Synchronous demodulator employing common base transistor amplifier input and base-emitter clamping action

Claim 1

In a detector for recovering the information content of a modulated input signal and of the type including a switching section formed from two transistors having a pair of base electrodes biased to a predetermined potential level by one terminal of a first source of energizing potential having a first voltage value and between which reference carrier switching signals are applied, a pair of collector electrodes to provide demodulated output signals representative of said information content and at which a direct voltage output of the detector is developed, and a pair of emitter electrodes commonly coupled to receive an intermediate signal corresponding to said modulated input signal from the collector electrode of an included amplifier transistor to which said input signal is applied, the improvement comprising: means interconnecting the base and emitter electrodes of said amplifier transistor in circuit with a source supplying said modulated input signals and with a second source of energizing potential of lesser voltage value than that of said first source of energizing potential to inhibit such forward biasing of the base-collector junction of said amplifier transistor by said input signals as would cause reverse biasing of the baseemitter junction of both of said switching transistors and an undesirable shift in the direct voltage output of Said detector; said means including: a. a first resistance means coupling a first terminal of said source of modulated input signals to the base electrode of said amplifier transistor for applying input signals thereto; b. direct current means coupling a first terminal of said second source of energizing potential to a second terminal of said source of modulated input signals; c. direct current means coupling a second terminal of said first source of energizing potential, a second terminal of said second source of energizing potential and the emitter electrode of said amplifier transistor to a circuit junction point; d. means coupling said junction point to a point of reference potential such that input signals are applied to said amplifier transistor between its base and emitter electrodes and output signals are developed by said transistor between its collector and emitter electrodes for coupling to said switching section transistor emitter electrodes; and e. a semiconductor junction coupled in parallel with the baseemitter junction of said amplifier transistor and poled in the same direction as said base-emitter junction, with the effective areas of said semiconductor junction and said baseto-emitter junction being selected in predetermined ratio to provide current flow in the collector electrode circuit of said amplifier transistor proportional to that through said semiconductor junction by a factor which is the inverse of said ratio, said semiconductor junction being rendered conductive by input signal excursions in the same direction as render said amplifier transistor conductive to effectively control the current flow through said transistor while limiting the input signal excursion at the base electrode of said amplifier transistor relative to the voltage developed at the collector electrode thereof via said first source of energizing potential.