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Old 2nd March 2009, 10:01 AM   #51
Bonsai is offline Bonsai  Taiwan
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These are all pretty standard shunt regulators, and I think its quite easy to get better than 80db of load rejection at 20kHz with a bit of effort. Whether or not you can consistently get down to below 100dB at 20KHz given the variables of layout etc is debatable. However, shunt regulators are an extremely useful tool, and op-amps along with FETS make ideal circuits (bipolar pass devices put a nasty pole in th e loop response, so you have to 'slug' the loop response - this leades to bad performance at higher frequencies, although you can mitigate against this a bit by configuring the circuit so the pass element is a follower).

A few points I noted reading through this thread.

1. Reference - noise in itself from the Vref should not be a problem - just use an RC to filter it before it goes into op-amp input reference pin. I guess 1/f noise is the main issue, but op-amps generally excel at PSRR in the sub 10Hz region.
2. LED's as a reference - nice and low noise to boot - but don't forget the -2.2mV temp co-eff. Good idea to stack 2 or 3 of them to raise the reference voltage.
3. The value and type of output cap (i.e. cap across the load) is a critical factor in performance. I ran some sims using the models in the LTSpice library and these include Ls and Rs and make a big difference compared to using a bog standard 'perfect' cap. My regulator performance went from -100dB at 20KHz to -80dB.
4. Op-amp. Make sure your op-amp inputs and output drive can handle the common mode input voltage (many high speed op-amps struggle with inputs within 1-1.5V of the rails. Similarly, use a mosfet with relatively high Vgs(th) to avoid problems where the op-amp output cannot swing low enough.
5. Test your regulator with a square wave load. Easy then to pick up any overshoot or ringing.
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Old 2nd March 2009, 10:18 AM   #52
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Hi Bonsai,

Yes, point 3) in particular is often misunderstood. In fact, you *want* a lossy cap for stability reasons - a standard electrolytic is fine. People often use very expensive boutique film caps here and then are surprised when it oscillates. This is one area where you can have your cake (low cost cap) and eat it too (stable reg)

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Old 2nd March 2009, 11:02 AM   #53
syn08 is offline syn08  Canada
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Excellent coverage Bonsai, I can't think right now of anything to add.

At point 4), though, this could be the reason why analog_sa was running in some issues (opamp motorboating). OPA627 has only +/-12.3V output swing, while IRF610 has a threshold of 2...4V. Certainly, there is a combination of OPA627 and IRF610 that would not work properly, and God knows what's happening when the output swing border condition is encountered. THS4031 I am using swings to +/-13.6V that covers even the worst case (including temperature effects).
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Old 2nd March 2009, 01:26 PM   #54
dodo is offline dodo  Poland
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Why Minnesota Engineering uses bipolar device instead of mosfets in their shunt regulator
I strongly dobut they don't know what they are doing , aspecially they do it for the profit.
(this shunt is a comercial product )

Any explanation ?
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Old 2nd March 2009, 01:27 PM   #55
dodo is offline dodo  Poland
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sorry , schematic of the Minnesota shunt
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Old 2nd March 2009, 01:32 PM   #56
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Because they use a BJT, they must also use R5 and C5 to avoid problems with the output voltage of the opamp.

Your suggestion that anybody who uses a MOSFET doesn't know what they do is just as wrong as suggesting that anybody who uses a BJT doesn't know what they do. The fact that you don't know why someone does something doesn't mean there isn't a good reason for it.

There are many roads that lead to Rome.

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Old 2nd March 2009, 01:36 PM   #57
KSTR is offline KSTR  Germany
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Relax, Jan,

you seem to have read Dodo wrong, in a hurry (double negation can be tricky)

- Klaus
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Old 2nd March 2009, 01:43 PM   #58
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Quote:
Originally posted by KSTR
Relax, Jan,

you seem to have read Dodo wrong, in a hurry (double negation can be tricky)

- Klaus
But Klaus, I am relaxed

I just wanted to indicate that there may be reasons for one or the other we are not aware of. I once used a particular transistor because I had 200 left from another project...

I had no intention to insult Dodo, if it looks like that I apologize.

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Old 2nd March 2009, 02:52 PM   #59
syn08 is offline syn08  Canada
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Guys, I agree that this little regulator is actually more complex than it appears at the first glance. There are literally hundreds of combinations you can build this, and here are a few variables I can quickly think of:

- Bipolar/MOSFET
- Polarity (NPN/PNP or Nchannel/Pchannel, with flipping the opamp inputs)
- Reference type
- Current source to feed the reference (yes, using a resistor to the output or a CCS like a JFET makes a significant difference)
- Ballast resistor or CCS
- Output cap value and ESR+ESL
- Reference bypassing cap value
- Reference bypassing connection (ground/output, yes it makes a very significant difference)
- Other bypassing (control input, like in the Minnesota shunt, yes, it makes a very significant difference).

If somebody with a simulator and enough time could try to optimize and find the best combination of the above, I would be the first to appreciate the effort.

Given the limited time I have, my intention was only to show something that works very good for me (actually probably as good or better than any discrete parallel regulator I can think of, at a fraction of complexity, size and cost).
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Old 2nd March 2009, 05:40 PM   #60
h_a is offline h_a  Europe
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Yes your little regulator is a real run for the money Such a nice and simple beast - I am really looking forward to put it onto my planned line stage. Just for the fun of it

Have fun, Hannes
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