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Two feedback paths (again)

Hi guys (which includes women in my vernacular)

I've got what looks like another two feedback loop thingie. (It's simpler so maybe I can get a result.)

I use this as a low noise voltage source.

Vin-+------------------+ | | +Vref-+ | p |\ | o~Rb-| | +|-/ |> GND ||/ | +---+-R-+--+---+--\ ^---out/load. | +-C-+ R | --- R/4 idle +Rf-+ | | | GND---------+ Cf +-------+

Rb~500, R = 10k, C = 10-100nF, Ridle-10k, Rf = 500, Cf = 100uF(Al)

It's a voltage reference/ opamp/ pass element/ filter.

There's some RC filtering between pot and opamp that is not shown.

So this has some problems. It's got an output impedance of Rf/beta several ohms, and the temperature variations in the uncontrolled cap mult. output filter. So I tried taking the RC feedback from the output. Putting the cap mult in the loop. Worked, but lots more noise at low frequency. (broad peak at ~2kHz.) The now uncontrolled node between pass transistor emitter and cap mult collector/base now had about the same broad

2 kHz. noise peak. So If I could split up the feed back task so everything slower than (say) 100ms was from the output, with the faster stuff via the pass element.... that might be nice.

So as I add HF feedback from the pass element node, I've got to subtract it from the DC path. Is that right? (sorry, just thinking out loud at the SED.)

George H.

Reply to
George Herold
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In the South, we have y'all as the nongendered second person plural. You Guys seems to work out west. Strangely, "Ladies" does too; pro quarterbacks have been known to use the term in a huddle.

What I do is take the DC feedback from the very output but take the AC feedback from somewhere sooner in the circuit.

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This works fine, but probably a bit of resistance in series with C3 would improve the load step response.

For best load transient response, you've got to manage the crossover frequency.

A huge polymer aluminum output cap is good, too.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
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Reply to
John Larkin

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

Sounds good. One more stage and you'd have a good old fashioned phase shift oscillator. With just two, it looks something like a Sallen-Key filter -- feedback delayed by another RC. The feedback phase shift isn't

90 degrees, so it's "guaranteed" not to oscillate...

In reality, a couple microseconds around the loop can fill in the last bit of angle required to make an oscillator. Effectively, as the filter's Q approaches infinity, that little extra bit will push it "just past infinity", loop gain goes >1, and it oscillates.

Real numbers are modular at infinity (well, in electronics they are), so "greater than positive infinity" is the same as "greater than negative infinity". So if you drew an RLC equivalent circuit, the loss would be a negative resistance. By extension, a negative resistance amplifier necessarily has a negative Q.

Seconded.

Incidentally, this is also handy with op-amps (notoriously bad at driving capacitance or capacitive cables), shunt regulators (like TL431) and LDOs (i.e., an upside-down TL431) -- the difference with LDOs being, they don't give you the internal FB node, so you're screwed and have to make do with ESR.

Tim

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Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

Load current requirements? Noise level target? ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
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I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Almost forgot... did you try a BOOTSTRAP ?>:-} ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Good idea.

--

John Devereux
Reply to
John Devereux

Rout(DC) = re + Rf/(beta + 1)

The 're' term is why I asked about load current.

...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

    ...Jim Thompson
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ide quoted text -

Hi Jim, Well no real 'target' spec's. I just scribbled down the circuit taking the feedback from the output... then tried it the next day.

The circuit in use (with feed back after the pass transistor) drives a 75 ohm heater from ~30V supply, so ~350 mA. The noise is in the

2-3nV/rtHz range. (I assume there is some 1/f corner at low frequency too, but I don't know where it is.)

George H.

Reply to
George Herold

"No!!!!!", Flails arms over head and runs screaming from room.

George H.

    ...Jim Thompson
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  |     et      |
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ide quoted text -

Reply to
George Herold

OK I hadn't thought that far ahead. re ~25mV/Ic ~ so most of the time less than an ohm.

George H.

    ...Jim Thompson
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  |     et      |
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ide quoted text -

Reply to
George Herold

to inverting input Hey I just stuck in a big ass cap in the feed back path. (The 20uF as shown above.) The output low pass has a tau of 0.5k ohm*100uF = 500ms. and a high pass feed back of R/4 (2.5kohm) * 20uF = 500ms.

This seems to work. Though I have to do some 'real' work today and can't test it totally.

George H.

Reply to
George Herold

[snip]

Why the need for low noise to a HEATER? Surely its mass would low-pass any "power" noise?

Or down to basics... what are you trying to accomplish? ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

You've discovered "lead", to compensate for a capacitive load ;-)

What OpAmp and transistors are you using? ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

No, it can be a good idea in voltage references! :)

You can often use the (boosted) output to source the drive to the reference. So it is almost completely immune to supply variations.

--

John Devereux
Reply to
John Devereux

t

Grin... yeah, Well that's a long story. But the heater is down the bottom of a LN2 probe that is used to measure johnson noise of resistors as a function of temperature. Depending on the resistors used there would be some capacitive pickup from the lines feeding the heater. (they shared a common cable.) So... cap multiplier to the rescue!

Anyway I might have a need in the future for a nice low noise power supply. (And just thinking of ways to improve what I've already done.)

George H.

    ...Jim Thompson
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  |     et      |
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ide quoted text -

Reply to
George Herold

 |

The opamp is an LT1013 (input voltage can go down near the negative power rail) and the transistors are TIP31C's... Both just pieces I have from other projects.

George H.

    ...Jim Thompson
   |    mens     |
  |     et      |
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ide quoted text -

Reply to
George Herold

t

Oh sorry I thought Jim was making a joke. So feed the reference from the output voltage.... Thanks I'll remember that too. In the current circuit I don't think it's noise on the supply rail that is the limiting noise source.... not really sure what it is... (the circuit works so I moved on. :^)

George H.

Reply to
George Herold

I was.

Did that around 1970, built with a uA741 chip and a zener. It's in the TOW missile. Only trick is to make sure it self starts... some OpAmp architectures won't.

...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

You have to make sure the circuit starts up, though.

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 

hobbs at electrooptical dot net 
http://electrooptical.net
Reply to
Phil Hobbs

[snip]

George, Your numbers are confusing. What is the voltage across the heater? What is the supply voltage? What is the reference voltage? ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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