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Low noise active mixer

Image Number 2 for United States Patent #5379457.

In a conventional Gilbert-cell active mixer, two local oscillator matched pairs of transistors receive a local oscillator input signal and are coupled to a radio frequency matched pair of transistors which receive a radio frequency input signal. The circuit generates an intermodulated output signal at the collectors of the local oscillator matched pairs. Noise degradation is reduced over the conventional mixer by replacing the standard radio frequency emitter degeneration resistor with a reactive element, thereby reducing thermal noise. Narrow-band input matching is achieved by insertion of a series inductive element and optional parallel capacitive element in line with the radio frequency input. Thermal noise contributed to the circuit is thereby minimized while circuit linearity is preserved in the narrow frequency band of interest.

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