US2401779A

US2401779A - Summing amplifier

Info

Publication number: US2401779A
Application number: US391331A
Authority: US
Inventors: Jr Karl D Swartzel
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1941-05-01
Filing date: 1941-05-01
Publication date: 1946-06-11
Anticipated expiration: 1963-06-11
Also published as: CA440710A; GB593354A

Description

Patented June 11, 1946 UNITED STATES PATENT OFFICE SUMMING AMPLIFIER Application May 1, 1941, Serial No. 391,331

11 Claims.

This invention relates to electrical calculating devices and particularly to a device for obtaining the sum of a plurality of electrical voltages.

The object of the invention is to obtain the sum of a number of electrical voltages, one pole of each of the voltages being grounded.

A feature of the invention is an electrical amplifier having a feedback of power from the output circuit to the input circuit of the amplifier of such magnitude and phase as to reduce the input impedance of the amplifier to a small value and to make the over-all gain of the amplifier a predetermined quantity.

In prior electrical circuits for the addition of quantities represented by electrical voltages, it has been necessary to place the voltages to be added in serial relationship in order to obtain the sum of these voltages. Such a connection has the serious practical disadvantage that only one of the voltages can be connected to ground. In accordance with the present invention all of the sources of voltages may be connected to a common point, preferably to ground, and supplied through individual high impedances to the input of an electrical network.

The voltages to be added may be of any desired frequencies, and may have zero cycles, that is, a direct voltage, as one limit. The network is designed to amplify voltages within the desired frequency range, without distortion or instability, and has a feedback from the output circuit to the input circuit of such polarity and magnitude as to render the impedance of the input circuit, as viewed from the voltage sources, low compared to the impedances in series with the sources, without rendering the amplifier unstable. In the specific embodiment of the invention disclosed in the present application, the network is capable of amplifying voltages from zero cycles to a comparatively high frequency, but the invention is in no-way limited to this specific disclosure, as the interstage coupling networks of the amplifier may be designed by known methods to amplify any other desired range of frequencies. The network is further adjusted so that the overall gain is a predetermined quantity, and during the operation of the circuit, the network will tend automatically to maintain this relationship.

Thus the sum of the voltages will be reproduced across the output load, without any interaction of one source of voltage upon another. Further, by adjustment of the impedances connected in series with the various sources of voltage, any one or more of the sources may be, in effect, multiplied by any desired factor and this voltage, multiplied by such factor, will be included as one element of the summation of voltages in the output circuit.

The drawing diagrammatically illustrates a circuit embodying the invention.

In the drawing the generators A, B, and C, diagrammatically symbolizing three sources of voltages to be added, are respectively connected in serial relationship with one of the impedances l, 2, 3, each having a relatively high impedance compared to the effective input impedance of the amplifier. The impedances I, 2, 3 will normally be resistors, unless the added voltages are to be modified in accordance with frequency, in which case the impedances may have any desired frequency characteristic.

The voltage sources, and their serially connected impedances, are connected in parallel. relationship to the input of a thermionic vacuum tube 4. This vacuum tube may, if desired, have the usual grid biasing resistor 5. The vacuum tube 4 is coupled to the vacuum tube 6 by means of an interstage coupling network comprising the three resistors l, 8 and 9. This coupling network is of the type disclosed in U. S. Patent 1,751,527, March 25, 1930, H. Nyquist, but any pther form of coupling network capable of operation without distortion for voltages covering the frequency range desired may be used in place of the network shown. The vacuum tube 6 is coupled to the vacuum tube III by a similar network comprising the resistors ll, 12 and [3, respectively. The vacuum tube I0 is coupled by means of a resistor I4 to the load I 5. The anode of the vacuum tube III is coupled by an impedance IE to the control electrode, or grid, of the vacuum tube 4, feeding back energy from the output circuit of the vacuum tube l0 to the input circuit of the vacuum tube 4. As this amplifier comthe energy fed back will be in the proper phase to oppose the voltage applied to the input of the vacuum tube 4, thus forming a reverse feedback.

An intermediate tap of the battery 25, or other suitable source of voltage, is connected to the grounded connection 26. Positive potential from source 25 is supplied through resistors 1, II, to the anodes of vacuum tubes 4, 6, and negative potential from the source 25 is supplied through resistors 9, l3 to the control electrodes of vacuum tubes 6, Ill. The screen grid of vacuum tube 6 is connected to a suitable tap in the source 25. Positive potential is supplied through coupling resistor I4 to the anode of vacuum tube I0, and negative potential from the source 25 is supplied to the cathode of vacuum tube Hi. The screen grid of vacuum tube In is grounded, thus making the screen positive with respect to the cathode of vacuum tube Ill.

The operation of vacuum tube In may be explained in different ways, some of which are set forth hereinafter but the scope of the invention is not thereby limited to these theories of operation. In the absence of an applied signal, the 60 constants of the circuit may be so adjusted that prises an odd number of stages of amplification 3 the positive potential from the source 25 is completely used up in driving the anode current through resistor It so that no voltage is applied to the anode of vacuum tube l0, which is thus at ground potential. The anode current of vacuum tube I is maintained by the negative potential from the source 25 applied to the cathode of vacuum tube [0. When a negative voltage is applied to the control electrode'of vacuum tube t, the amplifiedvoltage will cause the control electrode of vacuum tube ID to become more negative, reducing the anode current and the voltage drop in resistor IA, and applying a positive voltage to the load Hi. When a positive voltage is applied to the control electrode of vacuum tube Q, the amplified voltage will cause the control electrode of vacuum tube ID to become less negative, permitting the negative voltage applied to the cathode of vacuum tube In to increase the anode current, making the voltage drop in resistor It largerthan the applied positive potential from the source 25, and applying a negative voltage to the load.

From another point of view, a current can flow from the positive tap of the source 25 through resistor M, down through load 15, and connection 26 back'to the source 25. Another current can a flow from the tap of source 25, by connection 26 up through load i5, anode to cathode of vacuum tube In to the negative tap of source 25. By adjusting the bias on the control electrode of vacuum tube It, these curlents, in the absence of a signal, may be made e nal. Thus no current will flow in the load I5, and the anode of vacuum tube in will be at ground potential. When an amplifled signal is applied to the control electrode of vacuum tube H), the balance of these'theoretical currents is disturbed, and a resultant current will flow in the load l5.

Let quantities relating to the sources A, B, C be designated by subscripts a, b, c, and those relating to the output circuit by subscript d,

The control electrode, or grid, of vacuum tube t has a potential, preferably negative, suchthat the control electrode does not draw any appreciable current.

Applying the Kirchofi' relationship to the node at the control electrode of vacuum tube Q,

Let the impedances 1, 2, 3, 16 be resistances R1, R2, R3, Rm, the potential of the control electrode of vacuum tube 4 be 8g and the voltage across the output circuit be ed, then Solving Equation 3 for is, is, i0 and id and substituting in Equation 2,

where p. is the voltage amplification ratio of the amplifier.

If h be large compared to unity, the bracketed are unity, and that the ratio a is 6x10 the denominator of Equation 4 will be which differs from unity by about .007 per cent. The coupling impedance formed by the input circuit of vacuum tube 4 may be shown to be 1+1 If is 6x10 and rm is 1 megohm, the coupling impedance is about 15 ohms. If the input resistors are of the order of 1 megohm each, the interaction between the sources A, B, C is negligible. The output impedance of the vacuum tube 10 is reduced by the factor ia 10 "'10 1 z a) and is effectively less than 10 ohms. The amplifier, in effect, forms a voltage source of very low impedance, thus variations in the load impedance have little effect on the accuracy of the summation.

As the effective gain for each source is controlled by the ratio of the feedback resistorto the input resistor, the voltages from the sources may have different gains, thus multiplying or dividing one voltage with respect to the others.

For the purpose of illustrating the flexibility and utility of the invention and not as any limitation thereon, let the impedances 1, 2, 3 be resistances R1, R2, R3, and let the impedance 16 be a resistance R4, and let D be the voltage across the load [5.

If R =R -xR =R then D=A+B+:0C where m is any desired factor. Thus, if R be the feedback impedance, and R4 be the input impedance for, the voltage E1, the voltage gain G for that input will be and this relationship will apply simultaneously and independently for all inputs, thus for any When the network is designed for use with voltages having frequencies which may go down to' zero cycles, preferably the circuit constants are so chosen that in the absence of an input voltage the potential of the anode of the vacuum tube In is substantially that of the ground, or zero voltage. When voltages are applied to the input circuit of the vacuum tube 4 the potential of the anode of the vacuum tube ID will swing above or below the ground potential in accordance with the sum of the voltages applied to the input of the vacuum tube 4, consideration being taken of the signs of the voltages applied.

In order to adjust the potential of the anode of the vacuum tube l0 exactly to zero or ground potential, a correcting circuit comprising any convenient source of voltage E supplied to the potentiometer i1 may be connected through a serial resistance 18 to the input of the vacuum tube 4, and, in the absence of other applied voltages, the potentiometer may be adjusted to bring the potential of the anode of the vacuum tube In exactly to the zero or ground potential. As the eflective gain of the complete network depends upon the ratio of the resistance of the resistor in series with any source of voltage compared to the resistance of the feedback resistor IS, in order to secure a more accurate control of the adjustment, the resistance of the resistor I8 is preferably made say three or four times the resistance of the resistor I 5. This relationship will also improve the direct current noise situation.

In a practical embodiment of the invention the resistors I, 2 and 3 were 1 megohm. The potentiometer I! was of the order of 10,000 ohms and the resistor l8 was 3 megohms. The vacuum tubes 4, 5 and were commercial vacuum tubes having the type designations respectively BSC'I, 6SJ7, and 6Y6G. The resistor 5 was 1,500 ohms, the resistors 1, 8 and 9 were respectively and 2 megohms. The resistors ll, l2 and It were respectively V 1 and 1 megohms. The resistor 14 was 6,000 ohms and the load l5 was also 6,000 ohms. The resistor It was 1 megohm. A ZOO-volt battery grounded at the mid-point was used, plus 350 volts being supplied to the anodes of the vacuum tubes 4 and 6, plus 250 volts to the anode of the vacuum tube l0, plus '15 volts to the screen of the vacuum tube 6, ground or zero volts to the cathode of the vacuum tube 8 and the screen rid of the vacuum tube III, minus 135 volts to the cathode of the vacuum tube It, and minus 350 volts to the grid biasing resistors of the vacuum tubes 6 and I0.

To improve the stability of the amplifier, and to obviate high frequency singing due to parasitic capacitances, small capacitors i9, 20 may be connected from the anodes of the vacuum tubes 4 and 0 to the cathode circuit, and small capacitors 2 I, 23, respectively, in serial relationship with resistors 22, 24, may be connected across the input circuits of the vacuum tubes 0 and I0. The capacitors l0, 2|, 2|, 23 were respectively .25, .0001, .001, .03 microfarad and the resistors 22, 24 were 100,000 ohms.

What is claimed is:

1. In combination, a plurality of voltage sources, a plurality of high impedances respectively in serial relationship with said sources, an amplifying device having an input and an output circuit, said sources and impedances being connected in parallel relationship to said input circuit, a load impedance in said output circuit, and means for feeding back energy from said output circuit to said input circuit to make the impedance of said input circuit small compared to said input impedances and the over-all gain of said amplifier substantially unity.

2. In combination, a plurality of voltage sources, a plurality of high resistances respectively in serial relationship with said sources, an amplii'ying device having an input and an output circuit, said sources and resistances being connected in parallel relationship to said input circuit, a load resistor in said output circuit, and means including a high resistance for feeding back energy from said output circuit to said input circuit to make the over-all gain of said amplifier for any one of said sources substantially equal to the ratio between the resistance feeding back energy and the resistance connected in serial relationship with that one of said sources.

3. In combination, a thermionic device having a cathode, an anode and a control electrode, a coupling impedance, a source of current connected to said cathode and through said coupling impedance to said anode and having an intermediate tap, an output circuit connected to said anode and said intermediate tap, two impedances connected in serial relationship across said output circuit, and a connection from the junction of said two impedances to said control electrode.

4. The combination in claim 3 with amplifying means in said connection from the junction of said two impedances to said control electrode.

5. The combination in claim 3 with a source of control voltage in serial relationship with said two impedances.

6. In combination, an electron discharge device having at least a cathode and an anode, a source of direct current having the negative pole connected to said cathode and the positive pole to said anode, an output circuit connected to said anode and an intermediate point in said source, two impedances connected in serial relationship across said output circuit, said intermediate point being so chosen that the currents from said source flowing in said impedances are substantially equal and opposite and the voltage across said output circuit is substantially zero.

7. The combination in claim 6 with a connection from the Junction of said serially connected impedances to a control electrode in said electron discharge device.

8. The combination in claim 6 with a source of voltage in serial relationship with said serially connected impedances adjusted to make the voltage across said output circuit more nearly equal to zero.

9. In combination, an electron discharge device having at least a cathode and an anode, a source or direct current having the negative pole connected to said cathode and the positive pole connected to said anode, an output circuit connected to said anode and an intermediate point in said source, a plurality of voltage sources. a plurality of impedances respectively in serial relationship with said voltage sources, said impedances and voltage sources being connected in parallel relationship, and another impedance connected in serial relationship with said parallel connected impedances and voltage sources across said output circuit.

10. The combination in claim 9 with a connection from the junction of said serially connected impedances to a control electrode in said electron discharge device.

11. A network having a plurality of input and output terminals, a potential divider connected in serial relationship with a plurality of said input terminals and one of said output terminals, said divider having a plurality of impedances in parallel relationship connected in serial relationship with a single impedance, an electron discharge device having at least a cathode, an anode and a control electrode, a direct connection from the Junction of said plurality of impedances and said single impedance to said control electrode, a connection from said anode to said one output terminal, and a connection from said cathode to an input terminal and the remaining output terminal.

KARL D. SWARTZEL, JR.