Wideband RF PreamplifierThis circuit of radio frequency amplifier can be made in two ways:
(1) with a non-linear element operating with negative feed-back.
(2) with the aid of a linear active element
This circuit using an RF power transistor as the active element. noise figure is not increased because virtually no signal is lost. Feedback is also required to ensure correct termination (50 Q) of the aerial.
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 common-base rf 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.
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.