Tee networks in TIA feedback loops

The big nonlinearity on fast ramps is all the diode junctions across the small cap. The bow thing can dramatically improve net linearity, 5 or 10:1 maybe, and seems pretty predictable. It's simply a matter of having no sense of shame.

John

Gotcha. Being ashamed of things that work is silly anyway.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

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Well you could buffer one side of the feed forward, but then again, that device would have the same problem as the 2nd device in the bootstrap, so it is a no win situation.

I'll go back and look at the cap multiplier, but the thing with the shunt is at high frequencies, you end up with a battle of the caps. You drive the circuitry beneath the shunt with a current source from the top rail. [It's current has to be sufficient to drive all your circuitry, plus whatever the shunt regulator requires.] Whatever capacitance you have in that current source forms a divider with the bypass cap of the shunt regulator. The current source could be some of your BF862 in parallel. (Does anyone actually use these jfets in radios? ;-) ) Thus the attenuation at high frequencies is simply a matter of the quality of the bypass cap. The shunt regulator is out of the picture. It is bulletproof.

I ran into a Broadcom patent that looked similar to this, though I had the scheme in production years before Broadcom filed.

I see. Sure, using a shunt regulator would allow putting one or more RC poles on the supply rail without destabilizing the loop by trying to wrap a loop round a many-pole cap multiplier.

The bad news about shunt regulators is that the caps have to be beta times bigger to get the same rejection. It might be interesting to hang e.g. a TL431 on the base of the second transistor above, with feedback taken from the emitter. You'd still need the RC on the output, because the TL431 itself is so much noisier than a transistor.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Don't forget the reverse protection diode across the whole chebang and reverse Vbe protection. There is always a chance that something else shorts out the input rail and then the 47uF cap tries to feed things upstream.

Oh, and the DNLS160 is kinda pricey :-)

--
Regards, Joerg

http://www.analogconsultants.com/

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I guess it is a matter of at what frequency you need the rejection. That is the shunt regulator has good PSRR way beyond the frequency where transistors have a beta. ;-) For mixed mode chips or switchers, this scheme works well because the noise sources are so high in frequency. Probably that is the same reason Broadcom (doing mixed mode comm chips) reinvented what I probably reinvented (but never bothered to patent since I have some scruples).

I have no experience with off the shelf shunt regulators, so I can't comment on the noise. Since they have fallen out of favor, perhaps there are no quiet shunt regulators because there is no market for the product, noisy or quiet. Of course, you could roll your own. I have done bootstrapped voltage references when the shunt regulator is just part of the voltage regulator, but the PTAT part. The tempco isn't an issue since all the circuitry had a headroom with the same tempco. In many situations, you just need power supply rejection and not a tight voltage.

Let see, in 1969, Walt called it insensitivity to input transients. Talk about your prior art!

RC

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Thanks Miso, That's great. I was about to ask for a link or picture.

You see cap multipliers in old circuits too. (once your eyes have been 'tuned' to see 'em.)

George H.

Good points.

The one I posted above was for a 48V supply, which also gets a bit unpleasant when you power it from a +48V lead acid battery--lots of Joergish noises when you hit the switch. An inverted MOSFET in series, with a bit of RC delay in the gate, fixes that pretty well. Shunting the C-E of Q1 and Q2 with a couple of rectifiers in series each would probably do just as well. In principle, you still need that series diode to protect against input shorts

The one I'm using just now is for a +15V photodiode bias supply. The higher current supplies are +-5, so although I would have liked to use a cap multiplier, I didn't have the luxury. They had to be pi networks with 180 uF aluminum polymer electros and 390 uH toroids (you want to know about pricey...). Hopefully two poles is enough there.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

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Reverse Vbe protection! Ouch!

I think that may be what 'took out' a transitor in a fault I could never reproduce. (I designed in a replacable transitor, which is a bit of a cludge.) Some arc's 'n sparks tommorrow now that I have a 'theory' as to the cause.

Thanks Joerg.

George H.

[...]

If you don't need a lot of current, how about this series? Less than 20 cents:

Bigger, about 40 cents:

--
Regards, Joerg

http://www.analogconsultants.com/

We could always start one about global warming :-)

You are welcome. Those are the things that can sneak in via the back door. Very nasty when defects only occur once in a blue moon and none of the operators can remember what they've done differently than usual.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

It's going to hang off a large vacuum chamber with a 50 kW (average!) pulsed CO2 laser and a bunch of motors and other stuff, and it's providing the measurement data that everything else depends on, so I really want toroids. Nobody will notice the extra $5, but they'd sure notice if it were vulnerable to pickup. I'm really tempted to put in a two-stage pi network, just to get another couple of poles.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

If pickup is super-critical here is an old trick:

Assume the toroid core is ideal and keeps the field 100% in. Then you can still have one effective turn in air (the overall "turn" that follows the core) unless the winding trudges back to where it begun (but that in turn can result in unwanted capacitive coupling). Sometimes running the wire back as a loop can cancel most of that. Needs to be "experimented out".

Sometimes winding your own is better than buying. Had to do that on an RF pulser prototype late last week (after my right front paw had healed from poison oak).

--
Regards, Joerg

http://www.analogconsultants.com/

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Hmm, It was myself and not an operator that blew it up. There are a bunch of low noise power suppliles, with current limiting at the input. But behind that, nothing but transistors. I 'tested' the supplies by shorting 'em all, but there very well may have been a big cap on one of the supplies when I blew it. I never tested for that!

George H.

[...]

3rd party power supplies? Yeah, I really hate that. They never state a value for the output cap in their datasheets. That cap is usually behind any and all current limiting so if a short happens its energy is going to be dumped into your circuit at full gusto.

So when I get a new I often open it to see what and how much they've got in there.

--
Regards, Joerg

http://www.analogconsultants.com/

No, the power supply is all mine. (Of course pieces copied from everywhere.) A cap multiplier as Phil posted, but only one transistor.

I may have more questions tomorrow, if I can get it to blow again.

George H.

Thanks. The ones I'm using have the self-reversed winding, where the winding crosses over a core diameter and then keeps going, to get rid of most of the solenoidal field.

Cheers

Phil

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

Just crossing the core in a bee-line may not provide 100% compensation. Nothing ever will but it can pay to bend and experiment. Sometimes you cna get another 3dB for free.

--
Regards, Joerg

http://www.analogconsultants.com/

Please don safety googles when trying that :-)

Seriously, I've seen a close call once.

--
Regards, Joerg

http://www.analogconsultants.com/