US2013098A - Neutralization circuit - Google Patents

Description

Sept. 3, 1935. c w, A 2,013,098

NEUTRALIZA'IION CIRCUIT Filed June 8, 1932 Iz-r- ATTORNEY Patented Sept. 3, 1935 v t poration of Delaware "Al plicationJunel932g sei'ial-Nd;516,047 V H the-circuits connected to the electrodes ofhsuch It has beenthe practice thepast to neutralize the capacity coupling between the grid and platecircuits of an electron dischargedevice by I. pncviding a capacity coupling equal and oppo site; to the undesired coupling in such manner that a balance is obtained. Several such ar rangements' for eliectirig neutralization of un-' ,de'sirablereactions are described in United States Batents 1,334,l18"'grantedMarch 16, 1920 to C. W.

Rice,1,553,85 8 grantedfApril 14, 1925 to L. A. Hazeltine and*l,560,332 grantedNoverhberS, 1925 toIS. Ballentiner "A disadvantage inherentin circuits utilizing the prior art practice of neutralization by the methods outlined in the foregoing patents is that a true balance can only be obtained at a single frequency. This disadvantage permits parasitic oscillations to exist atbther irequencies, and is particularly noticeable atthe higher radio frequencies. As a' result, there is a tendency for aniplifiers to oscillate at frequencies appreciably r enrcvedfrom the resonant frequencies of the anode-and gridycircuits,inasmuch as the neutralizing arrangements in accordanw withthe foregoing methods are eifective andgive coinplete neutralization at only one frequency. failure of ordinary capacity neutralization to' give a periectbalance at more than one fre quencyhas benfound to be due to the presence of unequal inductances in the cennections to the electrodes of the'tubes and in the :connectioni to the neutralizing" condensers. In order to oom pensat for the differences inthe lead inductancesit has been thepractice in the 'pa'st-t'o make the neutralizing condenser larger or smaller in such a way that the inequality in inductance was compensated by the diiierenceih the ne'utralizing condenser capacity. Although this method of compensatiDnWi-ll balance a certain amount of inductance at one frequency only, it is impossible, however, to obtain neutralization over a wide band of frequencies in which parasitic oscillations may take place.

This disadvantage is overcome in the present invention which provides a system wherein a practically perfect balance may be obtained over a wide range of frequencies. This is accomplished by balancing the inductances as well as the capacities in the bridge circuit of an electron discharge device. More specifically, the

l sciaims. (01. 17991111 I present invention consists in the provision of an inductance'in the connection to the neutralizing condenser, or in series with the vacuum tube grid leads, in order toobtain neutralization over a wide frequency band.

An advantage 'of the present invention lies in the elimination of the necessity of making delicate adjustments of neutralizing condenser ca pacity when a transmitter requires a change in operating wave length. 1 Such a system of wave 10 changein connection with which the present invention has been found particularly useful is described in my copending patent application Serial No. 189,162, filed'May '5, 1927.

In the accompanying drawing, Figure l illustrates the present invention as applied to a push pull amplifying unit comprising two electron discharge device amplifiers, and Figure 2 shows, diagrammatically, the bridge circuit for the circuit arrangement of Figure l. l 20 Referring to Figure 1, there are shown two electron discharge devices"! and 2 arranged in the well known push-pull circuit and provided in a well known manner with neutralizing condensers 3, 4. Infpractice, a large portion of the distributed inductance whic'hexists in the connections to the grids of the electron discharge devices is usually foundin those sections of the circuit which areinside the tube envelopes. These inductances are represented in the drawing by numerals 5 and 6. The effect of these inductances is'to tune 011th portion of the capacity between the anode and grid electrodes of the devices I and Z in such manner as to make theapparent capac ity mm the anode circuit to the junction points 1 and 8 of the grid circuits larger. In View of this, -neutra1izingcondensers 3 and. 4 have heretof ore beenadjusted for a somewhat larger caparity in order to obtain neutralization at the operating frequency. For a frequency which ishig-her'than the operating frequency it will be noted thatthe inductive reactance of 5 and 6 increases while atthesarne time all capacity reactastes decrease, with the-result that inductances 5 and 6 tune out a larger portion of the capacity between the anode and grid, thus causing an apparent capacity which is larger than the apparent capacity at the operating frequency and which no longer is perfectly balanced by neutralizing condensers 3, 4. Consequently, in a system of this type it is possible for parasitic oscillations to take place at a frequency which is higher than the operating frequency.

At a lower frequency than the operating frequency, the reactances 5 and 6 decrease while tember 20,

at the same time all the capacity reactances increase. Consequently, at lower frequencies the apparent capacity through the electron discharge device is decreased and, similarly, in this case, also, the neutralizing condensers fail to balance the coupling between the anode and grid circuits. By introducing inductances 9 and I0, however, in series with the neutralizing condensers and adjusting them to an effective value equal to 5 and 6, respectively, in accordance with the present invention, there is obtained a condition such that for increasing or decreasing assumed frequencies past the operating frequency there results equal changes in the effective capacity of the tube and neutralizing circuits, and'the coupling between the anode and grid circuits'remains substantially at zero over a large range of frequencies.

The present invention may be applied to any neutralized radio frequency amplifier by inserting an adjustable inductance in series with either the neutralizing condenser or the grid lead depending upon which has the smallest natural inductance. This extra inductance which is inserted may be set at the time the equipment is tested and ordinarily will not require any adjustment on the part of the operator, but, if desired, provision may readily be made for adjusting the inductance at will by making it in the form of a trombone or small variometer, or by making arrangements for installing leads, or portions of leads, of various lengths. Such a variable inductance arrangement has found particular application in the construction of transmitting circuits adapted to operate at different times at frequencies which are widely different from one another. In such case, the inductance is adjusted until the setting of the neutralizing condensers remain fixed when the transmitter is changed from one operating frequency to another.

Figure 2 illustrates schematically the bridge circuit of the arrangement of Figure 1. It will be noted that the inductances 5 and 6 have a minimum value which is fixed by limitations in the design of vacuum tubes. In practice, the tubes are used at frequencies so high that this minimum value of'reactance becomes an important factor in the operation of the circuits. Consequently it becomes increasingly important to provide the adjustable reactances 9 and ID as the operating frequency is increased.

Although the invention is described only in connection. witha push-pull amplifier circuit it is to be distinctly understood that it is not limited in scope thereto, but may be used equally as well in single tube circuits, or in circuits wherein the tubes are operated in parallel. The invention is also applicable to vacuum tube frequency multiplier circuits; forexample, of the type described in my Patent No. 1,878,308, granted Sep- 7 1932, and application Serial No; 525,419, filed March 26, 1931, and to circuit-sdesigned to be changed from amplifiers to frequency multiplying amplifiers for changing the output frequency. Other numerous circuit arrangements to which the present invention may be applied will be apparent to those skilled in the art.

I claim:

1. In a neutralization circuit having an electron discharge device comprising an anode, cathode and control electrode, a condenser conductively coupled between said anode and. control electrode for neutralizing undesirable reactions between the circuits connected to said last two electrodes, and a balancing inductance directly connected between said condenser and control electrode for neutralizing over a wide range of frequencies undesired coupling between said anode and control electrode, said condenser having a value which is substantially equal to the capacity between the anode and control electrode, and. said inductance having a value which is substantially equal'to the inductance of the lead in said device connecting said control electrode with the other elements of the circuit.

7 2. A circuit as defined in claim 1 characterized in this, that said inductance is variable.

3. In a neutralization circuit having an electron discharge device comprising an anode, cathode and control electrode, and a condenser conductively coupled between said anode and control electrode for neutralizing undesirable reactions between the circuits connected to said last two electrodes, the method of obtaining more perfect balance over a very wide range of frequencies which comprises balancing, externally of said device, the inductances of the interelectrode connections within said device.

4. The combination with a push-pull circuit comprising two electron discharge devices whose corresponding anode, cathode and control electrodes are respectively connected together, an input circuit being connected between said control electrodes and an output circuit connected between said anodes, said push-pull circuit having a neutralizing condenser cross-connected between the control electrode of each said devices and the anode of the associated device, of a balancing inductance directly connected between each neutralizing condenser and its associated control electrode to which it is directly connected.

5. The combination with a push-pull circuit comprising two electron discharge devices whose corresponding anode, cathode and control electrodes are respectively connected together, and neutralizing condensers individually cross connected between the control electrodes of said devices and the anodes of the associated devices, of an inductance directly connected between each neutralizing condenser and its associated control electrode to which it is conductively connected, and an input circuit connected directly to both said control electrodes, said inductances being in series with their respective neutralizing condensers with respect to the terminals of said input circuit.

CLARENCE W. HANSELL.

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