US2282886A - Receiver and amplifier circuits - Google Patents

Description

-May 12, 1942. I H, ROTHE RECEIVER AND AMPLIFIER CIRCUITS Filed Dec. 21, 1937 INVENTOR HORSTZOTHE BY 7% ATTORNEY? Patented May 12, 1942 ngosrvnnann AMPLIFIER. cmcmzrs,

Horst Rothe, Berlin-Charlottenburg, Germany,

asslgnor to Telei'unken Gesellschaft fiir Drahtlose Telegraphic m. b. 11., Berlin, Germany, a

corporation of Germany Application December 21, 1937, Serial No. 180,947 I In Germany December 28, 1936 3 Claims.

The invention relates to receiver and amplifier circuits where it is essential to keep down the damping exerted upon an oscillating circuit through the grid circuit of an associated tube.

The invention primarily finds application in electrons) the field of high frequency amplification of very short waves. It is known that in the present day equipment, the amplification obtainable with one tube decreases as the operating frequency becomes higher. This was thought to be caused by a drop in the anti-resonant impedance of tuned circuits employed for coupling. However, it is now known that even at the highest usable frequency tuned circuits of sufiicient anti-resonant impedance may be obtained by the use of special circuits such, for instance, as sections of concentric transmission lines. Preferably,- thepresent invention is based on the realization that the cause of the unsatisfactory amplification isv to be sought in the grid damping action of the tube immediately following a resonant circuit.

The cathode and the control electrode of an amplifier tube can be looked upon as the plates of a condenser. As soon as a diiference of potential exists between them, the control grid carries a charge (static capacity and static charge, which is present even when no electronic emission takes place). Besides this, the electrons flying off the cathode through a control grid towards some intercepting electrode (anode) put a positive charge Q on the control grid whose magnitude depends upon density of space charge p in every portion of the discharge space. Assuming a certainpotential differencebetween the grid and the cathode, the charge .62 appearing on the control grid is greater in a positive sense (less negative), when the space in between is filled up with electrons, which means that the effective D. C. capacity is greater. If the potential between the control grid and cathode is an alternating potential ug=Ug sin mt,' tube. This charge is of course zero at zero grid potential and increases linearly in magnitude then in the grid circuit there flows an alternating may be considered as a complex capacity! which has a static component (discharge space without emission) and a dynamic component (effect of the charge influenced by the flow of For alternating potentials of the grid circuit the dynamic component AC; is chiefly important, while the..static component through suitable dimensions and position of the electrodes, can be kept down to a sufficiently low value. a

It can be shown by calculation, that the value of AC; changes as the operating point on the curve is shifted so that AC; increases with increasing plate current Ia when n is greater than 3/2jbut decreases with increasing plate current when n is smaller than 3/2, where n is a quantity that may be explained as follows: the plate current in the vicinity of any particular value may be represented as proportionalto the n power ofv the grid potential, the value of n-being determined by thenature of the curvature of the plate current-grid potential characteristic at the point in question. Thus n is the value of the power which best represents the characteristic at this operating point.

In the drawing, Figs. 1, 2 and 3 are curves which illustrate certain characteristics of a tube having a space charge grid or the equivalent thereof. Fig. 1 shows the anode current of such a tube plotted against its control grid potential, whereas, Fig. 2 shows the grid charge plotted against control grid potential. In Fig. 3 there is shown the dynamic grid capacity component plotted against control grid potential.

Referring to Fig. 2 the full line represents the static grid charge Q, ie e. the grid charge which would existin the absence of electrons in the tubes. This charge is of course zero at zero grid potential and increases linearly in magnitude with increasing negative grid potential. In addition to the static charge Q there is also the partial charge AQ on the'grid due to the presence of electrons in the vicinity of the grid and this charge is positive as the aforesaid electrons attract or induce on the grid a charge of opposite sign to their own. Adding the partial charge AQ of positive sign to the static charge Q of negative sign results in'the dotted curve of Fig. 2 which represents the total grid charge.

Finally in Fig. 3 is shown the shape of the dynamic capacity component AC which dynamic component is equal to the rate of change of AQ with respect to'grid potential and is a measurable quantity. Where the curve representing AC; crosses the zero line the total capacity between the control grid and the cathode is the same as the static capacity.

On grounds of the. invention it is suggested,

namic capacity AC;

tions simply devices, especially for waves below 5 meters, the operating point on the characteristic curve of the tube be so chosen, that the anode current is greater than zero, but the dynamic grid capacity AC; is small or zero.

A further analysis of the effect of the dynamic grid capacity AC; reveals that as a result of the finite speed of electrons there is a phase shiftof less than 90 between the induced grid current and the alternating potential of the grid. Due to this the grid to cathode admittance of the tube has a conductive component which is proportional to the dynamic capacity component AC when the latter has a negative sign the conductance is also negative as long as the frequency-is high enough to produce a time angle of the electrons from cathode to grid lying tween 0 and 180. Still on grounds of the invention, the operating point on the curve is shifted so far into the range of negative dy- (see Fig. 3) that while an undamping of the grid circuit results, an oscillation (self excitation) does not set in. The result of taking this measure leads simultaneously to a reduction of the effective grid to cathode capacity, to a value below the static capacity.

The realization of the desired working condidepends upon the selection of the proper grid bias of the tube inthe grid circuit of which a decrease brought about,and for this reason examples of circuit diagrams have been dispensed with.

' Another consideration in the selection of .the operating point is the value of alternating current resistance of the grid-cathode path for the frequency to be worked. It may be desirable to so choose the operating point on the curve, that the impedance to alternating current or at least its ohmiccomponent becomes infinitely large. Even though under these circumstances the effective capacity generally assumes zero 2,282,888 that in high frequency amplifying and receiving value, there might be cases where this does not happen, wherefore it is of practical importance to determine operating conditions with particular reference, to the effective resistance.

I claim:

i. In an amplifying system for operation at frequencies sumciently high 'so that the input conductance of a conventional amplifier tube has so large a positive value as to militate against the development of sighal voltage between its input electrodes, said amplifying system including an electron discharge tube having a cathode, a positive grid-like electrode, a control grid,

in the damping is to be and an output electrode arranged in the order stated, and means for exciting said tube electrodes, the method of operation which comprises adjusting the relative potentials applied to said electrodes to produce a variation of output electrode current with respect to control grid potential which in the vicinity of the applied grid potential is definable by the equation i=ke" where n is less than 3/2 and the operating potential of said control grid is negative with respect to that of said cathode whereby the input conductance of said device is less than it would be if said potentials had been so chosen as to make a value of n equal 3/2 applicable in said equation, wherein i of the aforesaid equation refers to plate current, e refers to grid potential, and k is a constant. a

2. The method of operation in accordanci with the preceding claim wherein the said po tentials are so-chosen as to make the value 0 n'which is applicable to the equation of the pre ceding claim sufficiently smaller than3/2 so tha the input conductance is rendered relativer small.

3. The method of operation defined in clair 1 characterized by that n is made sufilcientl less than 3/2 so that the input conductance i negative.

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