...or max gain. Or not... Why such a contorted circuit overall for just a gated common emitter amplifier? R18 is a minor player for the base's DC bias ...but it is not so for the grounding of the coil/antenna and attenuation of its AC amplitude at the base. When Q6 is conducting, Q6's base impedance is going to be much lower than any reasonable values for R17 or R18 and will, for the most part, determine the "input" impedance. When Q6 is not conducting, the base is held very close to ground by Q4's on resistance. That's only 0.4% of the maximum base current (1.5A) And your point? I doubt many applications run anywhere near 1.5 amp of base current. Half that would be a lot. The original R17=3.3K operated Q6 at about 3.8ma of base current. 50Ω would provide better match for the SA's input. If he chooses to use it. Using the SA, do we adjust for the "cleanest signal" or something "harmonically rich"? I don't think we know. Quite right, however measuring the base current means breaking into the PCB My point was that BJT's are current controlled devices, not voltage controlled like a FET. However, base current can be measured as the Vdrop across R17 divided by R17's value. R18 shunts 200ua to 300ua away from the base which is minimal at the base currents being used, but it too can be accounted for in the base current measurements/calculations if desired. I used 700mv as both the forward voltage of D1 and the Vbe of Q6 in my rough calculations. With R17 at the original 3.3K value, and using Hfe=80, I get around 310ma of collector current. With R17=8.2K, that works out to around 112ma of collector current. Both calculated values are a bit low (310ma calculated versus 350ma measured and 112ma. calculated versus 130ma measured). I suspect the the use of Hfe=80 is a bit too low for Itsu's Q6. Using Hfe=95 instead of 80 pushes the calculated numbers closer to the measured values. PW
« Last Edit: 20210616, 22:44:41 by picowatt »
