US2218902A - Thermionic valve amplifying circuits - Google Patents

Description

Oct. 22, 1940.

A. D. BLUMLEIN THERIIDNIC VALVE AIIPLIFYING CIRCUITS Filed llay 18, 1938 INVENTOR nuw DOM-R 5117mm! BY ATTORNEY UNITED;STATES THERMIONIC VALV AMPLIFYING omourrs Alan Dower Blumlein, London, England, assignor to Electric & Musical Industries Limited, Hayes,

Middleseilr', Britain England, a. corporation 1 of Great icati y 18, 1938, SerialNo. 203, 19

In Great Britain June 5, 1937 This invention relates to thermionic valve amplifying circuits and is particularly concerned With power amplifying. circuits.

It. is. usual in such circuits to employ as the last valve of the'stage a pentode valve or a power tetrode. These-valves are provided with screening. grids which are usually maintained at a constant positive potential with respect to the oathode. Valves of this kind give a greater output for a given anode potential than triodes, but they suffer fromthe disadvantage that their characteristic impedance to the anode is very much greaterthan the ratio'of the steady D. 0. Voltage to the steady D.-C. current. Consequently, in order to obtain a.goodpo wer output the impedance of the load-must be very-much less than'the characteristic impedance of the valve. Furthermore, particularly in'the-case of either a pentode or a power tetrode'valve, which is coupled to its load'through a transformer; it "is known that if the load is removed; thenf-for relatively moderate variations in potential of the input grid, excessively high output voltages will be produced which may besufficient? to cause disruption of'the valve or of the output transformer. 1

-It is the chief object of thepresent invention to provide an improved circuit in which the characteristic impedance-of valves of the'kind referred to can' be more nearlyTmatche'd to their v optimum loads and in which the danger of an excessive increase of 'voltageonremoval of the load may be substantially avoided.

According-to theinvention'a thermionic valve amplifying circuit is provided employing a pentode or tetrode valve in which the screening grid of the valve is so connected that the potential thereof varies in proportion to the A. C. potential variations of the anode of the valve. This result may be achieved by connecting the screening grid to a tapping point on the primary winding, of an output transformer, or alternatively, the screening grid may be connected to a source of potential through a resistance and connected through a by-pass condenser to a tapping point on the primary winding of the output transformer. Other alternative connections for causing the potential of the screening grid to vary in proportion to the potential of the anode may be employed. For example, the screening grid may be connected to a tapping point in a choke connecting the anode of the valve to the sourceof anode current.

The effect of causing the screen to vary in potential proportional to the variations in anode potential is to cause the valve to function somewhat after the manner of a triode. It is known that in atriodethe anode impedance isless than the ratio of the steady D. C. anode voltage to the steady D. Csanode current, Hence, by choosing a suitable tapping point for the screening grid the characteristic impedance of the anode and screen eifectively combined can .be given any desired value between the impedanceof a pentode or tetrode and the impedance of a triode. A suitable tapping point, in practice, may be found to be such that the voltage swing on the screen is between about a quarterand'ahalf the voltage swing-on.the.anode. v .1 1

In order that the invention may be clearly understood and readily carried into efiect it will now be more fully described with reference to the accompanying drawinginwhich: i

Fig. 1' illustrates the. application of the invention to a circuit embodying apentodevalve; and,

Fig. 2 illustrates the application of the invention to a tetrode valve. As shown in Fig 1, signals to be amplified are applied betweenthe icontrol grid 3 and cathode 4 of a pentode valve through a coupling condenser 5 and leak resistance 6, the cathode of the valve being biased by a dropper resistance 1 shunted by a decoupling condenser 8. The suppressor grid 9'is connected'to the-cathode in the usual manner. The anode I0 is connected through the primary winding ll of an output transformer to thepositive terminal of a suitable sourceof high tension current the secondary winding I2 of the transformer being connected to a load not shown. The screening grid. 13 is connected as shown to a tapping point on the primary'winding I I so that the screening grid [3 varies in potential in pro portion to the potential variations on the anode l0. As stated above, the tapping point maybe arranged so that the voltage swing on the screen grid l3 isbetween about a quarter and a half the voltage swing on the anode l0. 40

The arrangement shown in Fig. 2 of the drawing illustrates the invention as applied to a tetrode valve and, in this case, signals to be amplified are applied between the control grid l4 and cathode l5 through an input transformer IS the cathode I5 being biased by a dropper resistance l1 shunted by a decoupling condenser Ill. The anode l 9 is connected to the positive terminal of a source of anode current through a choke 20 and is connected through a by-pass condenser 2| to the primary winding 22 of an output transformer the secondary winding 23 of which is connected to a load, not shown.

In this example of the invention the screening grid 24 is connected through a resistance 25 to the source of high tension current and is also connected in the manner shown through a bypass condenser 26 to a tapping point on the primary winding 22 of the output transformer. If desired, in Fig. 2, instead of connecting the screen grid 24 to a tapping point on the primary winding 22, it may be connected to a suitable tapping point on the choke 20. Many other alternative connections are possible.

The circuits described also have the advantage that if the loads are removed the output voltage obtained for relatively small potential variations of the control grids is not as great as with the arrangements usually employed, since the screening grid which is partially effective in controlling the cathode current will swing in opposite phase to the grid and so prevent or reduce an excessive output voltage.

In some cases the decoupling condensers 8-and l8 of Figs 1 and 2 respectively may not be employed, in which cases negative feedback will occur tending to reduce the second harmonic output. The effect of such feedback will, however, be to increase the impedance of the valves so that in cases where it is desired to reduce the second harmonic output it should be arranged that the potential variations of the screen should be greater compared with the cases in which the decoupling condensers are employed in order to maintain the anode impedance at the required value. In each example described it will also be appreciated that the current which flows in the screening grid also contributes to the useful output of the circuit.

I claim:

1. In an amplifying circuit, a thermionic valve having an anode, a cathode, a grid electrode and a screening electrode, an input circuit for said valve connected between the cathode and grid electrode thereof, said input circuit including means for connecting to a source of signal voltage, an output circuit for the valve including a load impedance across which is developed the output voltage, and means for connecting the screening electrode to an intermediate point of said load impedance device to thereby produce a voltage swing on the screening electrode which is an amplitude reduced image of the voltage swing produced on the anode due to signal energy impressed upon the input circuit, the point of connection of the screening electrode to the load impedance being so chosen that the voltage swing on the screen is of the order of the voltage swing on the anode.

2. In an amplifying circuit, a thermionic valve having an anode, a cathode, a grid electrode and a screening electrode, an input circuit for said valve connected between the cathode and grid electrode thereof, said input circuit including means for connecting to a source of signal voltage, an output circuit for the valve including a load impedance across which is developed the output voltage, and means for connecting the screening electrode to an intermediate point of said load impedance device to thereby produce a voltage swing on the screening electrode which is an amplitude reduced image of the voltage swing produced on the anode due to signal energy impressed upon the input circuit, the load impedance comprising a transformer having a primary winding connected in the output circuit of the valve and a secondary winding adapted to be connected to a utilizing device, said screening electrode being connected to a point of said primary winding.

3. In an amplifying circuit a thermionic tube having an anode, a cathode, a grid electrode and an auxiliary electrode, an input circuit for said tube connected between the grid electrode and the cathode thereof and including means for connection to a source of signal voltage, a source of anode potential for said tube including a high tension terminal, a connection including a choke coil between the anode of the tube and said high tension terminal, an output circuit for said tube including a load impedance connected between the anode and cathode of the tube, means including an impedance device for connecting the auxiliary electrode to said high tension terminal and a by-pass condenser connected between said auxiliary electrode and an intermediate point of said load impedance whereby there is impressed upon the auxiliary electrode an amplitude reduced image of the voltage swing produced on the anode due to signal energy impressed upon the input circuit.

4. An arrangement as described in the next preceding claim characterized by that the output impedance comprises a transformer having a primary winding connected in the output circuit of the tube and a secondary winding provided with means for connection to a utilizing device, said auxiliary electrode being connected through the lay-pass condenser to an intermediate point of said primary winding.

ALAN DOWER BLU'MLEIN.

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