Design of an AM/FM Transistorized Receiver

When designing an integrated AM/FM receiver, two major problems that have to be considered are: 1. The transistors employed have to perform efficiently at both AM and FM frequencies. 2. The performance of the transistor should not be appreciably affected, when switching from AM to FM operation. Until recently the application of transistors in radio receivers had to be restricted to medium and low frequency operation. The introduction of the Amperex PADT (post alloy-diffused transistors) 2N1516/0C170 and 2N1517/0C171 has permitted the design of AM/FM transistorized receivers at frequencies to 100 Mc. In the Amperex post alloy diffusion process, alloying and diffusion take place simultaneously. The transistor is built up on a piece of P-type germanium. Two small pellets are placed on the germanium. Pellet B, the base pellet, contains only an N-type impurity. Pellet E, the emitter pellet, contains a P-type and an N-type impurity. When this assembly is heated at a certain temperature, the germanium dissolves into the metal pellets until saturation is reached, and the pellet impurities diffuse into the solid germanium. However, the P-type impurity in pellet E has such a low diffusion constant, that for practical purposes it does not penetrate into the germanium. The N-type impurity in pellets E and B has a much greater diffusion constant and readily penetrates into the solid germanium to form a diffused N-type layer underneath the pellets.