Application Date: July 4, 1936. No. 18597/36.
Complete Specification Left: June 30, 1937.
Complete Specification Accepted: April 4, 1938.
Improvements in or relating to Thermionic Valve Amplifying Circuit Arrangements
I, ALAN DOWER BLUMLEIN, of 32, Audley Road, Ealing, London, W.5, a British Subject, do hereby declare the nature of this invention to be as follows:-
This invention relates to thermionic valve amplifying circuit arrangements and has particular reference to push-pull amplifiers.
Amplifiers of this type are well known, the expression push-pull being used to describe the condition of potentials on a pair of conductors in phase opposition relative to earth. In contrast the expression push-push is used to describe the condition of potentials on two conductors in the same phase with respect to earth. Currents and voltages in push-pull are often described as being balanced with respect to earth. Transformers are usually used to couple the stages of push-pull amplifiers, this form of coupling giving amplification only of the push-pull potentials and not of those in push-push. Transformer coupling is undesirable in some applications of push-pull amplification on account of distortion which may occur due to the iron circuit, or again, in the case of an amplifier handling a wide range of frequencies. If a capacitative or direct form of coupling is employed instead of transformer coupling, any push-push potentials due to unbalances which may be set up and any signals in push-push applied to the input of the amplifier will be amplified. In such cases, also, a phase reversing valve or similar device must be used in order to obtain push-pull signals from a single input potential. Such phase reversing arrangements are shown in, for example, British Patent No. 325,833 which describes circuits enabling a push-pull output to be obtained from an unbalanced input.
According to the present invention, a push-pull amplifying current is arranged to provide discriminative amplification between push-pull and push-push potentials supplied to the amplifying circuit. In a particular amplifying circuit arrangement according to the invention, couplings to the input, output or intermediate stages, or to any or all of the stages, are designed to pass equally push-pull and push-push potentials, the amplifying circuit having greater amplification for the push-pull potentials than for the push-push potentials. The amplifying circuit may include one or more pairs of thermionic valves each pair having an impedance element common to the anode-cathode and grid-cathode circuits of the valves constituting the pair for the purpose of reducing the amplification of the push-push potentials. The impedance element may consist of a resistance, a choke, or both bridged between the cathodes of a pair of valves and earth.
An amplifying circuit arrangement according to the invention may for example be employed in converting a single input potential into a substantially push-pull output for supplying the input to a balanced cable. Again, an amplifier circuit according to the invention may be used to amplify a source of push-pull potential having superimposed unwanted push-push potentials, such a source being constituted for example by a balanced cable subject to interference.
In order to effect neutralisation of anode to grid capacity, the anode of one valve of a pair used in amplifying circuits according to the invention may be coupled by a condenser to the grid of the other valve of that pair, the anode of the latter valve being similarly coupled to the grid of the first valve of the pair.
In order that the nature of the invention may be more clearly understood, some amplifying circuit arrangements embodying the invention will now be described by way of example with reference to Figs. 1 to 4 of the accompanying drawings.
Referring to Fig. 1 of the drawings, input terminals 1 and 2 feed the control grids of valves 3 and 4. If these valves are similar and the input is purely push-pull no alternating current will flow through a resistance 5 connected between earth and the cathodes of the valves 3 and 4 which are connected together. If the input potentials are not in perfect push-pull relationship, or if the valves 3 and 4 are not similar, alternating current will flow through the resistance 5 tending to reduce the push-push currents by negative feed-back. If for example, the valves 3 and 4 have a magnification slope of three milliamps per volt, this will be effective for push-pull, but if the resistance 5 has a value of ten thousand ohms, the slope of the two valves in parallel will only be about 0.1 milliamps per volt for push-push signals. If the anode resistances 6 are of two thousand ohms in the case of each valve, the push-pull gain will be 6 as against 0.1 for the push-push channel, the effective anode resistance for push-push potentials being one thousand ohms. The two valves 3 and 4 are coupled to two further valves 7 and 8 which also have a common cathode resistance 9 which reduce the push-push gain. In the latter pair of valves, resistances 10 and 11 are introduced separately in the cathode circuits. These resistances serve to straighten the characteristic curves of the valves 7 and 8 at the expense of a reduction of both push-pull and push-push gain. The anodes of the valves 7 and 8 are connected to output terminals 12 and 13 which may be connected for example, to a cable through suitable coupling condensers. The usual coupling condensers and resistances have been shown in Fig. 1 and it will be understood that in this and in the subsequent figures to be described, the arrow heads at the ends of the resistances shown diagrammatically indicate that these resistances are connected to sources of high tension or grid biasing potentials as required.
The circuit described may for example be used to provide a substantially push-push output from a single input potential source. If such a source be connected to the terminals 1 and 2, the latter terminal being earthed, a substantially push-pull output will be obtained from the terminals 12 and 13. The input potentials may be considered as being a mixture of push-pull and push-push potentials. The push-push potentials are attenuated relatively to the push-pull potentials in the amplifier circuit so that the output potentials are almost entirely push-pull.
In Fig. 2 an amplifier circuit particularly adapted for terminating a cable is shown. Input terminals 14 and 15 are connected to the two conductors of a balanced cable. The wanted balanced signals arrive at the terminals 14 and 15 in push-pull and are amplified by valves 16 and 17. Interfering voltages arriving in push-push at the terminals 14 and 15 are attenuated due to the insertion of a resistance 18 connected in a similar manner to the resistance 5 shown in Fig. 1. Further attenuation of push-push potentials may be effected by adding further stages as in Fig. 1, or the valves 16 and 17 may be coupled to a phase reversing valve 19 which serves to reverse the potentials from the valve 16 and adds them to the potentials from 17. The valve 19 has a resistance 20 in its cathode lead, this resistance having a value such that the gain of the valve 19 is unity. The anode of the valve 19 is coupled to an output valve 21 which may conveniently be of the cathode following type described in Patent Application No. 448,421. The grid bias potentials for the valves 16, 17 and 19 have been shown as derived from the tapping points in the resistances 18 and 20 and although this is a convenient arrangement, it will be understood that this method is by no means essential.
It will be understood that in the arrangement shown in Fig. 2, the push-pull output potentials of the cable could be obtained to the exclusion of the push-push potentials by the use of a single valve such as the valve 19. However, the particular arrangement shown in Fig. 2 has the advantage that the relative attenuation of the push-push potentials given by the valves 16 and 17, reduces the critical nature of the adjustment required for the valve 19 for a given goodness of balance. Again, the valves 16 and 17 are capable of handling comparatively large push-push potential inputs without overloading due to the fact that for push-push potential inputs, their cathodes follow the potentials of their grids. If very large low frequency interfering input potentials are expected at terminals 14 and 15, a high inductance choke may be inserted at the lower end of the resistance 18 in order to raise this impedance for audible frequencies. Similarly, chokes may be employed in pace of any of the resistances 5, 9, 18 and 20 although if the circuit is to handle a wide range of frequencies, for example, of the order employed in television systems, series resistances will also be required if a low degree of amplification of the push-push potentials is required over the whole range of frequencies.
In Fig. 3 the invention is shown applied to a circuit arrangement suitable for supplying the scanning potentials to a cathode ray tube in a case in which it is required to apply substantially push-pull potentials to the deflecting plates. The anodes to two tetrode valves 22 and 23 are connected to two potentiometers 24 and 25 connected across the source of high tension supply for a cathode ray tube, one pair of deflecting plates 26 and 27, of which are shown. The potentiometers 24 and 25 serve also to constitute the anode loads of the valves 22 and 23. In a particular example, if the source of high tension supply is four thousand volts, the potentiometers 24 and 25 may each be of eight megohms. The anodes of the valves 22 and 23 are supplied from tapping points 28 and 29 half way along the potentiometer resistances. If each anode resistance draws 0.5 milliamps of current, the anode potentials in each case will be one thousand volts and the effective anode load will be two megohms. The screening grids of the valves 22 and 23 are supplied from a separate source not shown in the diagram the cathodes of the valves 22 and 23 are connected together and have a large common resistance 30 which may be of one hundred thousand ohms. Thus, if the screening grids also take about 0.5 milliamps each, this will give a cathode potential of two hundred volts. The control grids of the valves 22 and 23 are fed by condensers and leak resistances, the latter being fed from tapping points at slightly less than two hundred volts in the potentiometer resistances 24 and 25. Should either of the valves 22 and 23 tend to take less steady anode current than the other, the grid potential of that valve will rise and tend to equalise the steady currents. The grid condenser 31 of the valve 23 is earthed, and the grid condenser 32 of the valve 22 is fed with a canning potential wave such as a frame frequency sawtooth. The resistance 30 reduces the slopes of the valves in push-push to about 0.01 milliamps per volt, as against for example, 0.2 milliamps per volt for each valve in push-pull. The resultant output on the canning plates 26 and 27 is chiefly push-pull since the push-pull gain will be forty times the push-push gain. The scanning plates 26 and 27 are fed through condensers 33 and 34 with very high leak resistances 25 and 26