US2200055A - High impedance attenuator - Google Patents

Description

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HIGH IMPEDANCE ATTENUATOR Filed Feb. 23. 1938 i s 3 SOURCE OF k r-, To SIGNAL 3:

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Patented May 7, 194

PATENT 0mm I 2,200,055 I I I men mremuoa ATTENUATOR' Carlos E. Burnett, East Orange, J., assignor to Radio Corporation of America, a corporation of Delaware Application February 23, 1938, Serial No. 191,891

2 Claims.

My present invention relates to attenuators, and more particularly to an attenuator arrangement with high impedance.

Attenuators of various types are commonly used. foncontrolling the signal input to an amplifier to prevent overloading, and to add flexibility to the transmission network. For some purposes, such as in, connection with a cathode ray oscillograph, it is desirable to have an attenuator with high impedance. In general, the ohmic impedance of an attenuator can be sufficiently high for most purposes, but the shunt capacities which enter into the construction of the attenuator often limit the frequency range over which the latter can be used. It is well known, .for example, to employ various means of compensation so that the attenuator will have the same frequency. characteristics on difierent taps, If the ohmic, impedance of an attenuator is reduced the problem of compensation for the attenuator is reduced, and the problem of compensation for shunt capacities is somewhat simplified and the frequency range may be extended. Usually, it is undesirable to reduce the g5 ohmic impedance, because of the loading eifect it produces on the circuit to which the attenuator is connected. I

' It can be stated, therefore, that it is one of the main objects of my present invention to provide an attenuator arrangement which will permit an appreciable decrease in ohmic impedance in the attenuator construction proper and at the same time will provide an increase in ohmic and capacitiveimpedance in the input circuit supplying the attenuator.

Anotherimportant object of my present invention may be stated to reside in the provision of a degeneratively connected tube between an input circuit'and an output network; and the tube utilizing a cathode resistor of relatively low ohmic resistance, the output network being adjust-ably connected by a variable tap to the cathode resistor. I

Another, object of my invention is to provide a high impedance attenuator; the attenuator comprising a tube having a cathode load element of relatively low ohmic impedance, air-adjustable tap being arranged for sliding connection to the load element, and the output network being connected to the adjustable tap through a condenser.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organizastood by referenceto the following description, taken in connection with the drawing, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into efiect. I

tion and method of operation will best be under- In the drawing:

Fig. 1 shows a circuit, embodying the invention,

Fig. 2 illustrates a modification,

Fig. 3 shows another modification.

Referringnow to the accompanying drawing, wherein like reference characters in the'figures designate similar circuit elements, there is shown in Fig; 1 a tube whose control grid is connected by condenser 2 to aterminal of a source of input voltage. The numeral 3 denotes the alternating current voltage source, and it is to be understood that this source may be an input circuit of an amplifier, and that the frequency of the source 3 may be in the audio or radio frequency ranges. The cathode of tube l is connected to ground through the attenuator resistor 4, and the latter has a relativ'ely low ohmic impedance. The plate of tube I is connected to ground through a direct current source 5, while the grid of the tube is shown as provided with a normal negative bias by the source 6; the high impedance 1 being connected between the grid side of condenser 2 and the negative terminal of source 6.

The output circuit, or utilization network, is coupled to a desired portion of the cathode load element 4. The coupling is made adjustable by connecting one of the terminals of the utilization network to a slidable tap it through condenser 9 and the other terminal of the utilization network to ground. By varying the tap 8 between ground and the cathode end of resistor 4, the voltage amplitude at the utilization network may be varied from zero to any predeterminedmagnitude.

The ohmic value of the attenuator resistor 4 determines the amplitude of the input signal that can be applied without overloading. Because the circuit employed is degenerative, the input signal is not limited to the normal grid swing which could be used if the tube I were operated class A. Instead, it may be increased to a value E1 which is equal to E -i-E where Eg is the normal grid voltage permissible for class A operation, and E is the corresponding voltage which would appear in the plate circuit with class A operation if a resistance, the equivalent of the attenuator resistance, were used in the plate circuit. This characteristic permits the use of a tube suchas one of the 954 type, which has a very low input capacity, even though it has a low grid bias; and

the circuit can be designed to handle peak signals as that at the input electrodes of the tube. Themaximum amplitude of the output signals is governed by the magnitude of the resistance in the cathode circuit. "I'he gain between grid and cathode of tube I can not be made unity, but

can be made at least 0.9 without any difiiculty, depending on the size of the cathode resistor. In this circuit the gm L +gm L where gm is the mutual conductance of the tube under operating conditions, and Z1. is the cathode resistor 4. In general, the ohmic value of the resistor 4 which will permit a large signal input, and which will give a gain of 0.9 or better, will be appreciably lower than the values used in high impedance attenuators of 'known types. The ohmic value can be reduced to a much lower value if a reduction in input signal and gain are permissible, and sucharrangements will be obvious to those skilled in the art.

The attenuator can be used in either the audio or radio frequency range depending on the ohmic value of attenuator resistor 4 with respect to the stray shunt capacities of the wiring and the utilization network. In general, the ohmic impedance looking at attenuator d from point 8 should be about one-tenth of the impedance, ohmic or capacitive, looking toward the utilization network, for all frequencies desired to be transmitted uniformly. One advantage in the circuit is the possibility of placing attenuator ata remote point. Then, the majority'of the shunt capacities from wiring, etc., appear across the whole attenuator which is connected to the cathode and is shunted by the tube impedance. j The tube impedance between cathode and ground is approximately gain mote connection is about 500 ohms. However, if the line with its capacity were connected to point 8, the shunting effect would be more pronounced because attenuator 4 may be 10,000 to 50,000-

ohms depending on the input voltage on the tube. Thus, the design of the circuit influences the frequency range.

The impedance of condenser 2 should be about one-tenth of the impedance of resistor l at the lowest frequency it is desired to transmit. The impedance of condenser 9 should be about onetenth of the impedance of the utilization network at the lowest frequency. Resistor 1 may be of the order of 1 megohm; a normal grid leak. Resister 6 may vary from 500 ohms to 50,000 ohms depending on the use of the attenuator. The prior mentioned formulae for gain and voltage input, as well as utilization network impedance, and tube choice, all influence the choice of size of resistor 4. The high impedance is gained through using the input circuit of a vacuum tube. The low impedance in the cathode circuit perrnits a heavier loading. Actually, the tube acts like an impedance transformer.

' Another arrangement which is useful, particularly in video circuits where very low, as Well as very high, frequencies must be passed, is shown in Fig. 2. In this case the am of the tube is probably lower, but a heavy by-pass condenser is not necessary to give good low frequency response. In Fig. 2, the resistor 4 is in shunt with a, relatively higher (10 times) resistor l0. Point 1 on resistor I 0 is selected by trial to give normal cathode current. Point 1 could have been on resistor 4, but it is usually more convenient to use a separate resistor 10.

In Fig. 3 is shown another modifiedform of the grid bias network. Here, the resistor I0 is,

connected in series with cathode load resistor 4, the signal bypass condenser II is connected in shunt with resistor I0. Resistor I0 is of such value as to produce normal grid bias at point 1'.

While I have indicated and described several systems for carrying my invention into efiect, it

will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without deforth in the appended claims."

What is claimed is:

1. In a signal transmission "network, an input circuit, an output network, a high impedanceat tenuator arrangement comprising a tube provided with at least a cathode, control grid and a plate,

parting from the scope of myinvention, asi-set' an unbypassed resistive impedance disposed. in

the space current path of said tube and connected between the cathode and a point of relatively fixed potential, said input circuit being connected between the grid and said point of fixed potential whereby the total signal voltage developed across said impedance is fed back to thegrid in degenerative phase, means for 'maintain-ing the plate at a positive potential with respect tosaid fixed potential point, a second resistive impedance in said space current path inseries between said cathode and first impedance, .a direct current voltage biasing connection between the grid and v the junction of said two impedances whereby.

solely the direct current voltage across thesecond impedance biases the grid negative, a signalvided with at least a cathode, com r9 rid and plate, an unbypassed resistive impedance inthe tube space current path .and connected between the cathode and a point of relatively fixed potential, means for maintaining the plate positive with respect to said fixed potential point, said input circuit being connected between the'grid and said point of fixed potential wherebya 100% degenerative signal voltage feedback exists to said grid, a resistor in said space current path in series with said cathode and impedance, a signal bypass condenser shunted across the resistor, a

connection between the grid and the junction of c said resistor and impedance for applying to said' grid as its sole bias the direct current voltage developed across said resistor, and a capacitative coupling device slidable along said resistive'inipedance whereby said output circuit can be connected between any point on said resistive impedance and said point of fixed potential.

CARLOS E. BURNEI'I.

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