UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - 6-gang FM stereo tuner heads

regenfreak · 24th Oct 2022, 10:57 pm

Quote:

Originally Posted by G0HZU_JMR

I think any negative resistance will be avoided with the 47Ω bias resistor. To try and show this, I had a go at measuring some s2p data of a BF998 in common drain with the g2 pin strapped to the drain as per the probe design.

I then put this s2p data into a model of the probe circuit and compared it to the manufacturer's s2p data for the BF998. The manufacturer's data is really old, and it isn't taken with g2 tied to the drain. It's also taken at 8V. However, the results below were still fairly similar when I look for negative resistance at the input of the probe circuit. In the plots below I compare having 47Ω as the Rbias resistor and also 150Ω as the Rbias resistor when the g1 bias is re-adjusted for 10mA drain current in both cases.

To explore a realistic worst case, I assume the load at the far end of the coax cable can have a VSWR of 1.5:1 so I terminate with 75Ω rather than 50Ω. This will tend to worsen any negative resistance if there is any.

The plots show the 47Ω version looks OK, but the 150Ω version does appear to introduce some negative resistance (as expected).

There isn't much of it though. This amount of negative resistance would be fairly benign although it probably could cause instability if you went looking for it. You can see that the negative resistance is termination sensitive. If I remove the 75Ω load the negative resistance gets worse although it is far worse for the 150Ω bias version. See the third plot below for this.

Thanks! The series input resistance Rs can be negative. So Q becomes negative unless Xs is negative too! I learn something new every day! I can imagine it is difficult to model the parasitic capacitance and inductance at the probe tip using simulation.

I have attached the S22 of the Poorman's FET versus HP 8502A and Philips PM7515 probes from Hirschbuechler's thesis. They just show how well the probe matches the 50 ohm output. None of the S22 look particularly good. As i mentioned before, he added an extra 33 ohm resistor at the 50ohm output.