Solid State Circuits 1 -
1 MOSFET RF/IF Amplifier
The diagram to the right shows a single stage MOSFET tuned amplifier which may be used as a preamplifier, RF or IF amplifier. Depending on the frequency of operation, the gain will be from 15dB to 30dB -
Decoupling values are typical for operation on the HF bands.
Two bias options are shown -
2. As the 40673 is now very difficult to obtain it may be substituted with another type of dual gate MOSFET including a more modern device like the BF960.-
During tests, some UHF instability was noted which did not seem to be greatly affected by circuit layout or decoupling so a ferrite bead was put onto the emitter leg of the NPN device which cured the problem.
An IF amplifier consisting of three of these stages was constructed on a double sided PC board and produced about 100dB of gain at 9MHz. Care is needed with the layout to ensure stability.
Additional selectivity or filtering may be required to reduce the wideband noise at the output. The biasing may need to be adjusted for different supply voltages.