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Old 23rd Oct 2022, 1:56 pm   #175
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: 6-gang FM stereo tuner heads

The bias network for that poor man's probe does look to be quite aggressive as it places a positive voltage of about 2V at g1 and the source bias resistor is only 47Ω. I'd therefore expect the BF998 to operate beyond Idss so it could draw well over 20mA drain current. This is off the datasheet graphs and heading towards the 30mA max limit for the device. The magazine article does mention it can draw up to 30mA.

The 47Ω bias resistor will be in parallel with (1/gm) so it will spoil the source match into the 50Ω cable.

However, if it is increased to 150Ω then something else will also have to be done to try and offset the negative resistance this will generate at the g1 pin. A snubber network could be fitted at the g1 pin for example. This will have to be designed carefully because it could introduce excessive damping at the input if overdone.
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