High Level Wideband Rf Preamplifier

EE JULY/AUGUST 1987 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 sec- ond 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. 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 Tri. The input and output impedance of the preamplifier is 50 Q for optimum perform- ance. Network R3-05 may have to be added to preclude oscil- lation 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...