| 摘要 |
1,082,042. Gain controlled transistor amplifiers. SIEMENS A.G. Jan. 21, 1965 [Jan. 23, 1964], No. 2617/65. Heading H3T. The gain of a single transistor Tr1 amplifier is controlled by variation of the emitter circuit impedance, in the form of the input impedance of a second transistor Tr2 controlled by a temperature dependent resistor HL connected in its base circuit. The resistor HL has a negative temperature coefficient and is controlled by an independently adjustable or automatically derived D.C. control current Ih which causes the input resistance of transistor Tr2 to change and so vary the negative feedback applied to transistor Tr1. The input E is connected via resistor R1 to the load R2 of transistor Tr2 so that the input impedance which normally increases without feedback remains substantially constant as the negative feedback to Tr1is increased. The collector circuit of transistor Tr1 includes a tuned primary winding W1 of a transformer U, the secondary winding W2 of which can be connected to earth or to a source of reference potential. Alternatively the secondary winding as shown is connected to the emitter of transistor Tr1 the output voltage U2 being of opposite sense to that across the emitter U1 so that additional negative feedback provides a further reduction of amplifier gain. |
