GB677921A - Improvements in wide band amplifier coupling circuits - Google Patents

Abstract

677,921 Thermionic valve amplifiers. McINTOSH, F. H. Dec. 22, 1949 [Dec. 22, 1948], No. 32863/49. Class 40 (vi). [Also in Group XXXV] The anode of one valve of a wideband pushpull amplifier is connected to H.T. positive through an iron cored primary winding wound in the same sense as, and closely coupled to a second primary winding connected between the cathode of the second valve and H.T. negative. As shown, Fig. 1, valves 1, 2 are fed in phase opposition by sources 8, 9. The output transformer T has a bifilar primary winding 18, 21 and the winding 18 is connected between the cathode of valve 1 and H.T. negative while winding 21 is connected between the anode of valve 2 and H.T. positive. The windings have a small leakage inductance and, because the windings are closely coupled and connected at one end through the small A.C. impedance of the source, there is only a very small alternating potential difference between them, so that the effect of the inter-winding capacity is reduced. A condenser may be connected between the anode of valve 2 and the cathode of valve 1. Since the gains of valves 1 and 2 are unequal the amplitudes of sources 8, 9 are adjusted so that the outputs are equal. To enable inputs of equal amplitude to be used, two bifilar primary windings are provided, Fig. 2, not shown, the parts of one winding being connected to the cathode of one valve and the anode of the other, while the parts of the second winding are connected to the anode of the first valve and the cathode of the other valve. If pentode or tetrode valves are used, the screen grid of each valve is connected directly to the anode of the other valve. Since there is no alternating potential between the winding connected to the anode of one valve and that connected to the cathode of the other valve, the screen grid of each valve is at the same alternating potential as the cathode. Negative feed-back may be taken from taps on a resistor connected between the cathodes and applied in series with the inputs. In another embodiment, Fig. 5, the input is applied between the grid of valve 1 and ground, while the input to valve 2 is applied between the anode and grid. The heavy negative feedback thus applied to valve 2 compensates for the feed-back to valve 1 caused by winding 18 in its cathode circuit, so that equal inputs can be used. The valves are preferably operated in Class B. The impedance of each primary winding at the lower end of the band is of the same order as the internal resistance of the valves, and each pair of primary windings is unity coupled. Various constructions of output transformer are shown and described (see Group XXXV). The output may be taken from tappings on the primary windings.