Chuck,
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Thanks for lighting the fire under my rear. I have my Raspberry Pi Pico based jig working now, after a couple days of struggling with programming. Hardware design and soldering was the easy part, as I expected.
I finally got the idea that I don't need to measure the output precisely, if I can adjust the input in a linear way. Charles Wenzel adjusts the frequency.
I don't change the frequency, it is fixed at 10MHz. I change the input amplitude until the output is at 1.5V p-p (the maximum this kind of circuit can drive without much distortion).
Now I have all the data to calculate the current gain without calibrating the non-linear detector. Everything is automated, I just plug the device in and wait for a few seconds for the result.
Below are some teaser photos. I promise to put the actual documentation and software on my github page (eventually).
The Pico drives an R-2R DAC and generates sine wave at 10MHz with variable amplitude. The signal generator code is from this project:
The amplifier circuit is similar to Wenzel's, except that the base DC bias resistors are higher value and are connected straight to the base, not before the current source 1k resistor.? One is variable to adjust the collector current.
The detector is the voltage doubling circuit that you are also using. The diode is BAT54S.
I have one board that uses 12V for the collector supply, Vce=10V. The second one (pictured) uses 5V from USB, so the measurement is for about 3V Vce.
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Lower fT transistors show the expected values. The one plugged in when I took the picture is a fake 2N3904 2-cent part ($0.99 for 100), probably the same source you got yours from. Measured fT=195MHz. Genuine new 2N3904 from On Semi is not much better. An old stock Motorola part is 265MHz.
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Now the high fT transistors don't work as expected. Both 2N5109 and 2SC5551 that I tried show 450MHz fT. I suspect this is because 10MHz is below their "knee" frequency. Need to add another range which uses 25 or maybe even 50MHz , which will be pushing capabilities of the RP2040 an of my construction technique.
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73, Mike AF7KR
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