| 摘要 |
An improved RF amplifier system in which an undesired frequency such as the image frequency is rejected. The amplifier circuit includes a first tank circuit tuned to a first predetermined frequency by a first voltage variable capacitor and a second tank circuit tuned to a second frequency by a second voltage variable capacitor. A dual gate FET is coupled to the tank circuits such that the drain of the FET is across the first tank circuit and the source is connected across the second tank circuit. The second tank circuit is tuned to the image frequency or the undesired frequency while the first tank circuit is tuned to the desired frequency. When the image or undesired frequency is received the drain tank impedance of the FET becomes very small while the source tank impedance is relatively large which effectively degenerates the amplifier gain at the image frequency. When this occurs the current flowing into the first tank circuit is reduced causing the voltage appearing across the drain to become reduced and the undesired signals to be rejected in the IF section of the receiver.
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