTOR. W. S. BARDEN /0 2'0 To BY )fde q ATTORNEY Patented June 29, 1937 PATENT OFFICE AMPLIFYENG DEVICE William Stoddard Barden,

Grasmere, Staten Island, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 31, 1934, Serial No. 759,856

6 Claims.

An object of this invention is to provide a novel amplifying arrangement which is adapted to amplify radio frequency currents or voltages which cover a fairly wide range of frequencies substan- 5 tially equally.

A second object of the invention is to provide a novel coupling arrangement which is especially adapted to couple a circuit tunable over a wide range of frequencies to the grid and cathode of 10 an electron discharge tube.

' A still further object of the invention is to provide a novel form of voltage divider which is especially adapted for connecting across the terminals of a tuned circuit whose tuned impedance increases rapidly as its resonant frequency is increased.

Other and further objects of the invention will become apparent to those skilled in the art as the description thereof proceeds. For a better understanding of the invention however, refer ence is made to the following specification taken in connection with the accompanying drawing, in which;

Figures 1 and 2 are schematic circuit diagrams showing the invention used in a radio frequency amplifying arrangement,

Figure 3 is a circuit diagram equivalent to a portion of Figure l and illustrating the principle of operation of the invention and,

Figure 4 shows curves illustrating the advantages arising from the use of the invention.

Referring now to Figure 1 of the drawing, a substantially aperiodic amplifying arrangement is shown in which an aperiodic antenna circuit I includes a high resistor 2 which is connected to the grid 9 of the amplifying tube T through a coupling condenser 9. The grid 9 and cathode B of the tube are shown connected through a leakage resistance 4 and a circuit 5 which may in- 40 clude any known form of automatic volume control means. A normal bias may be maintained on the grid 9 by means of the plate current drop in the bias resistor 6 which is shunted by a by-pass condenser I. The tube T is shown as having a screen grid I0 but it will be understood that my invention may be used in connection with triode tubes also. The plate II of the first stage is shown connected to the positive terminal of the B battery I9 through an inductance coil :2 which may be tuned to the input signal frequency by the variable condenser I9 connected across its terminals AB, the terminal B being connected to ground through the by-pass condenser 29. Terminal A is shown connected to the grid 9 of the second stage through a condenser I l and the grid 9 and cathode 8 of the second stage are connected through an inductance coil I5 and a circuit H which may include any known form of automatic volume control means. As shown, a capacity I6 is shunted across the coil i5 and according to my invention these elements are so proportioned that the circuit comprising elements I5 and I6 is resonant at or close to the lowest frequency of the range to be amplified or, in other words, the lowest frequency to which the circuit I2-I3 is resonant.

As shown in the drawing, the capacity It is a physical condenser but this capacity may be the capacity between the grid and cathode of the tube together with the self capacity of the coil l5 and/or the wiring associated therewith, so that when the self capacities associated with the coil I5 are of the proper value to make the resonant frequency of the coil substantially equal to the lowest frequency of the range to be amplified, a physical condenser need not be used for the capacity I6. The plate II of the second stage may be connected to any suitable load circuit which may be the grid circuit of a third amplifying stage or the tuned input circuit of a detector stage by the same coupling arrange- 2 ment which I have shown between the first and second stages.

In the modification shown in Figure 2 the plate of the first stage is connected to the positive terminal of the battery I9 through the primary coil I8 which is inductively coupled to the sec ondary coil I2 tunable to the incoming signal frequency by the condenser I 3. The other elements I4, I5, I6 of the coupling circuit to the grid of the second stage are arranged the same as in Figure 1. The coupling arrangements between terminals AB and the grid and cathode of the second tube shown in Figs. 1 and 2 may be re-drawn and the equivalent circuit shown as in Fig. 3 in which the circuit I5, I 8 is replaced by a condenser C in series with a resistor R so that these elements together with the condenser I l provide a voltage divider arrangement connected across the terminals AB, the portion of the voltage developed across 0' and R being I impressed on the grid 9 of the second stage. It will be understood that the resonant circuit I5- it may be tuned to a frequency slightly below the lowest frequency of the range to be amplified or it may be tuned to a frequency within said range and near the lower end thereof. As shown in Fig. 3, the aforementioned voltage divider arrangement is obtained by effectively realizing a capacity C in series with condenser I4. The signal voltage developed across A, B is divided up between I4, C and R. C and R depend upon the frequency and in order to realize C at all the signal frequency must be above the parallel resonant frequency of 15-46. fore l5 and I6 are chosen to be parallel resonant at substantially the lowest frequency of the signal band. The capacity M acts entirely as an element in the voltage divider and has substantially no effect on the resonant frequencies of either circuit I2l3 or circuit l5-|6. When amplifying frequencies within the band between 10 and 30 megacycles, I have found the following circuit values to be suitable:

Capacity of condenser l4 equals about 5 micromicrofarads, capacity of condenser l6 equals about 10 micromicrofarads, inductance of coil l5 equals 25.3 microhenries. These values give the circuit l5-l6 a resonant frequency of substantially 10 megacycles. At this frequency the resistance of coil 55 equals approximately 32 ohms, and the circuit l2-i3 has a tuned impedance of substantially 6000 ohms. Also at this frequency the tuned impedance of the circuit I5-l6 is substantially a pure resistance having a value of substantially 79,800 ohms. This resistance is approximately 13 times the impedance of the circuit l2l3 and obviously may be disregarded as far as concerns its effect on the signal voltage developed by the first stage T across the points AB, i. e., across the terminals of circuit l2l3. The voltagedividing action involves the impedance existing between the points M and N of Fig. 3 in comparison with the impedance of the condenser I4. At a frequency above 10 megacycles, the value of R, is so small in comparison with the impedance of the condensers M and C that it may be disregarded. By calculation, the value of the equivalent condenser C at 20 megacycles has been found to be equal to '7 .6 micromicrofarads and at 30 megacycles its value has been found to be equal to 8.9 micromicrofarads.

It is thus apparent that my voltage divider arrangement is substantially equivalent to the series connection of two condensers connected across the resonant circuit, the value of one of these condensers l4, remaining fixed while the effective value of the other increases as the resonant circuit is adjusted to higher ferquencies. The net result of this arrangement is that while the voltage across the terminals A and B increases linearly with frequency as shown by curve D of Figure 4, the voltage across the points M and N remains substantially constant due to the automatic action of my voltage divider arrangement. The voltage across the points M and N is shown by the curve F of Fig. 4 which shows a small decrease near the lower end of the frequency range, nevertheless it shows that the output voltage is much more constant than that shown by curve D and for all practical purposes may be regarded as having a constant value.

Having described my invention what I claim as novel and desire to secure by Letters Patent is:

1. In combination, a resonant circuit comprising a coil and condenser connected in parallel and tunable through adesired range of "signal frequencies, means for impressing signal voltages on the terminals of said circuit and a voltage divider arrangement connected effectively across the terminals of said coil and comprising the series connection of a condenser and a parallel resonant circuit having a resonant frequency Theresubstantially the same as the lowest frequency of the range to be amplified.

2. In combination, a parallel resonant circuit tunable through a desired range of signal frequencies, means for impressing signal voltages across the terminals of said circuit, a vacuum tube having an input and an output circuit, said input circuit comprising a parallel resonant circuit having a resonant frequency substantially equal to the low-est frequency to be amplified and a condenser having a small capacity connected between the high potential terminals of said resonant circuits.

3. In combination, a parallel resonant circuit tunable through a desired range of signal frequencies, means for impressing signal voltages across the terminals of said circuit, a vacuum tube having a grid and cathode, a circuit comprising the series connection of two condensers connected across the terminals of said parallel resonant circuit, a conductive connection between one terminal of said resonant circuit and the cathode ofsaid tube and a conductive connection between the common terminal of said condensers and the grid of said tube.

4. In an amplifying arrangement, the combination of two stages, each comprising a vacuum tube having a cathode, a control gridand a plate, means for impressing incoming signal voltages including a desired band of frequencies across the grid and cathode of said first stage,

stage and the grid of said second stage and a circuit connecting the grid and cathode of said second stage, said circuit comprising a coil and condenser connected in parallel and having a resonant frequency substantially the same as the lowest frequency of the band to be amplified.

5. In an amplifying device, the combination of two stages each comprising a vacuum tube having a cathode, grid and plate, a circuit connecting the plate and cathode of said first stage including atransformer primary, means for impressing signal voltages between the grid and cathode of said first stage, a condenser connected across the secondary of said transformer to form. a circuit tunable through a desired range of signal frequencies, a parallel resonant circuit connecting the grid and cathode of said second stage and including a portion having a resonant frequency substantially equal to the lowest frequency of the range of frequencies to be amplified, a capacitive reactance connecting the high potential end of said transformer secondary and the grid of said second stage and an output circuit connected to the plate of said second stage.

6. In an'amplifying device the combination of a vacuum tube having a cathode, a control grid and a plate, a parallel resonant circuit tunable over a desired range of radio signal frequencies, a condenser connecting the high potential terminal of said circuit and the said control grid and a parallel resonant circuit connecting said control grid and cathode said latter circuit having an effective capacitance which increases as the frequency applied to said first named circuit is increased. I

WILLIAM STODDARD BARDEN.

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