Abstract :
This paper describes the application of the RCA 2N2857 (uhf) and 2N2708 (vhf) transistors in the rf-amplifier, mixer, local-oscillator, and if-amplifier stages of a 450-megacycle communications receiver. Block diagrams and circuit schematics are presented and described, the noise figure of the system is calculated, impedance matching is discussed, and data on “overload” signals are presented. Design examples in terms of admittance parameters are given for the rf stages, and calculations based on transmission-line relationships are made for the tuned circuits of the receiver. Receiver sensitivity, selectivity, and automatic gain control are also discussed. The 2N2857 and 2N2708 transistors are two of the most recent low-noise silicon planar epitaxial transistors developed by RCA for uhf and vhf applications. These transistors are packaged in a four-lead case which has the same dimensions as the TO-18 package, and their collectors are isolated. The fourth lead of the package, which is attached to the case, reduces feedback capacitance and rf radiation when its free end is connected to circuit ground.
