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Old 19th April 2021, 05:52 PM   #4321
Pete2 is offline Pete2
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Join Date: Jul 2019
That's perfect and quite reassuring , thank you .
I skip these for now and if needed I try normal ceramics with shortened legs before going SMD route - there seem to be quite few decoupling caps as it is .
How do you match input stage if you don't mind me asking , I have DCA75 but is limited to 12V and transistors work above 15V ?
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Old 20th April 2021, 12:27 AM   #4322
sq225917 is offline sq225917  United Kingdom
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They run at 15.4v, I batch them out with the dca to get the best match at 12v, the change in hfe is pretty linear at the Ic they are being run at , but you need to look at the curve traces for Hfe vs Ic and Ic vs Vce to weed out any wrong uns. I usually populate 3 on each side of the input stage and then swap out the 4th on each side until I get the lowest offset.

Matching them judiciously at 12v is usually good enough to get within a few 100mV without the action of the servo. Going the extra mile with the 4th transistor on each side can get you under 100mV, which is as good as it gets,

The hfe varies a lot with temperature, so the final matching in circuit is critical
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Old 20th April 2021, 12:39 PM   #4323
slav is offline slav
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Join Date: Mar 2010
Transistors matching is something putting me off building another one tbh
If I ever get to it I might try to match them within say 5% and make one of the resistors on each side adjustable, say 2k pot paralleled to 33R and try to adjust to give minimum voltage at the output , measure pot resistance and soldering similar value resistor.
Another thing to consider is transistors working temperature and temperature coefficient, maybe is better to measure Hfe after holding transistors body in the fingers for a certain amount of time as it would bring them closer to working conditions.
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Old 20th April 2021, 01:03 PM   #4324
sq225917 is offline sq225917  United Kingdom
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Slav, the swap out method works, I just use push fit headers for them.
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Old 20th April 2021, 02:50 PM   #4325
slav is offline slav
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Join Date: Mar 2010
Thanks Si , I might try your method on one board and adjustable pot on the other if I ever get to building one , have Martin from PFM been in touch with you ?
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Old 20th April 2021, 10:37 PM   #4326
sq225917 is offline sq225917  United Kingdom
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Yes, he got in touch. Seems like he's working through a few gear choices currently. I priced one out for him. I'll pop over to see him once covids done.
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