Patent GB-1298272-A (GB)
Title: Detector circuits
IPC: H03D1/18
Filed: 1970-03-02
Abstract: 1298272 Transistor amplifying and detecting Circuits RCA CORPORATION 2 March 1970 [3 March 1969] 9876/70 Heading H3T A detector circuit includes an input circuit for applying a signal modulated carrier and an unrelated direct voltage, an output circuit for deriving the modulation components of the carrier, a rectifier being coupled between the output and input circuits. Means are coupled to the input circuit for deriving a direct voltage responsive to substantially only the unrelated direct voltage, and means for applying the derived direct voltage to the rectifier. The embodiment comprises an integrated circuit 100, incorporating a video I.F. amplifier, the video detector, and video amplifier. The video I.F. amplifier comprises transistors 40, 46, 49, 50, 52, transistor 40 is of the double emitter type, being used as an emitter-follower with one emitter 43 coupled to the base of the succeeding stage 46 and the other 44 providing an A.F.C. output at terminal 111. The collector circuit is decoupled by means of a Zener diode 42. Transistor 46 is in common emitter connection and has the collector-emitter path of transistor 49 as part of its collector circuit; it is followed by emitter-follower 50. The final stage comprises transistor 52 in common emitter connection, its emitter circuit comprising bias resistor 59 shunted by a resistor-capacitor network 60 formed by a lossy capacitor. The collector circuit of transistor 52 comprises resistors 53, 54 the common point of which is returned to the base of transistor 49, providing negative feedback which stabilizes the amplifier. Detector circuit.-This comprises transistors 65, 66, 68, 70, 75. The actual detector is transistor 65, and this receives on its base the I.F. signal with in addition the D.C. potential at the collector of transistor 52: the A.C. component is removed from the latter by means of filter network 64, 67 and it is applied to the base of transistor 66, from the emitter of which the emitter of the detector 65 is biased, over load resistor 85. Carrier signal frequency is removed from the output of detector 65 by capacitor 69 and the D.C. and modulation components are passed by resistor 71 to an emitter-follower 70, the emitter circuit of which comprises resistor 73 in series with the collectoremitter path of transistor 75. The emitter of transistor 66 is connected via an I.F. decoupling resistor 67A to the base of an emitter-follower 68, the emitter circuit of which comprises resistor 74 in series with diode 76 which comprises a transistor with collector and base strapped and which shunts the input circuit of transistor 75: if resistor 67A can be disregarded and resistors 73, 74 are the same value, all the current through resistor 73 flows in the collector circuit of transistor 75. The collector of transistor 75 is coupled to the base of transistor 80, providing a feedback which prevents transistor 66 being biased to cut-off by the D.C. produced by rectification in the emitter circuit of transistor 65. The collector of transistor 75 is also coupled to the video amplifier comprising transistor 78, biased by diode 79 which also comprises a transistor with collector and base strapped, followed by emitter-followers 81, 82 which provide a video output on terminal 116: this may be connected outside the chip to a network coupling to a video channel, chroma stages and a sync. separator (Fig. 4, not shown). In order to further assure operating point stability with temperature changes, and to maintain large signal linearity, a D.C. loop is closed from the video detector circuit back to input terminal 110: the feedback is taken from resistor 90 via Zener diode 91,'to terminal 113 which is coupled to terminal 110 via the tuned I.F filter external to the chip (Fig. 4, not shown).