953,918. Transistor limiting circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. May 6, 1960 [May 9, 1959], No. 16126/60. Addition to 847,571. Heading H3T. In a limiter circuit comprising a parallel resonant circuit 1, Fig. 1, a symmetrical transistor 2 with its emitter-collector path connected across all or a part of the tuned circuit, and a parallel-connected resistor 4 and condenser 5 connected between the base of the transistor and a tapping on the resonant circuit midway between the connection thereto of the emitter and collector of the transistor, means are provided for supplying a current in the circuit between the tapping and the base of the transistor in opposition to the conduction direction of the base of the transistor. The means for supplying current comprise a battery 6 and current-limiting resistor 7. The arrangement reduces the damping imposed on the tuned circuit by the transistor at low signal levels. The resistor and condenser combination and biasing arrangement may be replaced by (Fig. 2, not shown) or connected in series with a suitably poled Zener diode. The Zener diode 10, Fig. 3, may be so connected as to apply automatic gain control to a preceding I.F. stage 11 when conductive; the time-constant provided by the condenser 12 may be made short so that the gain control circuit itself provides suppression of amplitude modulation. The Zener diode may be combined with the transistor (Fig. 4, not shown).
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