GB528893A - Improvements in or relating to circuits for the amplification and/or frequency-changing of electric osciallations - Google Patents

Abstract

528,893. Valve circuits for wireless reception. PHILIPS LAMPS, Ltd. May 16, 1939, No. 14647. Convention date, May 19, 1938. [Class 40 (v)] In an amplifier or frequency-changer in which gain control is effected by variation of mutual conductance and in which negative feed-back is provided, the feed-tack is taken from one or more current-carrying electrodes of a valve for which the alternating current increases or at least remains constant when the gain of that valve decreases. In the radio frequency amplifier, Fig. 1, the feed-back resistance 18 is in the circuit of grid 4 only, the anode and screen grid 6 being directly connected to the cathode, for high frequencies, over condenser 17. Volume control potentials are applied over lead 20 to a grid 5 between the anode and grid 4 so that increase of negative potential on lead 20 will cause an increase of current from grid 4 and therefore an increase of feed-back instead of the decrease which is produced when the feed-back is from the anode. Part of resistance 18 provides bias for the signal grid 3. In a valve in which grids 4 and 6 are connected together within the tube, Fig. 2 (not shown), feed back is obtained by coupling a coil in the circuit of these grids, in parallel with a resistance, to a coil connected between the signal grid 3 and oscillatory circuit 11, 12. Fig. 3 shows a frequency-changer with the feed-back obtained from the second grid 33 and local oscillations from a separate oscillator 42 fed inductively between the fifth grid 36 and the cathode. Negative bias for the signal grid 32 is obtained from a resistance 47 in series with a high-frequency choke 46 connected across the feed-back resistance 18. The current distribution between grids 4, Fig. 1, or 33, Fig. 3, and the other current-carrying electrodes of the valve, produced by the volume-control bias should be independent of the voltage of the signal grid. This may be achieved by proper choice of valve and bias potentials or by feeding part of the input signal voltage to the gain-control grid by connecting this grid to a point on the oscillatory circuit 11, 12 instead of over condenser 21 to the cathode. To obtain a feed-back which is substantially independent of the gain-control voltage, Fig. 4 (not shown), the feed-back impedance has passing through it current from all the current-carrying electrodes (anode and screen-grids). This arrangement can be used as a frequency-changer by connecting inductance 43, Fig. 3, between the gain-control grid and the junction of resistance 22 and condenser 21, Fig. 1. The invention is also applicable to low-frequency amplifiers.