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Okay, so I put a 20kΩ resistor between a 5V supply and the base of a 2N2222 with the collector connected directly to the 5V rail and emitter to ground. What I'd expect is that, roughly, 220μA (i.e. (5V-0.6V)/20kΩ) goes into the base and less than 22mA (i.e. 220μA*100) comes out of the emitter. This isn't what happens at all. I tend to get over 45mA coming out of the emitter.

First, I thought it was my resistor precision. I checked and the actual resistor value is 20.2kΩ (0.1Ω resolution).

I thought it might be the specific transistor but I tried two other 2N2222's. The first was from Mouser and the second two from China. All give roughly the same result.

I then thought maybe it was because I'm using a noisy buck converter. I switched to a linear regulator that gives 4.999V stable for several volts over the +1.6V.

I checked the circuit in CircuitLab using the exact values of my configuration and it gives me what you'd expect: 215.0μA in to the base and 23.17mA out of the emitter.

What's going on? Is this happening because I have no load? Is it because I'm doing this on a breadbord? Do I need capacative decoupling/bypass? Or do I seriously have 3 broken/fake transistors where one of them came from Mouser? If so, why are they all giving the same output?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ Which part of "somewhere between 35 and 300" doesn't match your observations? \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Oct 27, 2016 at 18:08
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    \$\begingroup\$ Where did you get the impression that a real physical 2N2222's gain is precisely and repeatably 100? \$\endgroup\$
    – brhans
    Oct 27, 2016 at 18:09
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    \$\begingroup\$ Congratulations, Anthony, you're measuring the actual gain (under these conditions) of each individual 2N2222. Note that the datasheet only specifies a minimum gain, and the conditions under which it has been tested. \$\endgroup\$
    – Ian Bland
    Oct 27, 2016 at 18:14
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    \$\begingroup\$ @brhans : from simulation ;-) \$\endgroup\$
    – user16324
    Oct 27, 2016 at 18:14
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    \$\begingroup\$ @Kaz: Monte Carlo analyses like the one you're describing are pretty easy to set up in LTSpice: k6jca.blogspot.ca/2012/07/…. The default distribution is flat, but the comments on that blog post describe setting it up for Gaussian. \$\endgroup\$
    – Synchrondyne
    Oct 27, 2016 at 22:14

2 Answers 2

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Here's a snippet from the Fairchild datasheet for the 2N2222.

enter image description here

As you can see, the h_FE (beta) is highly variable, ranging from 35 (at low temperature) up to a maximum of 300. So, the gain you're measuring is well within limits.

This is not unusual for a BJT - the current gain of these devices varies wildly from lot-to-lot and with temperature. This is why most BJT-based amplifiers implement some type of feedback to remove the dependence on h_fe at the expense of gain.

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    \$\begingroup\$ (+1) Especially for highlighting the wild variation from lot to lot. This is often understated even in university courses and books. They almost always introduce BJTs presenting the classic common emitter amp with NO feedback, which is practically an useless circuit in most situations. The only book I've read that actually points out this fact from the beginning is The Art of Electronics, from Horowitz and Hill. \$\endgroup\$
    – LorenzoDonati4Ukraine-OnStrike
    Oct 29, 2016 at 6:14
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Hfe or Beta is manufactured with a range of max:min over a 4:1 range and often binned to 2:1 ranges with suffix letter. This is at one test condition only and not limited to the dynamic changes in hFe with other parameters.

Getting hFE 200 when you expected 100 is unrealistic. Read the datasheet and look for Min Typ and max 200 is normal.

enter image description here

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