IE42040L - Current source/load for am-fm receiver - Google Patents

Abstract

1503171 Transistor current sources; AM-FM receivers; AGC and AFC circuits GENERAL ELECTRIC CO 22 Jan 1975 [22 Jan 1974] 2681/75 Headings H3T H3A and H3R In a current source 29 and a load comprising transistor circuits 10-12 forming part of an AM-FM receiver, the load transistors are of opposite conductivity type to an output transistor Q27 of the current source which supplies the collector-emitter currents of the load transistors and the current source has an emitter resistor 57 connected to a power supply line B+, a semiconductor diode Q25 and a resistor 51 series connected between the base of the output transistor and the supply line and a direct current amplifier Q24-Q26 having two inputs controlling the static and dynamic output current of the output transistor. As shown the static D.C. level is determined by the input to Q26 from an AM-FM selector switch and the dynamic D.C. level is determined by the AM-FM detector 26 output. Switching switch 32 from AM to FM reception biases Q26 to conduct and steps the current in Q23 to a new level thereby raising the voltage on line 18 by 0À75 v. This switches the AGC voltage on line 18 from a low gain setting for AM to a high gain setting for FM. The FM voltage range setting produces a higher IF gain for the IF amplifier Q1-Q8. The bus 18 provides B + bias for the FM mixer Q 11 and local oscillator Q12, B+ biasing for the IF amplifier stages Q1-Q6, automatic frequency control (AFC) of the FM local oscillator, AGC of the IF stages Q1-Q6 and AGC of the AM section including a mixer Q13-Q19 and a local oscillator Q20-Q22. A diode D1 functions to provide delayed AGC to the AM mixer. The IF amplifier has D.C. feedback via a resistor 23 for reducing drift and providing a secondary AGC bus 19 for the AM section and a balancing bias to the differential stages of the IF amplifier. The IF amplifier, the AM-FM detector 26, the audio amplifier 27 and the adjustable current source 29 can be formed as an integrated circuit. The FM detector 26 can be as disclosed in Specification 1,338,201. Specifications 1,503,172 and 1,503,173 are referred to. [GB1503171A]