High Vbr(ebo) transistors?

I get the picture.

I have a picture of how to do it with a pair of opamps and MOSFETs. I'll tinker with that a while, and see if I come up with any other ideas, then post something someday opening the door to critiques/enhancements/alternatives.

Thanks for the input.

Good day!

The base emitter breakdown is an avalanche effect IIRC. Usually occurs around

I think there's likely an inherent problem here.

Graham

Hello Chris,

That might win you a Nobel prize. Or a patent which you could then trade in for your own tropical island with a landing strip for your new Lear Jet.

I am afraid so.

Now that is like showing a piece of sausage to our dogs and then putting it away again...

Regards, Joerg

Hello Chris,

FETs would give you more voltage range. Most small ones are rated for

Or use a differential approach. For really precise clamp points you can still get arrays, mostly from JPN mfgs nowadays.

Regards, Joerg

How about:

----+------ Op amp inverting input GND ! ! ! \\ ! / ! \\ ! / ! C ! !/ +----! NPN ! !\\ Q2 \\ ! E / ! \\ ! / ! ! ! E ! !/ Vlimit --!

you

clamp

"collector"

It's crude, but why not simply add an emitter diode?

to summing node | | |/ -Vlimit ------| NPN + 2Vbe |> Q1 | '--|>|--------- opamp output

Cheers, James Arthur

Actually, Zetek makes some transistors with rather high inverse beta.

beta.

Yes, they do. The FMMT618, for example, which has a reverse beta higher than a lot of transistors' forward beta.

This simplified circuit, however, does the original job with just one extra component:

to summing node | | |/ -Vlimit ---R1---+---| NPN + Vbe | |>. Q1 - | D1 ^ | '------+-------- opamp output

Cheers, James Arthur

But this forbids a positive integrator output. Ideally we can produce an arbitrary (but more likely an arbitrary but symmetrical) clamp voltage about zero.

Good day!

Just add one more diode and you'll be parasitic action free...

to summing node | V D2 | | | |/ -Vlimit ---R1---+---| NPN + Vbe | |>. Q1 - | D1 ^ | '------+-------- opamp output

...Jim Thompson

beta

produce

Nice, but I don't see the parasitic of concern... oh, thanks, the collector - there it is. (blush)

Cheers, James

Well, if all you want to do is clamp to an arbitrary, but widely toleranced voltage, these guys come in a bipolar version:

Cheers! Rich

produce

toleranced

Back-to-back zeners are sometimes used this way, but they're leaky, soft-kneed, and the TransZorb variety's 100pF to 10nF of voltage-dependent junction capacitance in particular might befuddle a precision integrator.

But the *real* objecti>(Please don't suggest another specific method yet!)

So, no fair suggesting new methods yet ;-)

James

Yes, zeners are an option. At this point I am going to use Zeners because an adjustable option is added complexity I can't afford space for on the board. A later daughterboard add on with a little uC for thermal management will perhaps also include potentiometer settable current limits.

But Rich, Transorbs are a little hefty for this sort of application.

The Vishay BZX55-C series has 2% tolerances available in stock from Newark. I think a small signal zener is a better choice here.

Good day!

toleranced

Have a look at AD8036-37 + fixed gain.

Hello Chris,

Over the years I had to design out a lot of zeners for clients. Tolerances became unbearable. Other times their data sheets appeared to be a bit on the optimistic side, at least for high volume production. Mostly I use things like the TLV431 instead, maybe you could concoct something around two of these.

Regards, Joerg

Yes, I did give that a look. Awfully high current consumption, obviously since it is far wider band than I need. But the main spoiler is low voltage range.

It's too bad there are no limiting amps in lower power and "GP" implementations.

Good day!