Anode Bend Detection

Radio Record, Volume III, Issue 6, 23 August 1929, Page 28

Anode Bend Detection

For Local or Distance

(By

MEGOHM

Second Installment. | AST week a unit was describ- >} ed in which any valve suitable for use as a detector might be employed as an anode bend detector for local reception. Generally speaking, the lower the impedance of the valve: the higher will be the grid bias required. By using a-valve of not less than 50,000 ohms impedance, the required grid bias will be less than a volt, and provided that the unit is only to be used with a valve of such impedance} then the bias battery may be omitted, and the bias obtained from the potential divider across the A battery alone. This entails only the omission of the bias battery and joining together the two wires previously connected to it. This method will give a negative bias up to half the voltage of the A battery, variable from zero upwards. Another method is to omit the bias battery and potential] divider and obtain bias from the voltage drop across the filament rheostat, provided there is the necessary margin between the filament voltage of the valve and the battery voltage, as whén a 5-volt filament is run from a 6-volt battery. This gives a bias of one volt, which may be more than is required in some cases, Even the small drop available from a 8.7 or 4-volt valve on a 4-volt accumulator may prove sufficient, as some valves will give good results with practically no bias. The only objection to obtaining bias in this way is that it is not conveniently variable. For this purpose the rheostat must be in the negative leg of the filament, and grid return to negative also. An Experiment. BS shorting-out the grid-leak and condenser of a two R.F. Browning-

Drake, placing the detector rheostat in the negative lead, and grid return to negative, a valve of 55,000-ohms impedance gives excellent results, with much less tendency to overloading: the detector, and with greater stability. The anode head detector must be fed with strong signals, and these are obtainable with the two stages of R.F. Moreover, the reduced tendency to overloading in this class of detector, compensates in great measure for its less favourable conditions for amplifieation. -This is an experiment that any owner of a two R.F. BrowningDrake may test for himself. ° Alternative Biasing Methods. "THE kiasing circuit shown last week allows of fine adjustment, once the voltage of the bias battery has been determined, This method, also. does not put any potential divider drain upon the bias battery, thus greatly prolonging its life. Accompanying diagrams show three alternative methods of employing. a bias battery. No. 1 shows the placing of a battery where the required bias value is known approximately, but this method gives no more adjustment than the "O" battery taps. No. 2 shows how the potential divider may be connected across both "A" and "OC" batteries, giving a wide range of voltages, whilst No. 3 shows the connections with the potential divider across the "C" battery only. As a rule, the drain of the potential divider on an "A" accumulator will be negligible, but on dry cells eare must be taken that it is not of two low a value, ‘otherwise the life of the cells will be shortened.

In the diagrams, "B’ negative is shown jcined to "A" positive, but this may be connected to."A" negative without in any way affecting the remainder of the circuit. Action of the Anode Rectifier. REFERENCE to the diagram, which shows roughly the characteristic curve of a valve, we note that there is a straight portion, with a curved or "bend" portion at either extremity.

In this diagram plate current is measured vertically and grid potential horizontally. If we suppose that the grid voltage is fixed normally at A, then the corresponding plate current is shown at B. This is the plate current passing throug the valve when the grid is at hormal voltage. Now suppose that the alternating signal on the grid ren-. ders’ it ohne volt more negative, the position © will represent this value. and position D will represent a Value one volt more _ positive. Note the corresponding plate currents for these two positions, F and G, and it will be seen that when the grid is positive much more plate current flows than when it is negative. .. By applying to the grid an extra negative bias potential of suitable voltage it is possible to suppress the nega-

tive alternating impulses of the signal, and thus the rectifying. or "one-way" action is obtained from the positive impulses. . In other words, a positivesignal impulse on the grid at the lower bend in the curve increases the plate current more than an equal negative displacement decreases it. The net result is an increase in plate current and, as shown, at the right of the diagram, the wave form is made much greater on one side than on the other, whereby rectifying action is obtained. The rectifying action is very similar to that of a crystal, but there isthe ‘relay or amplifying action in addition. Changes in plate current caused by signal voltage on the grid do not in themselves give amplification; it is when these changes pass through a high resistance in the plate circuit. that they are changed to voltages much higher than those impressed upon the grid of the valve.: The similarity is in the method of production of audio-frequency from the high-frequency: signal impulses, differing greatly from the leak-condenser cumulative. method. The anode rectifier produces the sound in the ‘phones by an increase in the plate current, whilst the grid detector effects it by a decrease in the plate current. ' Although positive and negative impulses arrive at the grid alternately,. the grid need not necessarily ever be postive. When a negative impulse or cycle arrives the grid is negative to the extent of grid bias plus the voltage cf the signal. When a positive cycle are rives, the potential of the grid is that of the bias minus the signal voltage. As the plate voltageds increased, the difference between the plate current corresponding to positive and negative | signal alternations become greater, and provided that signal strength is good, ‘this system of detection becomes more efficient when a high plate voltage is used. Doubling the input to. an anode rectifier results in an increased output of three or four times the volume, so that reduction of loss in the tuning system or improvements in the aerial have a large effect upon volume, though usually this aspect is not important for local reception. Increasing plate voltage has the effect of moving the whole curve,,.and with it the normal grid voltage position A to the left, and this allows a greater negative bias to be applied.

Increase of. plate voltage always requires a suitable increase of bias. volttage, which should be carefully adjusted. By shifting the point A te wards the left and applying higher bias voltage,’ the signal alternations may have asegreater amplitude, which is equivalent to more’ volume without causing distortion. When the alter-

nating signal impulses make the grid alternately more and less negative instead of alternately negative and positive, the best effect is being obtained. Actually, when the plate voltage is low, and. the grid is positive during a portion of one half-cycle, then electrons are attracted by the grid from the filament-to-plate stream, forming what is known as "grid current." This ‘unwanted flow of current to the grid causes distortion, butvis absent so long #as a sufficiently high plate voltage is employed, along with suitable grid bias. In other words, if the maximum of the input voltage does not exceed the bias voltage, the grid will never be actually positive with regard to-the filament, and therefore grid current will not flow. It is. this reduced liability to the flow of grid current that gives the anode rectifier its superiority for quality over. the leak-condenser type. The employment of reaction has the same detrimental effect upon ‘quality with either’ system of rectification, unless used with extreme moderation. Anode bend ‘rectification is not put forward as anything new. The method igs quite old, but for a. considerable time has been superseded by the popular grid rectifier on account of the laffer’s greater sensitivity. Now that re@civers and valves have been much improved, and quality is considered essential, the anode rectifier is being revived, and when preceded by efficient radio-frequency stages will find: greater favour with many constructors than does the grid-leak type. It will, be , found that there is also a gain in quality from distant stations, apart from the usual interference of atmospherics and induction.