AM-FM receiver having improved mode conversion

In a radio receiver having AM and FM modes, the combination comprising:

• a. a d.c. coupled IF amplifier for signals at two intermediate frequencies, subject to amplification control by bias voltage variation and including a plurality of stages having wide band a.c. gain embracing both of said frequencies, each stage including an emitter coupled transistor pair,
• b. means comprising a primary control bus for applying a d.c. collector bias to the individual IF amplifier transistors, said potential being subject to variation in accordance with the mode setting of the radio receiver and/or a signal parameter,
• c. a secondary control bus for controlling the input bias level of the individual IF amplifier transistors and for applying a degenerative feedback to balance each transistor pair,
• d. means for establishing said secondary control bus at a potential which is a function of the potential of said primary control bus and for applying degenerative d.c. feedback to said secondary control bus, including
• 1. a collector load resistance connected between said primary control bus and the collector of the final transistor of one IF amplifier stage, and
• 2. a degenerative feedback resistance having one end connected to said collector and the other end to the base of said final transistor and to said secondary control bus, and
• e. mode switching means for conversion to AM or FM operation comprising a third control bus adapted to assume ground potential for FM operation and the potential of said secondary control bus for AM operation to provide a variation in bus potential sufficient to render transistors whose input junctions are coupled thereto conductive or non-conductive,
• f. a tuner for converting received AM signals to the lower of said two predetermined intermediate frequencies, including
• 1. a four quadrant multiplier mixer comprising two pairs of differentially connected transistors in an upper rank and single pair of differentially connected transistors in a lower rank, said AM signal being injected into said lower rank,
• 2. an AM oscillator having a transistor current source for amplitude control, the output of said oscillator being injected into said upper rank;
• the input junction of said lower rank mixer transistors and said oscillator current source being coupled to said third control base for turning on said AM tuner and for automatic gain control in AM operation and for turning off said AM tuner in FM operation.