TH-3 Microwave Radio System: The IF Main Amplifier

The T H - 3 IF main amplifier* provides an output of +1 dBm and normally operates at a gain of 9 dB. Under control of an AGC circuit, this gain can be increased to 49 dB or decreased to less than 3 dB. The amplifier uses ten silicon transistors in five gain stages and six silicon transistors in the AGC circuit. Figure 1 shows the block schematic of the amplifier. There are five gain stages in tandem to provide the 49-dB maximum gain. The three intermediate stages are variable gain to provide the 46-dB gain range. The design objective for the amplifier was a flat transmission and delay characteristic over the 53- to 87-MHz band. Gain deviations of less than 0.03 dB and delay deviations of less than 0.2 ns were desired. In order to meet these objectives a basic gain stage was needed having a very stable broadband transmission characteristic. Other desired characteristics were: (i) use of feedback to obtain gain stability, (ii) automatic gain control capabilities with the use of a single variable element, and (in) negligible interaction between stages to facilitate cascading. The use of transformers was to be avoided due to the variability of ferrites at these frequencies. This basic gain stage could then be used throughout the radio repeater where IF amplification was required. * This amplifier is also used in current, production of transmitter-receiver bays for the TD-3 system. 2195