A Question about hfe for the 2n5088

[chromesphere]

Hello!

In my quest to learn more i have been messing around with the lpb1 to try and learn as much as i can about this circuit and ultimately more theoretical understanding of electronics, which i feel im having (slow) success with. 

One question that arose last night.  I breadboarded the LPB1 circuit and i'm confident it's working how it should.  But i did notice something about the hfe that didnt add up.   Basically i have a 200mv squarewave (the only siganl i have access to at the time) on the input.  The voltages throughout the circuit seemed to be correct but alas, the circuit is at home and i cant remember them offhand.  Anyway, I measured Ic and Ib to work out the gain of the transistor and i got ib = 8ma and ic = 330ma.  So if hfe=ic/ib then the hfe of the circuit is 42.  Looking at the datasheet for the 2n5088, the transistor has a minimum gain of 300.  Im sure there is a logical reason for it, but im not sure how this works? (having a lower then minimum hfe for a transistor).
Thanks for any help as always!
Paul

[R.G.]

Anyway, I measured Ic and Ib to work out the gain of the transistor and i got ib = 8ma and ic = 330ma. 

330ma? A THIRD OF AN AMPERE??

I'm thinking that if that's correct, it also ought to be glowing in the dark from the excess heat given off.

If that's a correct measurement, I'm not surprised that the HFE fell off. Current gain in a transistor is not a fixed number, and it varies with collector current, among other things. At very low currents, HFE is low, and it increases with collector current til some point where the imperfections catch up with it, and then it decreases with increasing current after that. For a normally low-current 5088, that's way past the point of beta fall-off.

But I wonder if there may be a measurement error.

[chromesphere]

Anyway, I measured Ic and Ib to work out the gain of the transistor and i got ib = 8ma and ic = 330ma. 

330ma? A THIRD OF AN AMPERE??

I'm thinking that if that's correct, it also ought to be glowing in the dark from the excess heat given off.

If that's a correct measurement, I'm not surprised that the HFE fell off. Current gain in a transistor is not a fixed number, and it varies with collector current, among other things. At very low currents, HFE is low, and it increases with collector current til some point where the imperfections catch up with it, and then it decreases with increasing current after that. For a normally low-current 5088, that's way past the point of beta fall-off.

But I wonder if there may be a measurement error.

Aha-ha RG you found my typo!  Supposed to by microamps not millamps sorry
So 'minimum' doesnt technically really mean what it says? The minimum can go lower?   At 300 hfe thats a pretty significant boost...200mv x 300 = 60volts? But then im probably confusing voltage gain with current gain
Paul

[R.G.]

Aha-ha RG you found my typo!  Supposed to by microamps not millamps sorry

No problem. I'm pretty sure I've forced something like a third to a half an ampere through a 2N5088 before - but the didn't live through it. 

So 'minimum' doesnt technically really mean what it says? The minimum can go lower?

Remember that old quip, "Lies, damned lies, and statistics!"? Well, EEs say the same but sub in "datasheets" for "statistics".

Which datasheet are you using? Remember, current gain decreases at low currents too. Pretty much all transistors have a sweet spot where their gain is as large as you could reasonably expect it to be - and guess what current the measurements are taken at for the datasheet. 

At 300 hfe thats a pretty significant boost...200mv x 300 = 60volts? But then im probably confusing voltage gain with current gain

Yes, you are. Generally, a circuit can't provide nearly the voltage gain that the raw devices have current gain.

So - which datasheet? We'll go play some "interpret the datasheet".

[chromesphere]

Hey RG.  Thanks for the response!   I think it was this one:
http://www.futurlec.com/Transistors/2N5088.shtml

If im getting 300ua collector current, doesnt that mean that i would be getting a min 300x current gain?  WHich would basically set fire to the transistor?   I dont quite understand how the circuit produces less hfe then the minimum the datasheet specifies.

Edit: i mean clearly IT DOES.  I'm witnessing the perfectly functioning circuit doing what it does at 40~ hfe...

Paul

[R.G.]

Hey RG.  Thanks for the response!   I think it was this one:
http://www.futurlec.com/Transistors/2N5088.shtml

OK. In glancing at that sheet, the first thing I note is that it's from Motorola, and is dated 1996. Motorola was a solid, first-rate semiconductor maker, at least back when they made semiconductors. They make cell phones and radio systems now, having spun off their semiconductor operations to On Semi. Neither here nor there, but sad.

First question: what company made your specific 2N5088? Some companies have the idea that they can decide that they're selling bananas, only their bananas are spherical and red.

Beyond the gross questions of "is this really a Motorola 2N5088 and not something else entirely?", yep, a Moto 5088 should give you 300HFE at 330uA, according to that datasheet.

If im getting 300ua collector current, doesnt that mean that i would be getting a min 300x current gain?  WHich would basically set fire to the transistor?

300x current *gain* won't set fire to a transistor. It was the 330ma I was worried about. It's the heat that does it, not the gain. MOSFETs have an essentially infinite current gain. Their gates are high-purity glass 20 volts thick. So any current in the drain at all, no matter how small, makes a current "gain" of infinity. It's like saying "Danger: 50,000 Ohms!"

I dont quite understand how the circuit produces less hfe then the minimum the datasheet specifies.

It's one of several things, some of which I touched on above.
- who made this particular transistor?
- is the transistor marked correctly, or possibly counterfeit?
- if it's marked correctly, what does its real manufacturer say about it?
- what circuit conditions are you measuring it under?

This last is important. If you look at the second page of the data sheet you pointed out, at "On Characteristics", you'll find that for all of the DC current gain specifications, Vce is called out as 5Vdc. If you look down at collector-emitter saturation voltage, you find that the conditions are Ic=10ma, Ib=1ma, a gain of only 10. The difference is that in one condition the device has enough voltage to be actively amplifying, and in the other, it's being turned into a short circuit as nearly as possible. Gain drops precipitously when a transistor starts to saturate.

Looking at the top of page 4, you see some of what I was describing about gain varying with collector current. The 5088 (from Motorola...    ) has a gain that's low at low currents, rises to a maximum at between 3 and 5 ma, and then starts decreasing with more current flowing in it. They ... um... don't mention that high current gain decrease, partly because people use high-gain, low noise parts at low current and partly so you don't look at things outside where the wanted it to be used.

Edit: i mean clearly IT DOES.  I'm witnessing the perfectly functioning circuit doing what it does at 40~ hfe...

A lot of good circuit design practice is based on making the circuit work properly with gains between some minimum and infinity.

So - yeah, from the datasheet, you should be seeing higher DC gain. The questions then become do you have the right manufacturer/datasheet/device.

By the way, the persistent high AC gain is one of my reasons for Keen's Second Law: when in doubt, use a 2N5088.

[PRR]

> alas, the circuit is at home and i cant remember them

Paper. Pencil.

GOOD notes (and decimal placing) is essential to knowing what you are doing.

> I measured Ic and Ib to work out the gain of the transistor and i got ib = 8ma and ic = 330ma

How did you measure that base current? Break the base connection and insert meter? Or measure current in the 820K?

Was the circuit "working"?? The bias is pretty fussy and prone to drift into hardly-working operating point. If hFE is high, the collector falls until the transistor is "starved" and its hFE spoiled. If you got 0.1V across C and E, you sure could have very low hFE *at* that point.

[chromesphere]

Thanks for your responses guys! I had a look at the transistor and there's no definitive marking of the maker.  ALl it says is 2n5088-j01 and k3 is stamped inside the circle on the back.  Couldn't find an answer on Google.

Yep PRR, I broke the circuit and measured it at collector and base.  Unfortunately in my enthusiasm I have dismantled it on the breadboard (I need more then one bread board .  I'll have to return to this again.  I think I understand what you guys are saying, but it has fully clicked yet.

Thanks again,
Paul