GB813859A - Improvements in or relating to transistor amplifiers - Google Patents

Abstract

813,859. Transistor amplifier circuits. KOLSTER -BRANDES Ltd. Oct. 4, 1957 [Oct. 10, 1956], No. 30839/56. Class 40 (6). In a multi-stage transistor amplifier in which the input signal is applied to the base of the first stage, the biasing circuit for which is arranged so that negative voltage feedback from the final stage due to the impedance of the common supply source is eliminated, negative feedback is introduced into the input circuit of the second stage to eliminate positive feedback due to said impedance, from the final stage to the penultimate stage. The Figure shows a four-stage hearing aid amplifier in which the collector load R3 of the first stage is connected between the emitter and the battery so that the path from the negative side of the battery to the input of the second stage through which undesirable negative feedback due to the battery impedance occurs includes the relatively high impedance of the collector of the first stage so that the negative feedback is small. The bias circuit for the base of transistor T1 is decoupled by a capacitor C1 and sufficient negative feedback is reintroduced into the base circuit of transistor T2 through C1 to offset positive feedback to the base circuit of transistor T3 due to the common battery impedance. The amplitude of the output signal of the amplifier may be varied by means of a potentiometer P or a switch SW in the output circuit or by varying the resistor R6. In an alternative circuit (Fig. 1, not shown), the base of transistor T1 is biased by connecting two resistors in series between the collector supply and the base, the common point of the resistors being decoupled by a capacitor having its other electrode connected to the base of transistor T2 to reintroduce the desired small amount of negative feed-back. It is explained in the Specification that apart from capacitor C1 no damage will result to these circuits from accidental reversal of the battery (Figs. 3, 4 and 6, not shown). The reverse voltage which occurs across capacitor C1 may be avoided if the right-hand terminal of the capacitor is connected instead to the base of transistor T2 (Fig. 5, not shown).