BFR92AW, B-E junction saturation?

What do you mean "the base would not let go"? Do you mean it took a while to turn off current, or the thing went into latch-up, or just what your title suggests -- i.e., it saturated?

A long time ago I saw a Zetex app note that called out a network on the base that went something like this:

.----||----. | | in o---/\/\---o---/\/\---o---o to base

The capacitor was there to yank charge out of a saturated base. I've tried this, and as long as you can give the cap time to charge up it works a charm (having an input voltage that goes below ground works, too, but where do you find it?).

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Have you considered a Schottky clamp, base-to-collector? It could be that there's just too much charge in that Miller-capacitor junction. When the transistor saturates, the depletion region fills with charge (and that raises the Miller capacitance, 'cuz the insulator is thinner).

The base voltage roared to almost 800mV and then when the pulse of even duty cycle goes low for several hundred nsec it would only gradually drop in voltage, down to about 650mV. Since the next pulse came at that point that meant the transistor remained turned on all the time.

Very similar, but I don't have the left resistor. Well, except for the Rdson of the driver which is probably around 10-20ohms.

I can make that easily but it wouldn't help much.

Now that I got this working it turns out my single-diode sampler ain't sampling. Meaning no mountain biking this week until that's fixed. Hurumph!

--
Regards, Joerg 

http://www.analogconsultants.com/

Can't really do that here because any ever so small additional capacitance at the collector node is a problem. But that issue is fixed now by greatly reducing the DC base current. For fast switching the gooser cap is still the same and I get the expected sub-nanosecond pulse. It's just that the effect puzzles me since this is an RF transistor.

--
Regards, Joerg 

http://www.analogconsultants.com/

Check your wiring. With the base resistor bypassed like that the base voltage ought to yank low, unless your source can't sink.

Or is it really that the transistor saturated and is storing like crazy. Try a 2N2369, or equivalent. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

"RF" transistors aren't immune from saturation effects. ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

It sinketh well and hard but seriously, the base won't clear out. If I increase R from 5k to 100k it does and everything is smack dab on spec, as simulated. Including the sampler. It seems the sampler diodes my client got are bad because the next one was a dud as well. Soldered in another brand -> bingo, all woiks.

It must be, somehow. The 2369 is too slow for this. Anyhow, RF transistors used to not do this. Must be a new generational thing ...

--
Regards, Joerg 

http://www.analogconsultants.com/

[...]

P.S.: The sampler now works. The diode was bad, as was the next one out of the bag. Soldered in another brand and all is well. Unfortunately the footprint won't quite fit so we'll have to create some sort of custom combo-footprint for future situations like this.

--
Regards, Joerg 

http://www.analogconsultants.com/

Yep. You young bucks can never get anything right >:-} ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| San Tan Valley, AZ 85142   Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.

But it woiks :-)

Actually almost verbatim as simulated.

--
Regards, Joerg 

http://www.analogconsultants.com/

Is it sampling? What's the sampling rise time?

Oh, I have a Tek 25 ps step generator if that would help.

And these, too...

Non-Boat-Anchors, both.

--

John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com

500uA base drive to saturate at 3.3/10=330uA ??? Are you some kind of maniac?

Yes, you have too much cap on the base, it's holding you on but, all is not lost, try a bakers clamp.

Jamie

That's weird. Now I want to buy some of those parts and investigate. How'd they make it do that?

I'm working on one of those sorts of things too (there's a trade show involved in there someplace). Instead of feeling sorry for ourselves, we could agree to feel sorry for each other!

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

If it's designed for linear operation I'll bet they did something clevah with the doping profile or gommetry of the thing that makes it better in the expected operating range, but way bad when saturated.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Can't say for sure because my scope is too slow but it's a nice crisp

700psec wide pulse. This is the five-buck budget device, the $20 edition is much faster but may never get built if this suffices (which is likely).

Yes, a very nice tool. But I think I am almost much done with this project other than optimization, clean-up, software massaging, figuring our some more SW tricks and so on. Next one up is a fast PWM.

--
Regards, Joerg 

http://www.analogconsultants.com/

I have absolutely no idea, first time I experienced this. It has nothing to do with Miller because it also happens when there is nothing connected to the collector. Seems to be the B-E junction doing that.

Yes :-)

If you use HSMS-286B in there and it leaks like heck or won't sample, try another brand. I seem to have ended with up diodes from a bad batch. Now it all works.

--
Regards, Joerg 

http://www.analogconsultants.com/

Yeah, that sort of thing happens with off-label use of parts. But OTOH that's often the only way to design cost-efficient high-tech gear. In this case the 10c for RF transistors versus the 50c for SRDs did make a difference.

--
Regards, Joerg 

http://www.analogconsultants.com/

To be on the safe side. It's commonly done in pulsers and works, just not on this transistor. One has to keep in mind that they have very little gain.

[...]

--
Regards, Joerg 

http://www.analogconsultants.com/