GB1058556A - - Google Patents

Abstract

1,058,556. Loud-speaking telephones. GYLLING & CO. A.B. March 18, 1964 [March 19, 1963], No. 11550/64. Headings H4K and H4R. In a two-way speech amplifier, e.g. for a loudspeaking telephone, in which attenuators are arranged in the two channels the attenuation of the attenuators being differentially varied in dependence on the signals applied to the attenuators, the attenuators include the collector-emitter paths of a pair of transistors which have at least part of their emitter loads common so that, as the control signals applied to the bases of the transistors change the collector currents differentially vary and thus differentially vary the collector-emitter impedances of the two transistors, one of which is connected in the attenuator circuit of each path. Fig. 5 shows a preferred embodiment of the variable gain control stages of a loud-speaking telephone in which transistors T3 and T4 are included one in each amplifier path of the apparatus. The emitter and collector loads of transistors T3 and T4 are all shunted by the collectoremitter paths of further transistors TA and TB connected according to the invention, i.e. TA and TB have a portion of their emitter loads common and are fed on the bases with control signals Vs superimposed on a bias potential Vf. In addition a third transistor Tc is connected with its emitter current flowing through the common emitter resistor so as to provide, in the transistor which is switched on by the control signal, a greater change in current, and therefore a greater change in impedance, than would be obtained from the doubling of the current which would occur if only two transistors were used. With no control signal applied the current through TA and TB is the same and the circuits therefore produce a mean attenuation in both channels. Capacitors are connected between the collector and base electrodes of TA and TB to provide negative feedback at AC while further capacitors connected between the emitters and a point of fixed potential by-pass the emitter resistors so that they are effective only at D.C.