US2616988A - Self-balancing push-pull amplifier - Google Patents

Description

Nov. 4, 1952 K. RODENHUIS 2,616,988

SELF-BALANCING PUSH-PULL AMPLIFIER Filed May 2, 1947 K. RODENHUIS .I'NVE TOR MM ATTORNEY,

Patented Nov. 4, 1952 UNITED" STATE SELF-BALANCING PUSH-PULLHAMPLIFIER Klaas RodenhuisrEindhoven, Netherlands, as-

signor to Hartford NationalBank and Trust Company, Hartford, Conn., as trustee Application May 2, 1947, Serial-No. 745,616

In the Netherlands March'30, 1946 section 1, Public Law 690; August 8, 1946 Patent expires March,30,.1966

This invention relates to a circuit-arrangement for the transmission of electrical oscillations, more particularly of intermediate frequency and/or high-frequency oscillations, by means of at least two push-pull connected discharge systems, notably two discharge systems housed in a single bulb, of which the cathodes and the screen-grids, if any, are directly interconnected in pairs inside the bulb.

I In the case of low-frequencypush-pull amplifiers it is known to improve the equality of the direct current loads of thetwo discharge tubes used in the amplifier by causing each tube to, produce its own grid bias, namely by causing the anode current to bring about a voltage drop across a resistance includedin the cathode lead.

It is also known to attain the aforesaid object by including a resistance in the screen-grid circuits of each of the tubes.

These known methods, however, cannot longer 2 Claims. (01. 179-471 value of'the anode current is entirelyor partly supplied to a control-grid of the corresponding discharge system in such manner that equal or substantially equal direct current loads of the discharge systems are automatically ensured.

As a rule the direct voltages are each supplied, through one or more resistances, to the corresponding control grids. l 1 j Each' of the aforesaid direct voltages'is preferably supplied'to a control gridof the corresponding discharge syfstem through a potentiometer which is connectedbetween the anode and the junction of the cathodes or a point ofconstant'potential (earth). t

The undue positive nature of the voltage thus set up at the control-grids may be eliminated by inserting a resistance in the common cathodelead of the two discharge'systems, for'example, between the junction of the cathodes and the interconnected ends of the aforesaid potentiometers, the said resistance being proportioned in a manner such that the desired negative bias is set up at the control-grids of the discharge systems. e In order that the invention may be clearly I understood and readily carried into effect, it will connection of l the screen-grids is sometimes even necessary viz. if, in the case of short waves, there occurs resonance between the inductances of the screen-grid leads and the screen-grid capacities. In these cases the conventional circuit-arrangements cannot be used any longer, since the use of these circuit-arrangements requires the cathodes and the screen-grids respectively to be provided separately. g

' The invention provides a circuit-arrangement which also automatically ensures equal or substantially equal direct current loads of the two discharge systems, and which is also adapted to be used when the known circuit-arrangements are no longer serviceable for the aforesaid reasons. Under certain conditions the circuit-an rangementaccording to the invention yields better results than the known circuit-arrangements. According to the invention each of the anode circuits of the discharge systems includes a resistance and each ofthe direct voltages set up across these resistances and varying with the now be described more fully with reference to the accompanying drawing, in which one form of construction of the circuit-arrangement according to the invention is represented by way of example.

This drawing represents a circuit-arrangement for amplifying high-frequency oscillations by means of two push-pull connected discharge systems I, I each comprising a cathode 2', 2", a control-grid 3, 3", a screen-grid 4', 4", a suppressor 5', 5" and an anode fi', 6 respectively.

The two systems I, I" are housed in a single.

bulb. 1 The suppressor grids 5, 5", screen grids 4', 4" and cathodes 2', 2 are interconnected in pairs inside the bulb, the suppressor grids being, moreover, directly connected to, the cathodes. The two discharge systems may be so constructed as to have a common cathode.

A'push-pull circuit 7, which is tuned to the frequency of the oscillations to be amplified, is connected between the control grids 3 and 3". This circuit comprises a variable condenser 8 and two coils 9, 9", the latter being interconnected through the series connection of two blocking condensers ID, ID".

The amplified oscillations may be obtained from an output circuit l l connected-in push-pull arrangement between the anodes 6, 6". The

- push-pull circuit H is composed in the same 3 manner as the input circuit 7 and is constituted by a variable condenser l2, two coils l3, l3" and two blocking condensers l4, I4". The centre of the push-pull circuits 1 and II (the junction of the blocking condensers I, I0" and I 4', l4" respectively) is earthed.

The cathode-lead includes a resistance [5 and. the screen-grid circuit comprises a resistance It.

The circuit-arrangement so far described suffers, in general, from the drawback that very diiferent anode currents flow through the two discharge systems I and I", since, owing'to small disparities in construction, in contact potentials or in mutual conductances, the location of the working point of each discharge system may slightly vary. This is often undesirable, particularly if stringent requirements are imposed on the equilibrium of the output signal. It is therefore desirable to construct the push-pullcircuitarrangement in such manner that anode-current difierences are automatically neutralized.

However, the conventional circuit-arrangements used for this purpose cannot be used in the present case, since in these circuits use is made of the cathode-leads and the screen-grid leads respectively of each of the discharge systems, in contradistinction to the circuit-arrangement according to the invention, which may also be used in the present case and which, under certain conditions, ensures even a greater equality d of the direct current loads of the two discharge tubes than the known circuits- According to the invention each of the anode circuits includes a resistance IT, IT, at which direct voltages are set up which depend upon the anode current in question. Part of the direct voltage set up across resistance i1 and i7" respectively is supplied to the control-grid 3 and .3" respectively through a potentiometer I8 and 18f respectively.

The circuit-arrangement operates as follows: If for some reason the anode current of any of the discharge systems should exceed its normal value, the anode voltage falls ofi. Owing to this the potential of the associated control-grid decreases, so that the increase in anode current is counteracted by the decrease in control-grid voltage.

A simple calculation proves that by the circuitarrangement any anode current variations (which may, for example, occur upon interchanging the discharge systems) are reduced to a fraction designated f of the initial value, f being equal to .1 l+pRS in which S represents the mutual conductance of the dischargesystem, R, the value of the resistance in the anode circuit and p the potentiometer ratio.

In this case the circuit-arrangement automatically seeks to maintain the anode current value once adjusted.

The potentiometer sets up a positive voltage at the control-grid. In order to ensure that the control-grid has the required negative bias relatively to the cathode, the common cathode-lead includes the aforesaid resistance l5, through which the two anode currents flow. The resistance has such a value that the voltage drop across this resistance sets up a positive potential at the. cathode, which is so much higher than that of the control-grids relatively to earth that the latter acquire the customary negative bias for the normal working point.

From the aforesaid expression it appears that the neutralization of the relative difference in value of the anode currents will be better as the value of resistance R is higher and as the potentiometer ratio nears unity. The value of the available supply voltages determines what values of R and p are permissible. The circuit-arrangement will be more fully explained by giving a numerical example.

The elements of the circuit-arrangement as shown in the drawing, inasmuch as they are essential to the invention, have the following values:

the supply voltage being 300. The data of the discharge systems used are:

Va=Vg2=250 volts -Vg1=2 volts Ia: 10 ma. 92:1;8 ma.

S=6 majvolt Two sets (A and B) of these discharge systems are successively connected in push-pull arrangement in the usual manner, 1. e. without making use of the invention, the following anode-current strengths being measured:

In the case A: Ia1=13.6 ma.; Ia2=4.4 ma. In the case B: [04:85 ma.; Ia2=6.9 ma.

Thereupon the same sets of discharge systems are used in the circuit-arrangement according to the invention, the measuring results being:

A: Ia1=ll.9 ma.; Iaz=l0.2 ma. B: la1=1l.5 ma.; Iaz=9.9 .ma.

Thus it .is evident that the anode currents of the push-pull connected discharge systems have become substantially equal. In. one case the factor f has a value of 0.25, i. e. twice as low as that generally obtained in the conventional circuitarrangement comprising resistances in the screengrid circuits, which means that the circuitarrangement operates twice as eifectively.

What I claim is 1. A push-pull amplifier comprising a pair of electron-discharge systems included in a common envelope, each system having a cathode, a grid and an anode, the cathodes of said systems being directly interconnected within said envelope; a push-pull input circuit including a condenser connected between the grids of said systems, a pairof inductors and a pair of by-pass capacitors, one end of each inductor being connected to a respectlve grid, the by-pass capacitors being connected in series between the other ends of said inductors, the junction of said by-pass capacitors being grounded; a push-pull output circuit including a condenser connected between the anodes of said systems, a pair of inductances and a pair of by-pass capacitances, one end of each inductance being connected to a respective anode, said by-pass capacitances being connected in series between the other ends of said inductances, the Junction of said by-pass capacitances being grounded, means to couple said interconnected cathodes to ground, a pair of resistors serially connected across said series connected by-pass capacitances, means to apply a positive potential relative to ground to the junction of said resistors, whereby a voltage is developed across each reresistor to apply the voltage thereacross to a re-,

spective grid to maintain substantially equal direct currents in said systems.

2. An arrangement, as set forth in claim 1,

wherein said means to apply the voltages acrosssaid resistors to said grids is constituted by a pair tentiometers being connected in series across said .of potentiometers, each having a tap, said poserially connected resistors, the tap of each I, potentiometer being connected to a respective 6 7 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,993,479 Wise Apr. 23, 1935 2,123,241 Harley July 12, 1938 2,154,200 Dow Apr. 11, 1939 2,157,870 Sowers May 9, 1939 2,300,133 Nyquist Oct. 27, 1942

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