strange defect with MMBT2222

Hello Jamie,

Probably you changed more then just the transistor (also capacitors, resistors, inductors, etc). All of these will have different characteristics (ESR, self resonant frequency, etc).

Maybe the circuit is just at the point of oscillation or no oscillation. Whether or not is will oscillate may depend on some slight variation in transistor characteristics (as observed by your experiments). I would recommend you to trace the problem, when the product goes into mass production.

I don't know what type of circuit it is, but oscillation at 100..150 kHz is mostly not induced by little bit parasitic induction or stray capacitance. . When changing from conventional to SMT, I would expect oscillation at far higher frequencies (in the MHz range as this transistor has power gain above 100 MHz). How did you determine the oscillation? Be aware of anti-aliasing of Digital oscilloscopes.

In my opinion, it is to early to say the the Fairchild transistor is "defect".

Transistors are more likely to oscillate when both base and collector is inductively loaded. When the emitter (w.r.t. ground) is capacitive loaded, it is very easy to get XXXX2222 to oscillate in the MHz range.,

Many emitter follower circuits will oscillate when the base is inductive loaded in combination with a capacitive emitter load (for example the input capacitance of a digital device together with trace capacitance). I frequently make use this type of oscillation in oscillators into the GHz range.

Best regards,

Wim PA3DJS

In the circuit, it flat lines at what I assume is it's 15 ma Slew/Skew point of the ramp signal on the output. In circuit you don't see the 100k..150khz signal, you just see roll over instead of a linear ramp. At least for that time period. I contribute this to the parasitic resistors and caps I put in that is removing the actual oscillation and just producing this effect. This lead me to think there was something wrong in the transistor.

It's used as part of a bipolar output to drive a magnetic coil for a beam steering system to adjust the focus point of an irradiation unit.

I set up a simple common emitter test (DC), using a load resistor on the collector from a 15 volt source. Just biased the BASE via a 10k resistor from a variable trimmer reference, Place a by pass cap (.1) from the base to common. at about 15 ma's, You can see this OSC at the collector. In the real circuit the collector is connected to the + rail and the emitter is driving the next stage via an R. I used 3 different scopes to verify this, I found my 350 Mhz Tek give me the clearest reading . All 3 scopes show the problem at the same place. The other two were cheap digital LCD's.

I don't get oscillation at the final output because of rest of the circuit, I just get ramp level shift at ~ 15 ma's, and it's because the transistor is some how OSC at that point. It Almost act's like a tunnel diode effect.

Maybe, the other vender's worked fine, as well as the old stand by plastic type 2222's

Good point how ever, Since I strictly used a DC test circuit with no inductive components, I don't see this happening. The test circuit was a simple DC test jig.

Thanks

MMBT2222 is On Semiconductor, formerly Motorola. The '2222' makes it the surface mount version of the 2N2222A TO-92 NPN silicon transistor.

Actually, the 2n2222 was a metal-can TO-18 part, right? Not that it matters. Fab lines and therefore some BJT die designs have dramatically changed with time. If you check Fairchild's lineup you'll see many '2222 variants, no doubt positioned to grab sales from whatever p/n the buyer's BOM happens to have. Fairchild may also make '2222 parts at more than one fab, since they purchased or aligned with several other manufacturers.

Yes, the original version was metal. It's an old part.

The 'A' was the TO-92 version, wasn't it? I don't have my older reference books handy, today.

As to your comment at the end, your strictly DC test circuit has "stray" inductive and capacitive components which come from interconnect wires and the device. Can you post a schematic of this stage showing the driving stage and what this is driving? A picture of your test set up can be informative. Have you tried sticking a 47 ohm resistor in series with the base? Is your power supply bypassed near the circuit? As Wim stated, if you're using a digital scope, be sure its not aliasing. Digital scopes are notorious for giving wrong information when you are dealing with unknown signals.

Mark

We got a reply back from our supplier today, Fairchild verified a defect that I found. Guess I wasn't the only one. Our supplier is going to make good on it.. They're getting new stock. where? I don't know and don't care. Long as they're good.

Mean while, i'll used the leftovers I found in my junk drawer made by ON. They work fine, as did all the other non-smt type I had around here.

I haven't seen this possibility mentioned in others' postings--may have just missed it:

When I see "erratic behavior" mentioned (or observe it myself) I immediately think of oscillations at frequencies beyond the capability of the 'scope being used to look at the circuit. A '2222 could easily be oscillating in the couple-hundred MHz region or somewhat higher, and if it is, you can get all sorts of apparently strange things going on in an otherwise simple circuit.

Someone else mentioned trying some other brands, I believe. That seems like a good idea, too, at least for comparison.

We use lots of MMBT2222's, and I haven't seen any notes about strangenesses in them; not sure if Fairchild is an approved vendor for us for that particular part though.

Cheers, Tom

Motorola/On was our only approved source.

Yes, I agree, high-freq RF oscillation. I'd lay the blame to the simple wiring of Jamie's test fixture, which should have one or two judiciously-placed ferrite beads. He gets a transistor with slightly higher fT and lower capacitance, which is normally considered a better transistor, and calls it defective. I see he tells us Fairchild is replacing the defective shipment. It's good when a supplier jumps to say the customer is right, but sometimes they may be wrong.

They might still have the same problem with the new shipment, if it was casued by a slight improvment in the production process and their design is marginal. Using old samples only show that the design 'used to work'.

Likely. The real problem is lack of uniformity as to what an MMBT2222 really is. Everything that matches the DC characteristic is labeled MMBT2222, no matter what the fT.

...Jim Thompson

That was one of the reasons our 'Item Masters' stated what BRAND was acceptable for 99% of our inventory. In a few cases we had the same generic part number with different stock numbers for different products. The company was known for designing circuits with available parts to do the job. Their early designs had way too many 'S.I.T.' components for my taste, but it was the only way to build them, at the time. :(

Even passives that have the same specs can give you fits, at times.

On Oct 1, 7:03 pm, Tom Bruhns wrote: ...

Turns out they are, along with _several_ others (with variations on the prefix letters).

Since the MMBT2222 data sheets I have don't specify a maximum ft, I assume it's up to me to apply them in a way that they have a reasonable chance at stability. I haven't had much trouble with that, but suppose that one day I'll be bit by it. I have been bit by oscillations in other parts (op amps; inputs to high speed ADCs...) enough that I know some of the symptoms. And I'd expect that since the '2222 has some other specs it must meet, the ft isn't going to go too crazy high on me. Seems like for garden-variety small signal transistors, it's the unusual one that DOES spec a max ft -- counter- examples could prove me wrong, and I'd be happy to know about them.

Cheers, Tom