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Look at Figure 2 in the following datasheet: http://www.onsemi.com/pub/Collateral/MMBT3904LT1-D.PDF

enter image description here

Why are there two diodes and not just one?

Thanks.

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  • \$\begingroup\$ I think that 2 diodes gives a higher threshold than 1, eg: 2 100V diodes can barrier just under 200V. I've got nothing to back that up, though. \$\endgroup\$
    – tuskiomi
    Commented Jun 22, 2017 at 14:06

3 Answers 3

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The circuit includes two diodes to set the negative bias at the base during the turn-off phase of the measurement.

A single diode would only give about 0.7v reverse, two give twice that.

If they didn't have any diodes at all the voltage would exceed the reverse breakdown of the base-emitter junction that is in the region of 7v with silicon planar transistors.

I agree that it is somewhat unusual way to do the test rather than defining the voltage from the pulse generator.

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    \$\begingroup\$ so it's a 1.4V regulator of sorts? \$\endgroup\$
    – tuskiomi
    Commented Jun 22, 2017 at 15:05
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    \$\begingroup\$ @tuskiomi, yes that's right, you could make a shunt regulator this way and it's sometimes done. Regulators are for continuous power supply though. This is a simple clipping function. \$\endgroup\$
    – TonyM
    Commented Jun 22, 2017 at 18:09
  • \$\begingroup\$ @TonyM isn't this wildy unstable with temperature? \$\endgroup\$
    – tuskiomi
    Commented Jun 22, 2017 at 18:18
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    \$\begingroup\$ @tuskiomi, no it varies with temperature and slightly, not wildly. It's always stable. Let's not start a semantics debate in lots of comments, though. You can look up the temperature variance in the datasheet. \$\endgroup\$
    – TonyM
    Commented Jun 22, 2017 at 18:30
  • \$\begingroup\$ @TonyM i'm just a bit rusty, is all. not looking to debate. \$\endgroup\$
    – tuskiomi
    Commented Jun 22, 2017 at 18:32
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I would say the schematic is incorrect. They should be pointing "down". If they were pointing down, they would give a combined voltage drop (taking the average) of 0.63 *2 = 1.26, putting the voltage in the middle of the active region of the transistor.

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The diodes in such configuration are used for temperature compensation when such circuit gets to work in hot/cold environments so that the target output remains temperature invariant.

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