GB716442A - Improvements relating to thermionic valve circuits - Google Patents

Abstract

716,442. Valve amplifying circuits. ELECTRIC & MUSICAL INDUSTRIES, Ltd. Jan. 15, 1953 [Jan. 19, 1952], No. 1585/52. Class 40 (6) In an amplifier wherein signals are applied over a capacitor-resistor path to a valve having such a high resistance in the grid circuit that distortion would occur, the impedance seen by the grid of the valve is lowered to prevent distortion. The circuit is described as applied to a very wide band amplifier, Fig. 2, the signal applied at 10 being fed down an artificial line 13 tapped to provide signals for valves 7, 8 and 9 over small coupling capacitors 15, 16 and 17 having a small capacitance to earth. The signal is also fed over a capacitor 23 and resistor 24 of long timeconstant to a cathode follower 12 so that the grid resistors 18, 19 and 20 are fed from a low impedance source. The valves 7, 8 and 9 feed a second line as described in Specification 460,562. In a two-stage amplifier, Fig. 3, the output line 31 serves as the input to the second bank of valves 28, 29 and 30 feeding the output line 52. The low frequency signals are fed over cathode coupled pair 40, 41 to the grid resistors of the valves 28, 29 and 30 and are arranged to have unity gain. At very low frequencies the source impedance 55 tends to couple the stages and introduce negative feedback. The feedback signal appears equally at both ends of resistor 32 and the resulting signals are cancelled in the cathode coupled stage 40, 41, resistor 42 being adjusted to ensure this. Resistors are included in the cathode leads of all the valves to improve linearity. A cathode follower with A.C. coupling may be interposed between valve 41 and valves 28, 29 and 30 to allow the bias of the latter to be adjusted.