US2183707A - Resistance-coupled amplifier - Google Patents

Description

Dec. 19,1939. w. BUSCHBECK 3, 0

RESISTANCE COUPLED AMPLIFIER Filed Oct. 8, 1936 I 1 4 i c 3 l 1 L/ 5 Mdk/ABLE' "r1 @5255!"- 500/265 2 W3 INVENTCR ATTORNEY Patented Dec. 19, 1939 RESISTANCE-COUPLED AMPLIFIER Werner Buschbeck, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic m b. H., Berlin, Germany, a corporation of Germany Application October 8, 1936, Serial No. 104,632

In Germany October 17, 1935 Claims.

The operation of resistance-coupled amplifiers as is well known from actual practice, is attended with the difliculty that the amplification decreases with growing frequency. The underlying reason resides in the plate-filament capacitance connected in parallel to the plate resistance and the self-capacitance of the plate resistance and the leads. So far as the frequency bands heretofore used are concerned, it proved sufficient 1 to eliminate the influence of harmful capacitance by choosing either the internal resistance of the tube or the ohmic plate resistance-low in comparison with the capacitive reactance, or by resorting to both these steps at the same time.

Weakening of high-pitched notes is still further promoted in that a counter-coupling or degenerative coupling occurs by Way of the gridplate capacitance as the frequency increases.

For very broad frequency bands such as required in television work extending all the way from 16 up to several hundred thousand cycles, the measures and steps customary in the earlier art are no longer adequate to meet conditions, for if they were carried to the ultimate limit, amplification would become illusory since a fall of the frequency curve would result because of the counter-coupling or degeneration mentioned. If the number of tubes is increased, this would also raise correspondingly the grid-plate capacitance and thus also the counter-coupling so that in this manner the drawback of diminished gain could not be eliminated or offset. Attempts have been made to shift the droop of the frequency characteristic somewhat in the direction of the 35 higher frequencies by connecting an inductive reactance in series with the ohmic resistance.

However, it is not feasible to amplify a very large frequency band in this manner since the frequency curve rises and cannot be altered so far 40 as its form is concerned.

Hence, according to this invention the suggestion is made to provide a plurality of series circuits comprising ohmic and inductive resistances in parallel, if desired, with a further ohmic resistance being connected in parallel, and to so dimension the ohmic resistances and the inductive reactances that with increase of frequency the various parallel branches will become sequentially high compared with the total resistance of 50 the other parallel or multiple branches.

The drawing shows an exemplified embodiment of the basic idea of this invention. The undesired capacitance is here indicated in the form of condenser C. For low frequencies the chokecoils S2 and S; are substantially without effect, so

that the resultant plate resistance is predicated upon the paralleling of the three resistances W1, W2 and W3. The choke-coil S3 is so proportioned that its inductive reactance becomes high in com-v parison with the resistance We when the droop of the frequency characteristic starts as a result of the harmful capacitance C and the countercoupling or degeneration. As the frequency rises still more, the impedance of the branch comprising S3 and W3 finally becomes so high that it will no longer play any part compared with the other branches. In other words, the decrease of the effective plate resistance occasioned by stray or spurious capacitance and degeneration is compensated, as will be noted, by a rise of the other resistance.

As soon as the influence of the harmful capacitance makes itself felt again as a result of a further increase in the frequency and the absence of compensating action, the choke-coil S2 becomes operative and thus raises gradually the impedance of the branch S2, W2. Finally, in the presence of Very high frequencies, it will be only the resistance W1 and the spurious capacitance that are active. The fall of the frequency curve towards the end of the range or band could be still further retarded by the provision of another inductance in series with the resistance W1. As shown a source of potential of variable frequency is connected across the grid I and cathode 2 of the amplifier tube B and the amplified energy is taken off by a condenser 3 having one side connected to the plate 4 and upper end of W1 whose lower end is connected to the cathode through any suitable source of plate current A.

One merit of the present invention so far as amplification is concerned resides in the fact that the ohmic, resistance which is effective in the presence of high frequencies may be chosen equal to, or greater than, the capacitive reactance, while otherwise it would have to be made lower if a satisfactory shape or trend of the frequency characteristic is to be obtained.

Having described my invention, what I claim as resistor and an inductance coil, said coil being dimensioned in respect to the resistor so that its inductive reactance becomes high in comparison with the resistance of said resistor at frequencies of the electrical currents where degeneration caused by said inherent capacity between the anode and cathode of the tube is substantial.

2. Means for amplifying electrical currents covering a broad band of frequencies comprising in combination, an electron discharge device having a cathode, a control grid and a plate, a high resistance having one end directly connected to said plate and its other end connected to said cathode, and a circuit conductive to direct current shunted across the ends of said resistor and having two parallel branches, each of said branches including an inductance coil and the inductance of one of said coils being substantially greater than that of the other.

3. Means for amplifying electrical currents covering a band of frequencies comprising in combination, an electron discharge tube having a cathode, a control grid and an anode, a high resistance having one end directly connected to said anode and its other end to said cathode, a coupling means connected to said anode, the series connection of a first coil and a resistor shunted across said high resistance and the series connection of a second coil and a second resistor shunted across said high resistance, the

inductive reactance of said second coil being substantially greater than that of said first coil.

4. In a resistance-capacity coupled amplifier including an electron discharge device having an anode, a cathode and a grid electrode, an interstage coupling network comprising in combination an output circuit connected between said anode and cathode and including an anode resistor, a first circuit shunted across said anode resistor and including an inductance coil and ohmic resistance element in series, a second circuit shunted across said anode resistor and including an inductance coil, the inductance of one of said coils being considerably greater than that of the other thereof.

5. Means for substantially uniformly amplifying electrical currents covering a very broad frequency band comprising an electron discharge device having an anode, a cathode and a grid electrode, an output circuit connecting said anode and cathode and including a resistor element, a plurality of parallelly arranged circuits shunted directly across said resistor, each of the last named circuits including an inductive reactance and an ohmic resistance in series, said circuits being dimensioned so that with increasing frequency the resistance of the various circuits becomes sequentially high compared with the total resistance of the other parallel circuits.

WERNER BUSCHBECK.

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