US2594912A - Electrical driving circuit - Google Patents

Description

April 29, 1952 J. w. GRAY 2,594,912

ELECTRICAL DRIVING CIRCUIT Filed Feb. 27, 1945 INVEN TOR.

JOHN W- GRAY Patented Apr. 29, 1952 UNITED STATES PATENT OFFICE/'3 mesne assignments, to the United States of America as represented by the Secretary of War Application February 27, 1945, Serial No. 580,021 a 4 Claims. (01. 179-171) circuits and more particularly to electrical circuits used to supply a controlling voltage to a load which draws a heavy current. This type of network is called a driver circuit.

According to conventional circuit practice, the driver is a cathode follower, which inherently has a high input impedance and a low output impedance. The high input impedance is desirable in order to match impedances with the usually high output impedance of the stage from which the driver controlling voltage comes. The low output impedance is desirable because of the linearity which is expected of a driver, and in order to match impedances with the load, which draws a heavy current and which usually has a low input impedance.

.As is well understood in the art, however, a

'cathode follower has a non-linear responsewhen a heavy load is drawn from it. Also, to obtain. "a sufiiciently low output impedance from a cathode follower, it is often necessary to use a plurality of tubes in parallel, this involving extra expense and design difficulties in suppressing "parasitic oscillations, which often build up between two or more tubes connected in this manner.

v Among the objects of this invention, there- Q fore, are:

' 1. To provide a driver circuit having a very high input impedance.

2. To provide a driver circuit having a very low output impedance.

3. To provide such a driver circuit which will jhave' a'linear response over a wide swing of the "input voltage.

4. To provide such a driver circuit without the use of a plurality of tubes in parallel.

In accordance with the present invention there is provided a driver circuit comprising a pentode voltage amplifier, with the driver load making up pull amplifier circuit such that the gain of this circuit is low. In this case the circuit would not have 100% degenerative voltage feedback This invention relates in general to electrical a. and the input and output impedances would differ from their aforementioned extreme values accordingly. My invention will best be understood with reference to the drawing, in which is shown a circuit diagram of the complete driver circuit.

Referring now to the drawing, tube I is a pentode amplifier tube into which the controlling voltage is fed, in the cathode circuit of which are un-bypassed resistor 2 providing some current degeneration, and the driver load 3, the latter having connected in parallel therewith a series circuit consisting of condenser 4 and resistor '5. The load is shown grounded at the point Ii. The other elements associated with tube I are a plate resistor I, a screen-dropping resistor 8 and a screen by-passing condenser 9. The plate' of tube I is coupled by means of condenser In to the grid of one triode tube I I of the cathodecoupled push-pull amplifier. The grid is connected through resistor I2 to a positive bias from voltage divider I3. Tube II has itscathode directly connected to the cathode of tube 22, the cathodes being connected to ground through coupling resistor 23. The plates of tubesII' and '22 are connected together through the primary of transformer I4. The secondary of transformer is connected in parallel withthe load 3. The plate of tube I I is coupled to theungroundedside of the secondary of transformer I I by condenser I5. A portion of the output voltage is fed back to the grid of tube 22 by a voltage divider network, one branch of which is condenser I6 and resistor I! in series, and the other branch oi. which is resistor I8 and a portion of voltage divider I3. Referring now to the operation-of the system, it can be seen that the cathode-coupled pushpull amplifier is a positive feed-back amplifier. Consider an increase in voltage on the=grid of tube 22. This causes a decrease in' the-plate voltage of tube 22. This voltage change is applied to the primary of transformer I4 and fed to the secondary in the same polarity, as shown by the polarity marks. Since-the grounded side of the secondary of transformer I4 cannot decrease in voltage, the ungrounded side of the secondary increases in potential, increasing the" load voltage and thus increasing the grid voltage of tube 22. The feedback voltage is therefore in phase with the input voltage and so the feedback is positive. An increase in grid voltage onjtube 22 will cause an increase in current through cathode resistor 23, an increase in voltage across resistor 23, and the grid-cathode voltage of tube is lbK where Kn represents the gain with feedback, K is the gain with no feedback, and b is the fraction of the output voltage that is fed back. Neglecting the resistance of the portion of voltage divider l3, which is made comparatively small, I) in this case is: a

resistance 18 b resistance 17+ resistan cc 1 8 This ratio is made equal to in this invention. Therefore,

1a,: =infinity For some purposes the value of b is made much less than In these cases the gain of the cathode-coupled push-pull amplifier is low. Thus when b is made equalto K the gain of the cathode-coupled push-pull amplifler with respect to the grid of tube 22 is theoretically infinite, because it is at this grid that the voltage is ied back. But the gain of the amplifier with respect to the grid of tube H is infinite also, because of the cathode-coupled push-pull arrangement. In practice, the circuit is made to have a very high, not infinite, gain. To demonstrate that the driver has a very high input impedance, consider what happens when there is a'positive change in the grid voltage of tube I. The plate voltage of tube 1 will decrease, and-this decrease is coupled to the grid of tube l l.

Thcgoutput of tube i l is a large voltage increase, which change also occurs at the bottom of resistor 2. :The value of resistor 2 is so chosen so that the increase in current flowing through it will produce a voltage across it which almost overcomes the increase in load voltage.

The change iii-current in tube 1 is therefore quite small, and with a finite change in grid voltage, the efifect to provide. a very high impedance.

, a To demonstrate that the driver has a very low a very low impedance looking back at point l9, consider a decrease in the current drawn bythe load. This tends to .momentarily decrease the potential at point l9.

output impcdancei. e.,

This causes the cathode of tube l to lower, and the grid-cathode voltage to increase.

The resultant increase in plate, voltage of tube l l tends to greatly increase the voltage at point IE), but the voltage drop across resistor 2 holds down the voltage at this point so that the resultant change in voltage This plate-- voltage therefore. decreases, and this change is coupled to the grid of tube 1 l.

at point l9, 1. e., across the load, is very small. Since the load current change was assumed finite, the output impedance is seen to be very low.

The driver circuit as a whole has substantially percent degenerative feedback when b is made equal to due to the fact that the gain of the cathodecoupled push-pull amplifier and the resistance of 2- are so chosen so that the gain between the input and. point [9, the output, is unity.

Condenser 4 is put in parallel with the load in order to increase the efiective load impedance. Resistor 5, whose resistance is very small compared to the reactance of condenser 4, is used to lower the time constant of the load circuit. I

Condenser I5 is used to couple changes in plate voltage of tube ll very quickly to the secondary of the transformer 14, without the usual transformer time lag. Without condenser IS the cathode-coupled circuit tends to oscillate.

I claim:

1. A variable voltage, impedance transforming driver circuit comprising first, second and third electron tubes each having an anode, cathode and grid electrode, means to impress a control voltage between said first tube grid and ground, a load having one end thereof grounded, an impedance coupling the ungrounded other end of said load to said first tube cathode, a common grounded cathode bias resistor to which said second and third tube cathodes are coupled, a push-pull output transformer to the primary ends of which said second and third tube anodes are coupled. the secondary end of said transformer having the same polarity as the primary end of said transformer to which said second tube anode is coupled being connected to the junction of said load and said impedance, the other end of said secondary being connected to the grounded end of said load, said first tube anode being coupled to said second tube grid, and a voltage divider network across said load to which said second tube grid is connected to derive a positive feedback voltage therefrom, said second and third tubes serving to amplify the variations in control voltagev impressed upon said first tube grid and impress them across said load, whereby. the voltage across the load will vary linearly with variations in control voltage.

2. The circuit recited in claim 1, wherein said voltage divider network comprisestwo resistances in series having a ratio such that said positive feedback voltage impressed upon. said third tube control grid substantially equals the reciprocal of the gain of said second and third tubes functioning as a push-pull amplifier Without feedback.

3. The circuit of claim 1, further including capacitor means for connecting said second tube anode with said junction of said load and said impedance.

- feedback thereto, the ratio of the feedback voltage to the output voltage of said high gain amplifier being equal to the inverse of the gain of said high "gain amplifier without said feedback, the gain of said high gain amplifier with said regenerative feedback thus being substantially infinite, load impedance means having one end thereof grounded, means for coupling the output of said phase inverter amplifier as an input to said high gain amplifier, said high gain amplifier being connected across said load impedance means, and means coupling the ungrounded end of said load impedance means to said cathode impedance so that the potential developed across said load impedance means is applied as a negative feedback to the cathode circuit of said phase invertor amplifier, whereby said control potentials Will produce a linear variation in potential across said driver load means.

JOHN W. GRAY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,027,054 Miessner Jan. 7, 1936 2,153,756 Hunt Apr. 11, 1939 2,361,198 Harmon et a1 Oct. 24, 1944 2,383,351 Smith Aug. 21, 1945 2,383,867 Koch Aug. 28, 1945 2,411,706 Berkoff Nov. 26, 1946 2,510,683 Carpentier June 6, 1950 16 2,516,181 Bruene July 25, 1950

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