| 摘要 |
<p>Variable-gain amplifier stages, such as may be used in a television receiver intermediate-frequency (IF) amplifier, each comprise a pair of transistors connected as an emitter-coupled differential amplifier with resistive collector loads. An electrically controlled conductance is connected between the collector electrodes of the transistors, to shunt the resistive collector loads in controllable amount to set the voltage gain of the stage. The electrically controlled conductance in each variable-gain amplifier stage is constructed so as not to change the DC operating conditions of the emitter-coupled differential amplifier and its resistive collector loads with variation in the conductance of the electrically controlled conductance. Front end portions of television receivers or video tape recorders are described each of which uses separate intermediate-frequency (IF) amplifiers for the FM sound carrier and vestigial AM picture carrier. In each front end the two IF amplifiers comprise matched cascade connections of the variable-gain amplifiers with tracking automatic gain control (AGC). The AGC signals are generated by an AGC detector supplied composite video signal from a video detector, which video detector responds to amplified IF signal supplied by the IF amplifier for the vestigial-AM picture carrier.</p> |
