| 摘要 |
1526916 Gain control; amplitude modulators SIEMENS AG 27 May 1977 [31 May 1976] 22391/77 Headings H3R H4R and H3G A variable gain emitter coupled circuit comprises two transistors with their emitters connected by respective D.C. paths to a reference voltage and through respective capacitors and separate series cascades of diodes to a controlled direct current source responsive to a control voltage, the junction point of the capacitors and their diode cascades being connected by a respective resistor to a reference voltage point. As described a doublebalanced modulator Fig. 1 in which the modulating signal U E is applied to the bases of transistors 21 and 22, which have their emitters coupled for A.C. by diodes 15-20. The D.C. through these diodes and hence their dynamic impedance is controlled by a transistor 30 receiving an AGC voltage derived from a pilot signal. Potential divider 35, 36 applies a mean control voltage to transistor 30 in the absence of a pilot signal. In another embodiment, Fig. 2 (not shown), an unbalanced modulating signal is applied to the base of transistor 21 through an adjustable capacitative voltages divider to provide manual gain control, the base of transistor 22 being A.C. earthed. In this case a choke or active equivalent is inserted betwen resistors 11 and 12 and -U B to prevent asymmetrical loss of signal. |
