# DC lab amp.

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Well mostly the front end of amp*

I'm at home so I can't post a pic of the lab amp in AoE3, (pg 274?8?)

Win, I hope it's OK to post a dropbox link to a figure from your book?

In this case, I (think, I) need both ends floating, and low current**

I've been trolling Digikey for FET instrument amps. (I should make a map of Vos vs Ibias***) My ascii art is (a bit) better than me trying to explain it. one side of IA, in total four(or two?) R's and four diodes.

V(+)supply-+ | _ D ^ |\ Vsig-+-Rs--+--|+\ | | | \ Rin _ | \ | D ^ | >--- Gnd | | / V(-)-------+ (etc. traditional input protection.) So Rin = 10Meg (or infinity?) and Rs is 1k (at the moment, 10k is ok, but adds noise... most IA's stink noise-wise, I've hardly looked at the (IA) noise.)

So if Vsig is 100V then Rs has to dissipate Vsig^2/ Rs... 1W @Rs=10k!

OK Win, I was going to complain that your 50k resistors were too big.... never mind about that.

Hmm, what about resistors as fuses?

George H.

*Though I'm happy to talk about how to organize the back end of the signal chain.

**And low V offset, though in some sense the offset is OK if you can measure it.

*** there's also an Ioffset number, (not sure what it means)
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I'm thinking 1/4 W through hole, with nice big pads... (for the replacement... 1% TH R's are \$0.01-0.02!!) GH

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Certainly, that's what fair use allows, in order to discuss something n a copyrighted publication.

A solution to the noise issue is to bypass all but say 1k of the resistor with a cap.

Yes, 0.38W dissipation at 150V, in a 0.5W resistor, maybe it's too small?

230V blokes could try out the fuse concept.
```--
Thanks,
- Win```
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(Thanks I figured it would be OK, but since you are here I could ask.)

Oh, I didn't think of that. But I'm not all that worried about the noise.

Well that seems fine to me. I have run resistors a bit hot... (mostly by mistake.) (a 1k ohm 1/4 watt MF TH at ~25V... I noticed it was getting that charred look, fortunately only a few in the field.)

I'm going to design against 60 V overload. So 5k ohm @1W. My biggest unknown is what happens if the circuit is not powered, and the over-voltage is applied. I'll have to test that.

Well better not to have fuses. Well maybe a poly fuse and a resistor... but a higher power resistor is not that expensive.

George H.

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Depletion MOSFETs make good input protection resistors. The LND250 looks like about 1k until you get up to about a milliamp of drain current, then levels off at around 1.5 mA. You need two of them back to back for bipolar protection. That reduces the dissipation and the noise, at the price of a bit of capacitance. (I learned that trick from JL long ago.)

Cheers

Phil Hobbs

```--
Dr Philip C D Hobbs
Principal Consultant ```
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There are higher current/lower resistance depletion fets, like DN2530, to keep the Johnson noise down. Adding one resistor to the back-to-back pair (A Win trick?) allows the resistance/current limit to be controlled.

New trick, for higher currents: Ixys makes SSRs that self-protect, up to hundreds of volts. You can use them as switches/muxes, or just run some LED current all the time and use them as pure current limiters. CPC1540 is one part. Off leakage is picoamps.

```--
John Larkin         Highland Technology, Inc

lunatic fringe electronics```
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Huh, OK thanks. LND250's are rare. I added some LND150's to my power resistor order. They look to max out at a bit less than 2mA at zero Vgs.

And they come in to-93!

Do you still add diodes to the power rails with this scheme? Seems like the internal protection might be able to handle 2mA???

(After scribbling..) Oh you mean back to back in series.. I drew them in parallel first (like an analog switch) but then the body diode is going to be a problem. That also makes JL's added resistor trick work too. (with gates cross connected from the resistor.... cute.)

--D-+-S--+-RRR-+--S-+-D--- G----(-----+ G +----------+ (bad ascii art...)

George H.

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Worse when Google gets through with it!

"Back to back" is inverse series, as in "back to back zeners". The other wa y is "anti-parallel."

The source resistor trick is cute but you have to watch that an ESD event d oesn't blow out the gates via C_dg.

Cheers

Phil Hobbs

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The LND150 and 250 are about the same part. Consistantly close to 1.6 mA.

See AoE3 page 361. Dual LNDs protect an opamp, with the over-range current going into the opamp ESD diodes. Win added R4 between the fets.

```--
John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  ```
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Thanks, AoE3 is a work. I've got some LND's on order from DK. (some beefer ones too... a ~20mA current limit might be useful.)

When there's a high voltage across the fet, what's going on at the drain (HV) end... is there all sorts of breakdown? Does the current get noisier? Hey, (you've probably already thought of this) Phil was talking about capacitance... which I'm picturing as the gate capacitance in parallel with the D-S "resistance". If fast stuff was getting thorough, maybe some R (~1k?) and a ferrite bead in the gate lead... more parts.

George H.

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picturing as the gate

was getting thorough,

Nah, just connect the gates and sources together and the problem goes away. A couple of milliamps into the protection diodes is no big deal if you p ick decent op amps.

Cheers

Phil Hobbs

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Supertex thought to label the sot-23 version of the LND150, to LND250. But they also had the LND150K1, which was a sot-23 package. Distributors stuck to the LND150K1 for inventory, rather than the LND250. So the LND250 died.

```--
Thanks,
- Win```
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y. A couple of milliamps into the protection diodes is no big deal if you pick decent op amps.

OK... I think I should go abuse some opamp inputs. (The only problem with DK orders is it takes a few days for parts to arrive .)

George H.

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e.)

Parts I ordered Monday afternoon arrived Tuesday morning at about 9:30. (It was a pretty big order by my standards, so the extra \$60 was worth it.) No t all distys are that good.

Cheers

Phil Hobbs

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