| Electronics > Beginners |
| Matching transistors room temprature |
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| soldar:
This thread might interest you. https://www.eevblog.com/forum/testgear/transistor-curve-tracer-kit-on-ebay/ |
| floobydust:
Are you matching beta or VBE? Touching the transistors to put them in the breadboard then takes time for them to cool off. Using a high test current heats them up, this is the most frustrating because readings are always moving. Add some capacitance like 0.1uF to prevent RF pickup from getting amplified, which also makes readings wander. A box to prevent convection currents, drafts, heating system, sunlight, IR etc. helps. I don't see a point to super-matching (<2%) because your end circuit will have thermal gradients and they'll end up running at different temperatures anyway. Like in an audio power amplifier. You can apply heat-shrink to a diff. pair though. |
| Jan Audio:
I am measuring for HFE for exponential converters. Those 1 part measuring devices i think are no good. Like you say you cant toutch them. I add a whole row in heat isolating material and measure from a distance. I dont know why the readings change, i will try a 100n cap, it wont work, the readings change over time, you have to measure really fast before it changes again. |
| Jan Audio:
--- Quote from: floobydust on May 07, 2019, 09:06:12 pm ---I don't see a point to super-matching (<2%) because your end circuit will have thermal gradients and they'll end up running at different temperatures anyway. Like in an audio power amplifier. You can apply heat-shrink to a diff. pair though. --- End quote --- I use heat paste from computer, 2 transistors + tempco resistor together. |
| floobydust:
Dr. Leach simple blurb on matching diff. amp transistors for his power amplifiers. http://leachlegacy.ece.gatech.edu/lowtim/part2.html I add a 0.1uF E-B capacitor to shunt RF if the test fixture has long leads. You can also put a scope on the transistor under test to see if anything odd is happening. Cheap DMM's apply maybe 3V to measure hFE so that is not so great if you have high VCE expected. If you use multiple power supplies (one for base drive, another for collector, or bi-polar), be careful of the sequence to turn them on/off so you do not reverse or damage a transistor. |
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