On Fri, Mar 21, 2025 at 12:23 PM, Tom - VE3PSZ wrote:
This reduced IMD noticeably. So the benefit in my opinion is not noise reduction, but large signal handling.
A better way to improve large-signal handling is to replace the source resistor of the input FET by an active current source. This increases linearity and also reduces the gain loss of the source follower. I've done that with all my E-field Whip amplier designs (see link below) . Paralleling FETs is IMHO counterproductive, because you increse the input capacitance by the number of FETs. This input capacitance forms a cpacitive voltage divider with the capacitance of the Whip, which can be assumed as between 5 and 10 pF. With 10 pF antenna capacitance and 20 pF input capacitance of 4 x FETs, you get only 1/3rd of the EMF input voltage from the field probe. In combination with the negative gain of s source follower and the emitter follower you end up with an overall gain of -10 dB. These 10 dB have to be added to the inherently bad noise figure of an Hi-Z FET amplifier.
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I've uploaded two of my active-whip amplifiers. Many of them have been built and they perform greatly.
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regards
Fred
