While troubleshooting a simple single-transistor amplifier circuit recently, I found that I have what appears to be a bad batch of 2N2222As.
The DC biasing of my original circuit is attached (bias1.png). The base voltage should be at 3.6v, putting the emitter voltage at 3v, and emitter/collector current at .5mA. The base bias resistors are much stiffer than they need to be, so the impedance looking into Q1’s base (R3 * (hFE + 1)) should have negligible effect on the base voltage, even with abnormally low values of hFE (the lowest minimum hFE listed in the data sheet is 35).
With Q1 omitted from the circuit, I do in fact read 3.66v at the junction of R1 and R2. However, with Q1 inserted into the circuit, the base voltage drops to 3.39v. This means that R2 || (R3 * (hFE + 1)) must be ~2.4K ohms; since R2 is 2.7K, then (R3 * (hFE + 1)) must be ~24K ohms, making hFE only 3.3 (5600 * (3.3 + 1) = 24000)!
Granted, the hFE values listed in the data sheet are given for a particular VCE and IC, so just to check, I replicated the data sheet’s specified configuration (bias2.png). The base voltage should be ~11v, putting the emitter voltage at ~10.4v, making the emitter/collector current 1mA. VCE is then ~10v, matching almost identically to the conditions specified in the data sheet for a minimum hFE of 50.
Again, the measured base voltage with Q1 removed was as expected (11.00v); however, with Q1 inserted, the base voltage rose to 12.30v. This means that there must have been some base-collector current, and the base-collector resistance was low enough to drop the total resistance of R1 || Q1 significantly.
All voltages are well within spec, so there shouldn’t be any breakdown of the collector-base junction, but clearly there is. Changing the power supply voltage for the circuit in bias2.png, it seems that the base-collector break down voltage is about 9v (a noticeable increase in the base voltage occurs when Vcc is at about 18v).
Now, if this was happening to one transistor, I’d just figure that it was one that maybe I had blown at some point in the past and had somehow managed to find its way back into my parts bin. But in fact I have five transistors that exhibit this exact behavior, all marked “P2N2222A E02” (picture attached). Their markings seem to be consistent with those from ON semiconductor (
http://www.onsemi.com/pub_link/Collateral/PN2222-D.PDF), though I couldn’t tell you where I got the transistors from; most stuff I get from digikey, but I do have parts in the junk box from various “grab bags” and the like.
I’ve replaced the offending P2N2222As with other BJTs (Fairchild 2N2222As, 2N4401, MPSA18, 2N5210), and they all work as expected in the described circuits, having practically no effect on the base voltage.
I've never seen this before, so I was wondering if anyone here has any insight/experience with something like this? Is it not unheard of to get a bad batch of transistors? Is there some plausible way they could have all been damaged in the same way? Or is this just the price you pay for those “grab bag” specials? (I’m guessing the latter…
)