Re: Types of diode-connected BJTs

Does E connect anywhere? Or leave open?

Grant.

I'll leave it for Larkin to pontificate, then I'll say :-)

...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
Obama isn't going to raise your taxes...it's Bush' fault:  Not re-
newing the Bush tax cuts will increase the bottom tier rate by 50%

Many thanks to Jasen Betts, George Herold, whit3rd and John Larkin. I learned something useful.

Chris

That's great! It would be useful as a over-voltage protection 'diode' on a low noise front end. Any idea how much current it can handle. Seems like if used as over-voltage protection you might want to short the E and B and get a bit of current gain.

George H.

I'll try the BFT25 with the emitter open and shorted to the base, when I get a chance. I'd be interested in comparing both leakage and capacitance. As I mentioned, fA leakage testing is tedious.

John

Reverse beta lowering the voltage drop? Probably so. C would go up.

I'd always assumed that "microwave" transistors would be leaky for some reason. As Phil pointed out, they make good low-leakage diodes because the junctions are so small.

We created PADS schematic and PCB symbols for the BFT25 as a diode. Schematics get weird and ugly when you use a bunch of transistor symbols as diodes.

I did the testing for a couple of projects. One is a photodiode amp where we want to prevent windups and inject some test currents, and the other is an FTMS preamp where we have a kilovolt of transmit RF millimeters away from a nanovolt receive antenna, and we need to recover quickly but add minimal leakage and capacitance. The resulting circuit is cute but un/fortunately too good to publish in the open. I

*did* Spice it because I *didn't* entirely understand how it would work; too damned nonlinear, too diode dependent, no hard definition of "best."

John

hen

I was just thinking that if E and B were shorted then not all the current would have to flow through the base. There must be some small amount of current gain in this 'backwards' transistor.

George H.

My Fluke seems to output 0.6 mA on the "diode" range. A BFT25A reads

0.843 for the C-B diode, down to 0.771 if I short the base to emitter, so there is some advantage from reverse beta. Capacitance is 0.55 and 0.83 pF respectively. Those are the easy measurements; maybe I'll get around to doing leakage, too.

The high voltage drops at low current suggest a very small chip, no surprise.

Hmmm, delta-V is about 70 mV. Does that imply a reverse beta around

10?

John

ion but then

og

n 'diode'

andle.

That's Great! I'll try and remember to measure a 2N3904/6 on Monday. How did you measure the capacitance? We've got an SRS RCL 'meter' (box) but I've never tried it on an active device.

70mV looks like a bit more than ten at room temp, but at low gain there must be an 'extra' factor of one floating around somewhere.

Can I 'measure' the forward current gain the same way? I've got some old BK meter in the shop.....(I won't know the current)

gonna have to find a transitor

Thanks, another hammer looking for a nail.

George H.

I used my AADE capmeter, with the surface-mount adapter. I'm not sure what the drive volage is, so maybe I should check it with one of my old pale green Boonton analog c-meters; I know it runs about 0.1 volts.

John

That '2N3904' only means a JEDEC specified part, it could vary widely from one manufacturer to another (and from one year to another) because any transistor that meets the loose specifications can be so labelled. If you want a meaningful result for future guidance, it's better to pick a manufacturer-specific part, and maybe even one with some kind of premium specifications, like low noise or high bandwidth.

Hopefully, if NXP or Zetex redesigns, the 'newer' parts don't have the same ID number as the ones they replace. Unless the part is a JEDEC standard and they want to bid on the big jobs that use those standard parts.

BFT25A is Philips/NXP part with 5 GHz bandwidth, only available in surface mount SOT-23 (low stray capacitance due to leads). I'd expect its character to be more consistent than that of 2N3904.