Radio Broadcast (May 1929-Apr 1930)

Data on Performance of Various Tube Types GRID-LEAK VS. BIAS DETECTION By J. M. STINCHFIELD Engineering Department, E. T. Cunningham, Inc. Many recent advances in the theory of detection of a modulated r.f. carrier have given the development engineer a good theoretical basis for improving the performance of the detector stage in radio sets. The work of Carson, Llewellyn, Chaffee, and Ballantine in this country, and of Colebrook in England, was of fundamental importance. The relation of tube and circuit components has been investigated and the influence of these factors on the performance of the detector stage has been indicated. The theory indicates the following factors to be of importance in the design of the detector stage. Grid-leak Detection (1) The modulated r.f. voltage is applied from a resonant circuit through a grid condenser to the grid and cathode terminals of the tube. A loss of r.f. voltage will occur in the grid condenser. Usually this loss is determined principally by the effective input capacity of the tube to r.f. and the capacity of the grid condenser. (2) Rectification takes place in the grid circuit due to the change in slope (where the action is essentially that of high-vacuum electron conduction) of the grid-current grid-voltage curve. Small internal grid resistance and small external grid-circuit impedance to the r.f. increase the rectification. The grid resistance is that due to electron conduction and is entirely analogous to plate resistance. The tuned r.f. input circuit will be damped some depending upon the grid resistance of the tube and to a less extent upon the grid-leak resistance. This will affect both the gain and selectivity from the r.f. stage. (3) Among the rectified components will be direct current and audio frequencies. The direct-current component of the rectification will flow through the grid leak producing a small change in the effective d.c. bias voltage. In traversing the grid circuit the audio frequencies will encounter the high impedance of the grid condenser and the high resistance of the grid leak. The tuned r.f. circuit obviously offers negligible impedance to the audio components. The audio-frequency voltage developed between the grid and cathode terminals, due to the impedance of the grid leak and condenser combination, are amplified by the tube in the usual way. When considering the plate circuit load impedance of an amplifier tube, a load impedance that is much larger than^ the tube's internal plate resistance at all frequencies will give uniform output. Since nearly all of the voltage in the circuit is developed across the load, no change can occur as the load impedance changes with frequency. In the same way, when the a.f. load in the grid circuit is high at all frequencies with respect to the tube's internal grid resistance, the voltage across the load will be independent of frequency. The a.f. input capacity of the tube must be added to the grid condenser capacity when calculating the impedance. (4) The tube will also amplify the r.f. This r.f. in the plate circuit will produce some plate-circuit detection, depending upon the change in slope of the platecurrent curve. The plate detection is R.F.Input — A.F.Output / D M P etection C odulation = ate Voltag irves 0.22 :+45 Ira /ci J A V r "0 0.04 0.08 0.12 0.16 0.20 0.24 RADIO:FREQUENCY INPUT (VOLTS R'M.S.) Fig. 1 nearly in phase opposition to the grid detection, though usually much smaller in magnitude, so that a small decrease in the audio output results. A by-pass condenser for the r.f. is usually connected from plate to cathode. While this tends to increase the plate rectification, it has a large effect in reducing the input capacity and increasing the input resistance resulting from feedback through the grid-plate capacity. For example, with a c-327 gridleak detector and a 50,000-ohm resistance s DETECTOI Cunning R.F.Input 0.7 Modulat SENSITIVITY lam C-327 37 Volts R.M.S. on =0.22 / Plate Load lmpe ance-200,000 ohms / t -V 4i \*/. i i \ ! / X \ L \ // / V / / / 1 i / J i / -15 -10 C0NTR0LGRID BIAS VOLTAGE Fig. 2 plate load, removing the plate-filament by-pass condenser had a negligible effect on the output when the r.f. input was supplied by a potentiometer, but when supplied by a tuned stage the voltage gain in the r.f. stage was changed from 7.4 to 3.1. The r.f. amplifier was a c-327 giving a gain of 10 when feeding a biased detector. The r.f. bypass and an r.f. choke in the plate circuit also help to prevent the r.f. from loading up the audio amplifier and from being radiated back to the input. If the impedance of the r.f. bypass at the highest a.f. is not several times larger than the impedance of the a.f. load and the tube resistance, the output at higher audio frequencies will be reduced. Bias Detection (1) When the grid is biased negatively beyond the point at which grid current begins to flow, no rectification will occur in the grid circuit. The modulated r.f. voltage applied to the grid and cathode terminals is amplified by the factor mu and appears in the plate circuit. The rectification takes place in the plate circuit. Since the amplification factor is not absolutely constant, particularly in the region of the plate-current cut-off, some nonlinear amplification results. This increases the rectified a.f. (2) Small internal plate resistance and external plate circuit impedance increase rectification. The external plate-circuit impedance can be kept low to r.f. by a bypass condenser connected between the plate and cathode terminals. This also helps to maintain a high input impedance in the grid circuit by reducing feedback through the grid-plate capacity. The rectification produces a number of component frequencies which include direct and a.f. currents. If the d.c. resistance of the plate circuit load is high there will be an appreciable decrease in the effective d.c. plate voltage. The a.f. voltage output will depend upon how large the a.f. load impedance is with respect to the internal plate resistance of the tube. The r.f. bypass may reduce the load impedance at the higher a.f. In the following the overall performance characteristics of some tube types widely used as detectors are compared. Fig. 1, shows a comparison of a cx-301a tube and a c-327 tube operated in a grid-leak detector circuit. The voltages and the circuit constants are favorable to both types. It is evident that the detector sensitivity is greatest with the c-327. The lower curve of Fig. 1 shows the a.f. output of the c-327 as a bias detector. The load of the biased detector in this figure was a 150-henry choke shunt by an 0.25-megohm resistor and an 0.0001-mfd. condenser. The other detectors looked into an Amertran DeLuxe first-stage transformer. The sensitivity, i.e., the a.f. output for any given r.f. input, 350 • • OCTOBER 1929 •