Protecting inputs of transistors of a low noise amp

In message , dated Fri, 4 Aug 2006, snipped-for-privacy@gmail.com writes

Watch out for onionization, too.

Probably not, since your FETs probably don't draw appreciable input current.

That was a very shallot thing to say.

John

Why not regular diodes?

How much capacitance can you stand? How much leakage?

John

C'ld use if they are more suitable!

No more than couple of pFs. The leakage has to be less than few nAs.

What would be unsuitable about diodes?

If you run them the against the rails and the bias voltage is far enough away from there diodes should be able to fulfill that.

Joerg, Thanks, Also i found this one, with very low reverse leakage currnet.

In message , dated Fri, 4 Aug 2006, John Larkin writes

Yes, I'm a real rapscallion, aren't I?

Or just back-to-back diodes to ground, if the TIA input is nominally at zero volts.

John

I'd look for lower capacitance versions and SMT. Even the old standby, the BAV99, is around 1.5pF. It is one of my favorites for such purposes, mostly because of low cost.

Hello John,

That is often done in radio applications but leads to poor 3rd order intercept, harmonics and all kinds of nasties. I'd pre-bias them in that case.

But the op is not using a 10 element beam on 20 meters - it's a TIA.

Wouldn't pre-bias increase the leakage currents and reduce the SNR? Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:

Oh, that was a real pearl.

John

Hello Mike,

True but he wrote "high voltage transients due to a RF source in proximity". That can modulate in and make a pudding out of the signal to be measured. Unless it's a pulsed source and he can measure in between pulses.

Very little, at least at room temperature. For the BAV99 it's a few nA. The nice thing is that with a few volts reverse bias the typical capacitance is around 0.5pF. Not bad for such a low cost part.

OK, I just got the "high voltage transients" part and skipped the RF source. But that doesn't say much about what kind of transient he wants to protect against.

Isn't that about the same order as the signal he is detecting? I thought TIA used gigohm feedback resistors to get some kind of useful signal from very small currents from a photodiode. He mentioned

"Or can there be leekage issues in the zener which can suck my signal current away from the FETs."

So leakage noise might be an issue.

I wonder if the input of a 74AC04 CMOS gate might be useful as a low- leakage clamp. The spec claims the max leakage is pretty high, say about

Regards,

Mike Monett

Antiviral, Antibacterial Silver Solution:

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That isn't a a very low leakage current. Check out the Calogic PAD5 - 5pA leakage - and the cheaper JPAD50 with 50pA leakage current.

Farnell stock both.

The PAD5 offers a worst case capacitance of 0.8pF at 5V reverse bias, while the JPAD50 can go up to 2.0pF.

Hello Mike,

If it's a radar pulse then often all you can do is gate it out and protect against it so nothing will be fried.

It can be but he wrote in the answer to John Larkin that it should be less than a few nA. So the old BAV99 might still fit the bill.

Maybe that could be done. I don't know his circuit but maybe he could even re-design it for a MOSFET tetrode from the BF9xx series. Those are very low capacitance and often include protection diodes because their market segment is TV tuners which need similar protection. IIRC the BF998 contains diodes and those are pretty cheap.

It's been rather peasful around here with John Woodgate away.