GB492407A - Improvements in or relating to electric amplifiers - Google Patents

Abstract

492,407. Valve amplifying circuits. GENERAL ELECTRIC CO., Ltd., and BIGGS, A. J. June 10, 1937, Nos. 16168 and 16897. [Class 40 (v)] The invention relates to circuits of the kind in which two phase opposed output voltages for application to a push pull amplifying stage are derived from a single input and seeks to avoid the inherent disadvantages of one known arrangement comprising a valve with high resistances in anode and cathode circuits, output voltages being taken from anode-HT negative, and cathode -HT negative. According to the invention as embodied in Fig. 3, the cathode resistance R<1> is made small compared with the anode resistance R of valve 1, the phase-opposed voltages across 4, 6 and 5, 6 being balanced by amplifying the voltage across R<1> in an auxiliary valve 7 without change of phase. The anode load resistance S of the valve 7 is greater than R and the Specification shows mathematically that the condition for balanced output voltages is where g2 is the mutual conductance of 7. Resistance 10 is large compared with R<1> and 9 is an blocking condenser with small impedance at audio frequencies. The condition of balance depends on the characteristic of valve 7 ; in order to prevent replacement of this valve from disturbing this condition, the output circuit may be modified, as shown in Fig. 1, by connecting two equal resistances r1 with a common portion r2 in the control grid circuit of valve 7 It is shown mathematically that, while the condition for balanced voltages is still as for Fig. 3, provided the term r1 + 2r2 is large compared with the right-hand side of equation (10) of the Specification the balance will be almost perfect even if the equation of Fig. 3 is not fulfilled ; r1 and r2 need not be exactly equal and may constitute the grid leaks of following amplifier valves.