How do you measure bias with no source resistors?

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How do you measure the bias of an undegenerated output stage when trying to set it?

Say, for the XA-25, or for an HPA-1-inspired lateral output power amp such as the attached....

Thanks,
Jeff.
 

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2 picoDumbs

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2013-09-06 9:35 am
Just adding another cat skin to the list.
If you’re making your own pcb then add a place for a 0.1R resistor at the drain of the output mosfets to take the measurement.
Then once satisfied with your bias setting either short them or remove them.
Some people even leave them in but I personally hate the idea of leaving them in.
 
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@woody, no, it's just poorly drawn because I never cleaned it up after playing with a few different ideas. The resistors are in parallel.

(I've also discovered that I can't find any metal films in 4W either, so they need doubling up again -- at which point there's not much point in showing them that way in SPICE.)

Here's a clearer version:

attachment.php
 

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Consider the Hantek CC65 from eBay. I have one and I am pleased with the results I get from it.

I can certainly imagine other instances where being able to view the waveform would be very useful (such as determining the class A envelope -- assuming the output devices are on flying leads).

I did some Googling and Pico also sells one[1]. Looks like it was made in the same factory, but with slightly higher-spec parts (at least there's a strain relief on the cable, and a metal latch ring on the BNC). A bit more dosh, but maybe better quality control too?

Thanks for the idea, Mark!

Cheers,
Jeff.

[1] https://ie.farnell.com/pico-technology/ta018/current-probe-clamp-60a-oscilloscope/dp/2677035
 
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