Finer Details of Radio

Radio Record, Volume II, Issue 47, 7 June 1929, Page 30

Finer Details of Radio

Correspondent Comments on Valve Curves

‘*RRHEOSTAT" (Wadestown) writes. -I was much interested in your article on "The Three-Dlectrode Valve as an Amplifier,’ appearing in the "Radio Record" for May 10, but am somewhat at a loss in reconciling one or two points with what I have learnt regarding alternating currents. I must admit that my knowledge of radio is rather limited and not nearly so extensive as my grounding in A.C. work, but it seems to me that the principles are identical. Your article leads one to the conclusion that the "operating point" of the valve describes a line transverse to a number of curves corresponding to the static curves of the valve with differing plate voltage-that the operating point is always on that line, only its extent of travel varying according as the signal voltage varies. Possibly you intended to imply that the "operating curve’ varied as to slope according to the frequency of the applied A.C. Would you please make me clear on this point? It seems to me that the frequency must alter the

slope, as it certainly alters the impedance of the load in the plate circuit of the valve. That isnot my real difficulty, however. What is, is this: The load in the plate circuit of the valve is surely almost wholly inductive, since the grid to filament resistance of the next succeeding valve is, or should be, practically infinite. This will result in the current being considerably out of phase with the voltage. Since the load is inductive, the current will lag the yaltage, the extent of the lag being dependent on, among other things, the frequency of the applied voltage since the ratio of inductive to resistive load will be varied thereby. Assuming that it is true that the current lags the voltage it seems to me impossible that the path described by the "operating point" can, even for a single frequency, be in the form of a single finite line. The voltage across an inductive load is a maximum when the current is changing most rapidly. In dealing with a sine wave, therefore, the voltage is a maximum when the current is a mini-

mum-that is, when the operating point has, after its excursion towards positive. grid voltages, returned to. its initial position, or, to be more accurate, a point vertically above its initial position. Since the load is not wholly inductive, the maximum voltage . will be reached somewhat before this point, but my idea is that the voltage applied to the plate, and consequently the plate current, must be a maximum, not when the negative grid voltage is a minimum, but some time after the grid voltage has commenced to retreat to its former position. Applying the same reasoning at different points of the cycle, it seems to me that the path traced by the "operating point" for any single frequency and applied voltage must be of an oval shape. "Pentode" Replies. UR correspondent is, of course, correct on both ‘counts.. It will be obvious, from an elementary consideration of resistance-capacity coupling, that the slope of the operating curve of a valve varies with the resistance (or impedance) in the plate circuit, the slope becoming less with an increasing resistance. Since an inductance possesses an impedance varying with frequency, the slope of the operating curve will clearly lessen at. the higher frequencies when the impedance is a maximum. The fact that the "operating point" of a valve with an inductive load deseribes an ellipse. is also well-known. At the same time, we do not think that a discussion of a question of this character has its proper place in a journal having a popular appeal; and since the "operating curve" described in the article in question lies wholly within the described ellipse for any particular frequency, it seems that the simplification attempted has some justification. In passing, it may be pointed out that the shape of the ellipse varies with frequency, so that to attempt a full treatment would involve considerable space and the risk of confusing readers unduly.