High Level Wideband RF Preamplifier

A linear RF amplifier can be made in two ways: (1) with the aid of a linear active element, or (2) with a non-linear element operating with negative feed-back. This circuit is of the second kind, using an RF power transistor as the active element. Feedback is also required to ensure correct termination (50 Q) of the aerial, since bipolar transistors normally exhibit a low input impedance. Also, the noise figure is not increased because virtually no signal is lost.

Circuit diagram :

High Level Wideband RF Preamplifier Circuit Diagram

The common-base amplifier is based on a UHF class A power transistor Type 2N5109 from Motorola. The feedback circuit is formed by RF transformer Th. The input and output impedance of the preamplifier is 50 4 for optimum perform-ance. Network R3-C5  may have to be added to  preclude oscillation outside the pass-band, which  ranges from about 100 kHz to 50 MHz. The gain is  approximately 9.5 dB, the noise figure is between 2  and 3 dB, and the third-order output intercept point  is at least 50 dBm.

The input/output transformer is wound on a Type FT37-75 ferrite core from Micrometals. The input winding is 1 turn, the output winding 5 turns with a tap at 3 turns.