Solid State Circuits 1 - Discrete RF & IF Amplifiers


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 - higher gain at lower frequencies.

Decoupling values are typical for operation on the HF bands.

Two bias options are shown - use A if AGC is not required and B if AGC is required. The no-signal AGC bias should be around 4-5V - check the data sheet.


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 or one of the 3N20x series. In this case check the operating characteristics to ensure that the voltage and current ratings are not exceeded. Alternatively a cascode circuit using a low noise n-channel JFET and an NPN bipolar device may be used as shown below. The gain is typically 33dB at 9MHz and the Hi-Z output starts to clip at 4.5v pk-pk on a 9v supply and 7.2v pk-pk on a 12v supply.

During tests, some UHF instability was noted in oth the BF199 and a test MPSH10 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.

As there are two devices in series fed from the 9v line the output level of the upper device will be relatively low so operation may be better on a higher voltage supply. The 47K and 22K bias network will require changes for different supply line voltages and devices. On a 9v line I use 3K9 and 4K7 respectively which also reflects the lower hfe of the BF199.

*The ferrite bead is intended to suppress UHF oscillation but if this proves impossible then try a BF173 or a 2N3904 instead of the BF199 - the BF173 has a lower internal feedback capacitance and should therefore be more stable but did show slightly less gain than the BF199.