GB734060A - Improvements in or relating to negative feedback transistor amplifier interstage coupling circuits - Google Patents

Abstract

734,060. Transistor amplifying circuits. MOULON, J. M. May 22, 1953 [March 10, 1953], No. 14521/53. Class 40 (6). In a two-stage negative-feedback amplifier having a transformer coupling between the stages and fed in series from a common power source, the coupling between the stages is so arranged as to reduce the phase-shifts occurring in the coupling circuit at frequencies below the pass-band of the amplifier and thus increase the stability thereof. In the circuit shown in Fig. 3, which is stated to show a known arrangement, the primary winding of the interstage transformer 13 is connected between the collector and base electrodes of the first transistor 11 in series with a condenser 53. The directcurrent path for the power supply is provided by a connection from the junction of the primary winding and the condenser 53 to a variable tapping point on a potentiometer 14 connected in series with the secondary winding of the transformer 13 between the emitter and base electrodes of the second transistor 12. To provide a low-impedance path for signal currents the potentiometer 14 is shunted by a condenser 15. The condensers shown in the circuit have large values (e.g. 25 microfarads) so as to reduce phase-shifts at low frequencies, below the pass band of the amplifier which in the example shown extends from 300 to 3000 c/s. Negative feedback is provided by a connection made from the primary winding of the output transformer 18 to the input terminal 16 through components 24, 25. (In the circuit shown the feedback connection is not completed and is terminated instead in an impedance z equal to the input impedance of the amplifier.) Fig. 4 shows the arrangement according to the invention wherein the condenser 53 is not connected directly in series with the primary winding of the transformer 13 but is connected instead to the secondary winding as shown. It is stated that by this arrangement the phase-shifts occurring at low frequencies are reduced and the stability of the amplifier is improved. The transformer 13 shown in Figs. 3 and 4 effects a phase inversion; in a modification illustrated in Fig. 6 (not shown), employing a non-phaseshifting transformer, the condenser 53 is connected to the end of the potentiometer 14 connected to the base electrode of the transistor 12 while in order that the feedback may be negative, the feedback connection is taken from a suitable point on the secondary winding of the output transformer 18.