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Old 20th August 2015, 06:12 AM   #1
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Default Need to know Vbe of some power BJT

Hi

I need to your help to tell me what power BJT you use and measure the Vbe at idle and post it/them.

I am looking for Vbe higher than the MJL1302/3281 I am using. I have been searching around, the most common candidates are

Fairchild 2SA1943/2SC5200

Fairchild 2SA1962/2SC5242

ST Thomson 2STA1962/2STC5242

On Semi NJW0302/0281

Toshiba 2SA1943/2SC5200

If anyone has one of these transistors please help me in measuring the Vbe and post here. Also tell me your bias current for the measurements.

If you have any other transistor, please tell me the Vbe and part number also.

Thanks a million in advance.

Alan
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Old 20th August 2015, 06:30 AM   #2
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Do you have a DMM. Connect it in diode mode, connect in forward bias to B and E and what you see is the vbe. Assume the decimal point.

Gajanan Phadte
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Old 20th August 2015, 06:37 AM   #3
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vbe information is useless without knowing temperature it was measured at
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Old 20th August 2015, 07:24 AM   #4
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I mean at room temperature. If I know the bias current, it should be enough for me to compare. Too bad I cannot find spice model of some of the transistors, or else the Is can tell more about the Vbe.

I use a breadboard tester to do that. I bias to the idle current of my amp and measure the Vbe.
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Old 20th August 2015, 03:13 PM   #5
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You generally get higher Vbe from either higher current density ie a lower current device or higher voltage rating
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Old 21st August 2015, 05:10 AM   #6
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Can anyone help me measuring some transistors?

Thanks
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Old 21st August 2015, 07:20 AM   #7
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Don't worry about it, I ordered 5 of each of the transistors I listed, don't look like there is any other ones as good as those already.

Actually the selection is very limited in power BJT. all the other ones are much slower.
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Old 25th August 2015, 05:28 AM   #8
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Received the transistors, sadly, they at within 15mV of each other at 100mA. So this is a total flop and waste of money. Guess this is the end of the road in finding power BJT with higher Vbe. I looked at TO-220, a lot of the higher current ones are just the same transistors in TO-220 package, so it's going to be same difference.
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Old 25th August 2015, 02:25 PM   #9
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Why do you need higher Vbe?
If you have something measuring 1V plus, suspect a darlington
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Old 25th August 2015, 04:27 PM   #10
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My design is a 3EF with folded pre-driver. My spreader is less than 1.2V. I have to use CFP spreader with two trim pot to balance tempco and spreader voltage. Even I already choose transistors with low Vbe of 0.58V@10mA for spreader, I am cutting very close to saturation. Right now, I am using low Vbe 2SC4793 pair that have Vbe=0.58V for the folded Pre-drivers. But the Cob is 26pF that can load the VAS. I am trying to go back to the KSC3503 pair that have Cob=2.6pF. Even calculating the distribution capacitance, the total capacitance of the KSC3503 pair is going to half the loading capacitance to the VAS.

Problem is the KSC3503 pair has Vbe=0.66V. I need to increase the driver or the power transistor to get back some of the voltage.

I am going another route right now and see whether I can get some voltage suggested by Mr. Cordell.
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