GB840976A - Improvements in or relating to electrical circuit means employing transistors - Google Patents

Abstract

840,976. Transistor automatic gain control circuits. PHILCO CORPORATION. Oct. 22, 1956 [Oct. 20, 1955], No. 32108/56. Class 40(6). A base input transistor amplifier is controlled in gain by applying an automatic gain control voltage to the base to vary the D.C. current through a non- linear impedance connected in the emitter circuit and arranged to carry the both signal and D.C. components of the emitter collector current. The Figure shows a radio receiver comprising a transistor R.F. amplifier 70 having a diode 84 connected in series with the emitter circuit. The capacitor 110 of the input tuned circuit is chosen so that the apparent source impedance of the input signals is equal to or slightly less than the input impedance of the transistor so that approximately optimum power transfer takes place. As the input signal increases the collector voltage of the second detector transistor 40 becomes more positive and this voltage is applied through the AGC line 30 to the base of the transistor 70. The base to emitter impedance and the impedance of the diode 84 accordingly rise causing an increase in input impedance of the transistor circuit and thus a reduction in the power transfer from the aerial circuit so that the output of the transistor remains more nearly constant. A capacitor 94 connected between the base and emitter of the transistor 70 reduces the initial input impedance of the transistor circuit and accordingly causes the change in input impedance with changes in AGC potential to increase. Diode 84 may be replaced by a thermistor. The AGC arrangement may be applied to an IF amplifier or to regenerative or super regenerative amplifiers. The # network in the aerial input circuit may be replaced by a T-network or a transformer.