- #1
yungman
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- TL;DR Summary
- This is not homework.
Funny that if I pull the transistor out and plug it back in, the Vbe decrease by over 1mV
THIS IS NOT HOMEWORK
I design audiophile power amps as hobby. I need to match power transistors. It is A LOT HARDER than I thought. I built a test fixture to hold up to 50 transistors at a time. I try to match sets of 13 to about 1mV which is very tight as Vbe change is about -2mV/deg C. I have to move transistors around to get matching set all in a roll, not from far apart.
I set Vc=1.4V, emitter resistor is 910ohm and goes to -5.6V to get about 5mA of current through. I keep current and collector voltage Vc low so I won't cause big rise in temperature. Also I keep current low so the contact resistance of the proto plug in board will not cause problem. Below is the pictures to show:
One thing I notice that is very strange, if I pull the transistor out, then plug it right back in, the Vbe decrease by about 2mV. If I repeat on the same transistor again, it's will further decrease. It will take about 10mins sitting with power on to stabilize back to the original value.
I don't think my fingers will cause the temperature to rise so fast, I measure, pull out, plug in and measure, Whole process is less than 10sec. Another thing more important that convince me it's not the problem is it will take at least 10 minutes to go back to normal. It it is thermal issue, the heat up and cool down time is very different, 10sec to heat up, 10 minutes to cool down. I don't buy that at all.
Any explanation on this?I can assure anyone that is interested, matching is A LOT harder than it sounds.
I design audiophile power amps as hobby. I need to match power transistors. It is A LOT HARDER than I thought. I built a test fixture to hold up to 50 transistors at a time. I try to match sets of 13 to about 1mV which is very tight as Vbe change is about -2mV/deg C. I have to move transistors around to get matching set all in a roll, not from far apart.
I set Vc=1.4V, emitter resistor is 910ohm and goes to -5.6V to get about 5mA of current through. I keep current and collector voltage Vc low so I won't cause big rise in temperature. Also I keep current low so the contact resistance of the proto plug in board will not cause problem. Below is the pictures to show:
One thing I notice that is very strange, if I pull the transistor out, then plug it right back in, the Vbe decrease by about 2mV. If I repeat on the same transistor again, it's will further decrease. It will take about 10mins sitting with power on to stabilize back to the original value.
I don't think my fingers will cause the temperature to rise so fast, I measure, pull out, plug in and measure, Whole process is less than 10sec. Another thing more important that convince me it's not the problem is it will take at least 10 minutes to go back to normal. It it is thermal issue, the heat up and cool down time is very different, 10sec to heat up, 10 minutes to cool down. I don't buy that at all.
Any explanation on this?I can assure anyone that is interested, matching is A LOT harder than it sounds.