THE AMPLIFICATION FACTOR OF RADIO VALVES IN RELATION TO IMPEDENCE

Radio Record, Volume I, Issue 10, 23 September 1927, Page 2

THE AMPLIFICATION FACTOR OF RADIO VALVES IN RELATION TO IMPEDENCE

{By

J. W.

Muir

Borough Electrical Engineer, Palmerston North).

When the average man buys a radio valve, he is buying something he knows very little abont. He will, naturally, think a valve with an amplification factor of 8 will amplify more than a valve with an amplification of 6. He buys the 8 factor valve, whereas he wonld, very probably, have got louder and clearer reception had he bought the 6 factor one. Firstly, the valve is the means of coupling two electrical circuits together. ‘The first circnit with its weak current flowing is by means of the valve ' xeproduced exactly in the next circuit, but at higher current values. There are two circuits or paths in the valves other than the circuit through the filament. See any simple diagram ot a transformer coupled set. No. 1.-I’rom the secondary of the transformer to the grid across the space in the valve, to the filament and back to the secondary of the transformer, "called the grid circuit." No, 2.-Krom the B battery term.ual through the primary of the next transformer to the plate of the valve, across the spece to the filament, and back fo the neg battery terminal, "called the | plate circuit." You will note that in each case I have said "‘across the space.’? Current usnally will not flow across space, but in a valve when the A battery is turned ow, and the filament flows it gives off

small particles of matter, and it is these that conduct the current across the space mentioned. Immediatzty you light your valyes up, current starts to flow im che dy battery cisenit, Fius cure rent will be a constant value until you tune in a current from the aerial that traverses the grid (No. I circnit). This ‘causes an alteration in the conductance of the plate (No. 2) circuit, in exact relationship, but this circuit having a powerful B battery in it, consequently the strength of the current is greater than that in the grid cirenit. I’rom this it should be quite clear that the valve amplifies the signal received. Each valve has an amplification factor which may be anything from 5 to 20 or even 30, and the average person would think that a valve with, say, an amplification of 8 will amplify 8 times, or give just twice as much volume as one with a factor of 4, This is not necessarily so. Probably this last valve will give the best amplification in an ordinary transformer coupled set, and to try and explain this is the main object of this article, Every valve has an impedance factor, or resistance, which may be anything from 3000 to 20,000 ohms, and it is this factor as well as the amplification factor, that affects the amplification, especially in the audio stages of the ordinary transformer conpled set. We wilt consider what takes place in

each case with the following valves :- No. 1 case, a valve with an amplifia-| tion factor of 8 and an impedarice factor of 12,000 ohms. No. 2 case, a valve with an amplification factor of 6 and an impedance of only 4000 olims. No. 1 case.-The circuit is made up of valve impedance 12,000 olims, and transformer impedance (on a given note) 10,000 ohms.

Now assume the incoming signal to| the grid had a value of 3 a volt. In case 1 it would be amplified 8 times, making it 4 volts to expend in the plate circuit. 2.2 volts of this will be expended in the valve impedance and 1.8 volts in the transformer primary. It will divide in proportion to the impedance in each case. The transformer is, say, 3 to I and the useful signal passed on is now 5.4 volts. The amplification is from $ a volt to practically 54-a total amplification of 11 for the stage. No. 2 case.-The circuit is made up of valve impedance 4000 ohms, transformer on same given note 10,000 ohms. The incoming signal had a value of 4 a volt. It would be amplified 6 times, making it 3 volts to expend in the plate circuit, for the reasons stated before, 8 volts wilt be expended in the valve and 2.2 volts in the transformer primary. The 3 to 1 transformer will pass on 6.6 volts to the next stage and the amplification for the stage is from 14 volts to 6.6 volts, a total amplification for the stage of 18, against 11 for the other valve with a higher amplification factor. This example is of two wellknown valves in general use, and not only will the valve with the lower impedance and amplification factor give higher or louder reception, but it will also give clearer reception, reception in |

which the lower base notes are not dis-torted-reception in which the drums of the band are andible. The explanation of this can be quite easily understood. I vill not attempt to describe it in full, as it requires graphs, eurves and calculations. It will be sufficient to say that the transformer impedance alters with every note received because every note has a different frequency. The bottom line of the base cleft has a frequency of 100,. while

the C above the treble cleft has a frequency of 1000. The impedance of the transformer will vary in reproducing these two notes from approximately 4000 olims, to 40,000 ohms, whereas the impedance of the valve practically remains the same It is quite apparent from this, that of the voltage expended ‘in the plate circuit, the proportion ex- ) pended across the transformer (which is the useful part) will be very much greater when the freqnency is bigh than on the low frequency note, and the proportion expended on the valve impedance will be greater in ratio on the low frequency notes. It is this unequal ratio effect that causes the low notes to be imperfectly reproduced, and you. will be able to ‘reason that the lower the loss, in the -yalve impedance, it naturally follows ‘that there will be less difference in ‘ratio between, the nseful part of the ‘voltage expended, and the wasted volt‘age in the plate circuit over the full range of frequencies, and conseqnently | clearer reception over a wider range. These low impeslancge valves are called power valyes by some makers, I suppose because they will pass on more powerful current fluctuations or signals | without distortion, but a considerable number of makers sell these low imped-. ance valyes as ordinary valves,