H. F. AMPLIFIERS

Evening Post, Volume CIX, Issue 117, 21 May 1925, Page 15

H. F. AMPLIFIERS

STABILISATION SYSTEMS.

Several times in these notes the writer nag referred to.the objections which exist to the system of radio-frequency amplication which involves the use of a potentiometer to prevent self-oscillation of the circuits. -This is by far the most-frequently used stabiliser, when the tuned anode method of coupling is used, and is., also commonly introduced in transformer-coupled circuits to overcome the tendency to oscillate when the receiver is tuned to the natural frequency of the transformers. The system, though it gives results which seem to satisfy its users, is a bad one theoretically. The function of an amplifying valve is to reproduce in its plate circuit a mat;nified true copy of the electrical changes in its grid circuit, and this can only be done when the grid of the valve is kept negative to the filament, and the plate voltage is sufficiently high to give a "straight line characteristic," The use of the potentiometer makes the potential of the grid positive to the filament—with the result that there is a flow of electrons from the filament to the grid; indeed, the stabilisation effect results ■ solely from-the equivalent resistance sot up by the existence of this current. The added effects are twofold. First, the "resistance" set up by the grid-current acts as resistance in a grid circuit always does—it broadens the tuning of the circuit, and reduces its selectivity. Secondly, it decreases the available amplification. This phenomenon is due to the fact that the waveform is "distorted." One half cycle (that in which the grid is more negative than' the normal potential) is more amplified than the other. That means that the waves are to some extent rectified as in a, detector; and as the rectified current variations so produced are of audio-frequency, they cannot be passed on by the coupling with the nest valve, and their effect'is thus lost.

It should be ;understood,that the distortion of wave-form does not affect the quality of the sound produced after rectification, because all that actually passes from stage to stage is a symmetrical wave, and one which will in general be closely approximate to a true "sine wa.ve."

The sacrifice of volume caused by the wave-distortion is not usually serious, and probably it is never noticed. But, apart from the loss of energy already referred to, which affects the- high frequency output of the valve, there is a heavy lobs of B battery current. It hais been pointed out in these notes before that the greater part of the B battery current is sheer waste whatever is done. All that is of value is the varying component. The plate current naay be. Jikened to a stream of water in a trough, with means for taking energy, not from the stream, but fros ripples on its surface. Evidently water will be wasted if the water is deeper than is necessary for the formation of the ripples. But a positive grid means a higher value of plate current than a negative grid, and with an appropriate plate voltage—say, 45 volts—the plate current through an amplifier valve will be at least 50 per cent, higher than if the correct negative bias is used. This waste of say half a milliampere of plate battery current is not perhaps a serious matter, but added to the other defects of the postive grid,' it strongly increases the arguments in favour of some other method of stabilisation. Probably the best method is that known as the neutrodyne system, which has several variations.

A method of stablisation recently introduced for amplifiers of more than one stage, up to any desired number, has been worked out by Mr.' J. Scott-Tag-gart, the well-known English expert, and has been used with success by Borne local experimenters. This is called the "T. A. T." system. The initials stand for "tuned, aperiodic, tuned," and the name implies that the alternate circuits are resonant and non-resonant. The input of the first valve is, of course, tuned in the ordinary way, but its output is coupled to the next valve by a choke-coil and condenser (reactance capacity) for short waves, or a resistance and condenser for long waves. The coupling to the third valve is another "tuned anode" circuit, and so on alternately for any required number of stages. The rationale of the. system is that each tuned circuit can be set exactly to the received wave-length, but, no matter how strong the voltage oscillations in it are, they cannot make the .preceding untuned choke or resistance circuit osculate, and the set remains stable. .The.circuit'is a most interesting one. Its originator describes is as of high efficiency; he says it does not give as high amplification as an equal number of tuned stages carefully stabilised, but it is free from the uncontrollability of the fully-tuned series, and is more efficient than a series of transformed coupled stages. A great deal of interesting experimental work is possible with . this system; for instance, the use of choke-coils of copper wire or of resistence wire may be com ; pared. A well-known method of stabilisation, used in some of the well-known commercial sets, uses a tuned transformer of a special type. The secondary of the transformer is tuned with a condenser. The primary consists of a very few turns, and the effect of the combination is that the amount of oscillating energy in the transformer' exists almost wholly in the secondary: ' The oscillations in the primary are so weak that, though it may be possible, for them to react through the valve and oscillate its grid circuit, the, tendency is small, and the circuit is comparatively stable. This is a good system, but the amplification is likely to be less than with a more liberally wound primary with the capacity effect of the valve neutralised.