US2164899A - Wide band amplifier - Google Patents

Description

July 4, 1939- w. BUSCHBECK WIDE BAND AMPLIFIER .Filed Oct. 8, 1936 MODULATOR 0R AMPLIFIER 7'0 SIG/VA L SOURCE INVENTOR WERNER BUS BECK ATTORNEY Patented July 4, 1939 PATENT OFFICE WIDE BAND AMPLIFIER Werner Buschbeck, Berlin, Germany, assignor to Telefunken Gesellschaft fiir Drahtlose Telegraphic in. b. H., Berlin, Germany, a corporation of Germany Application October 8, 1936, Serial No. 104,633 In Germany October 22, 1935 8 Claims.

My invention relates broadly to an amplifier adapted to amplify a wide frequency band and more particularly to a type of amplifier employing a combination resistive and impedance coupling.

The operation of the well known resistance coupled amplifiers is attended with the drawback that due to the high ohmic resistances included in the plate circuit, a high direct our- 10 rent potential drop is caused by the plate current and as a result the potential impressed on the plate is correspondingly decreased.

While it is true that the use of choke coil coupling avoids this drawback, such coupling has the inherent disadvantage that the gain in the amplifier becomes more dependent upon the frequency of the input signals to the amplifier.

In the prior art it has, therefore, been usual to parallel a resistance with the choke coil.

For the amplification of particularly wide frequency bands, an audio-frequency choke and .a radio frequency choke may be connected in series. But, even with this arrangement, if merely an ohmic resistance is connected in parallel relation to such a connection of chokes, no useful fre quency characteristic is secured for such wide frequency bands as are common in television work. As, for instance, from 16 to 1,000,000 cycles per second. The trouble has been that resonant points are reached in the series connection and this connection then becomes merely a short circuit across the resistance.

According to my invention, a resistance is joined in parallel to the series connection of chokes, that is, the audio-frequency and the radio frequency chokes, and one or more of said chokes are damped by resistances connected in parallel thereto, these resistances usually being higher than what is usually termed the outer resistance which is joined in parallel to the series circuit comprised of the audio-frequency and radio frequency chokes.

My invention will best be understood by reference to the figure which shows one embodiment thereof.

Referring to the figure, the outer resistance W is joined in parallel to a series circuit comprised of radio frequency choke H and audiofrequency choke N. One side of the series parallel arrangement is connected to the plate of vacuum tube K, while the other side is joined to a series of positive potentials (not shown) which is indicated by a plus sign. The potentials developed across the output circuit of vacuum tube K are impressed. through a coupling condenser L on to the grid of the next tube which may be, for instance, a modulator tube or an amplifier tube.

Joined in parallel with radio frequency choke H is a damping resistor R. It is, of course, 5 realized that a similar resistance may be joined in parallel with audio-frequency choke N only, or both of the chokes may be damped. The value of the resistance W is so chosen that in spite of the frequency response of the choke coils, 10 the resultant impedance of the output circuit will be practically constant.

The audio frequency choke which acts as a choke coil at low frequencies will act as a capacitance at frequencies above its natural period (so- 15 called stopper circuit effect). Inasmuch as the inherent ohmic resistance in the choke coils is very low, nearly the full potential of the positive potential source is effective at the plate of vacuum tube K. 20

Short-circuiting of the ohmic resistance W in the presence of low frequencies is avoided by the audio-frequency choke which, as has been stated, will act as a capacitance above its natural period and as the frequency rises, short circuit 25 resonant points are produced and between the bands the choke acts alternately as an inductance and as a capacity. At high frequencies at which the audio-frequency choke has a capacity, the radio frequency choke coil H becomes operative. 30

Consequently, it may not be avoidable that the series circuit comprised of chokes H and N will become resonant at definite frequencies and act as a short circuit across the resistance W. In order to prevent this short-circuiting, there 35 is illustrated in the drawing a damping resistance R joined in parallel to the radio frequency choke. It might just be joined in parallel to the audio-frequency choke. A resistance connected in series to the choke coils would not 40 be suitable because the potential drop would be developed across it, occasioned by the plate current of vacuum tube K, thus cutting down the potential impressed on the plate of the tube.

The value of the damping resistance is chosen 45 so that when a short-circuiting condition is reached, that is, to say when the series circuit comprised of chokes H and N becomes resonant, it is equal to the reactance of the radio frequency choke coil. Therefore, there is occasioned 50 by this parallel connection of the choke and resistance, an effective resistance of one-half of the damping resistance. This is naturally in series with the plate of the vacuum tube K. Hence, the values of the various circuit elements 55 must be so chosen in such a way that this series resistance will still be high in contrast with the ohmic value of the plate resistance of the tube.

What I claim is:

1. In a Wide band amplifying circuit, a coupling circuit comprising a series electrical circuit con.- sisting of a radio frequency choke coil and an audio frequency choke, a resistance connected in parallel to said radio frequency choke, and a resistance joined in parallel with said series circuit.

2. In a wide band amplifying circuit, a coupling circuit comprising a series electrical circuit consisting of a radio frequency choke coil and an audio frequency choke, a resistance of such value as to be substantially equal to the reactance of the radio frequency choke where said series circuit is resonant joined in parallel to said radio frequency choke, and a resistance joined in parallel to the series circuit. a

3. In a wide band amplifier, an electron discharge tube including anode, cathode and control electrodes, a series electrical circuit joined to the anode of said tube comprising a radio frequency choke, and an audio frequency choke, a resistance joined in parallel to said radio frequency choke, a resistance joined in parallel with the series circuit, and a source of positive potential joined to said series circuit to furnish anode biasing potential.

4. In a wide band amplifier, an electron discharge tube including anode, cathode and control electrodes, a series electrical circuit joined to the anode of said tube comprising a radio frequency choke and an audio frequency choke, a resistance joined in parallel to said radio frequency choke, said resistance having substantially the same value as the reactance of said radio frequency choke when the series circuit is resonant, a resistance joined in parallel with the series circuit, and a source of positive potential joined to said series circuit to furnish anode biasing potential.

5. In a wide band amplifying circuit, a coupling circuit comprising a series electrical circuit consisting of a radio frequency choke coil and an audio frequency choke, a resistance connected in parallel to said radio frequency choke, and a resistance joined in parallel with said series circuit, the value of said resistance being so chosen as to substantially obviate a change in frequency response of said radio and audio frequency chokes.

6. In a wide band amplifying circuit, a coupling circuit comprising a series electrical circuit consisting of a radio frequency choke coil and an audio frequency choke, a resistance of such value as to be substantially equal to the reactance of the radio frequency choke when said series cir; cuit is resonant joined in parallel to said radio frequency choke, and a resistance joined in parallel to the series circuit, the value of said resistance being so chosen as to substantially obviate a change in frequency response of said radio and audio frequency chokes.

7. In a wide band amplifier, an electron discharge tube including anode, cathode and control electrodes, a series electrical circuit joined to the anode of said tube comprising a radio frequency choke, and an audio frequency choke, a resistance joined in parallel to said radio frequency choke, a resistance joined in parallel with the series circuit, the value of said resistance being so chosen as to substantially obviate a change in frequency response of said radio and audio frequency chokes, and a source of positive potential joined to said series circuit to furnish anode biasing potential.

8. In a wide band amplifier, an. electron discharge tube including anode, cathode and control electrodes, a series electrical circuit joined to the anode of said tube comprising a radio frequency choke and an audio frequency choke, a resistance joined in parallel to said radio frequency choke, said resistance having substantially the same value as the reactance of said radio frequency choke when the series circuit is resonant, a resistance joined in parallel with the series circuit, the value of said resistance being so chosen as to substantially obviate a change in frequency response of said radio and audio frequency chokes, and a source of positive potential joined to said series circuit to furnish anode biasing potential.

WERNER BUSC'HBECK.

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