Two feedback paths (again)

[George Herold]

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<--|+\ |/
t | >~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.

 

[Tim Williams]

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.

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

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

 

[John Devereux]

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

Good idea.

 

[George Herold]

Load current requirements?  Noise level target?

                                        ...Jim Thompson
--
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I love to cook with wine.     Sometimes I even put it in the food.- Hide quoted text -

- Show 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.

 

[George Herold]

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

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

George H.

 

[George Herold]

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

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

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

George H.

 

[George Herold]

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<--|+\     |/
          t   |  >~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)

Vin-+------------------+
| |
+Vref-+ |
p |\ |
o<--|+\ |/
t | >~Rb-|
| +|-/ |>
GND ||/ |
+--20uF(?)-+---+--\ ^---+---out/load
| R | --- |
R/4 idle +Rf-+ |
| | | |
GND---------+ Cf |
+-------+ |

to inverting input <----R=10k----+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.

 

[John Devereux]

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

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.

 

[George Herold]

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

George H.

[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?

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.

 

[George Herold]

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

What OpAmp and transistors are you using?

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.

 

[George Herold]

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.

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.

 

[George Herold]

On Mon, 28 Jan 2013 18:07:11 -0800 (PST), George Herold
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<--|+\ |/
t | >~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.
Load current requirements? Noise level target?
...Jim Thompson

[snip]

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.

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

Oh sorry I was a bit loose with the numbers. (And a bit loose with
the schematic.)
The reference is an LM4040 - 5.0 volts
The supply is 30 volts, 0-25 volts across the 75 ohm heater


It's really a +/-15 volt supply, but the above circuit treats the -15V
rail as ground.... well that's not quite correct either. There's a
cap multiplier on the negative rail...and the output of that is what
I've called ground above.


I didn't want the heater to put any currents through the 'real'
ground.

George H.

 

[John Devereux]

Wed, 30 Jan 2013 07:47:52 -0800 (PST), George Herold

Looks good. Your "big ass" lead capacitor fixes the day. I did a
round with the load current as 100mA...

http://www.analog-innovations.com/SED/GeorgeHeroldsCapMultiplier_NoiseAnalysis.pdf

I tested it with substantial (up to 1uF) cabling capacitance and the
phase margin stays put.

Student question for the day... how did I make the noiseless resistor?

Voltage controlled current source?

 

[John Devereux]

Jim Thompson <[email protected]> writes:

[...]

Voltage controlled current source?

Yep. Otherwise known in Spice parlance... a G-source.

Well now I feel clever; I "invented" that one myself couple of years ago
for same reason (noiseless).

 

[George Herold]

Looks good.  Your "big ass" lead capacitor fixes the day.  I did a
round with the load current as 100mA...

http://www.analog-innovations.com/SED/GeorgeHeroldsCapMultiplier_Nois...

I tested it with substantial (up to 1uF) cabling capacitance and the
phase margin stays put.

Wow, cool.
I was wondering if I needed some resistance in the highpass feedback
path..?
(Would that add more gain?)

Say if you look at the emitter of Q1 do you see a noise peak at about
~2kHz?
I see just about the same level of noise with either feedback
topology.
I've no idea where it comes from... not a big deal 'cause I see no
sign of it on the output.

Student question for the day... how did I make the noiseless resistor?

In real life or in spice?

George H.

 

[George Herold]

On Wed, 30 Jan 2013 18:57:05 +0000, John Devereux

Jim Thompson <[email protected]> writes:

[...]

Student question for the day... how did I make the noiseless resistor?
Voltage controlled current source?

Yep.  Otherwise known in Spice parlance... a G-source.

Well now I feel clever; I "invented" that one myself couple of years ago
for same reason (noiseless).

Ahh, in real life? If so how?

George H.

 

[Jeroen Belleman]

@devereux.me.uk> wrote:

[...]



Student question for the day... how did I make the noiseless resistor?

Voltage controlled current source?

Yep. Otherwise known in Spice parlance... a G-source.

Well now I feel clever; I "invented" that one myself couple of years ago
for same reason (noiseless).

Ahh, in real life? If so how?

George H.

Perhaps not altogether noiseless, but it *is* possible to make
active room-temperature circuits presenting a resistive impedance
with noise temperatures (well) below ambient.

Jeroen Belleman

 

[John Devereux]

@devereux.me.uk> wrote:

[...]



Student question for the day... how did I make the noiseless resistor?

Voltage controlled current source?

Yep.  Otherwise known in Spice parlance... a G-source.

Well now I feel clever; I "invented" that one myself couple of years ago
for same reason (noiseless).

Ahh, in real life? If so how?

Well, in real life in LTSpice.

You mean LTSpice isn't real life?

Another one is those handy 1E9H inductors as noiseless loads

 

[George Herold]

]
Student question for the day... how did I make the noiseless resistor?
Voltage controlled current source?
Yep.  Otherwise known in Spice parlance... a G-source.
Well now I feel clever; I "invented" that one myself couple of years ago
for same reason (noiseless).

Ahh, in real life?  If so how?

George H.

Perhaps not altogether noiseless, but it *is* possible to make
active room-temperature circuits presenting a resistive impedance
with noise temperatures (well) below ambient.

Jeroen Belleman- Hide quoted text -

- Show quoted text -

Are you talking about active damping type circuits?
(The few I've seen always seem to have some other trade off...
like less noise but over a reduced bandwidth.)

Got any examples?

George H.

 

[Jeroen Belleman]

@devereux.me.uk> wrote:

[...]

Student question for the day... how did I make the noiseless resistor?

Voltage controlled current source?

Yep. Otherwise known in Spice parlance... a G-source.

Well now I feel clever; I "invented" that one myself couple of years ago
for same reason (noiseless).

John Devereux

Ahh, in real life? If so how?

George H.

Perhaps not altogether noiseless, but it *is* possible to make
active room-temperature circuits presenting a resistive impedance
with noise temperatures (well) below ambient.

Jeroen Belleman- Hide quoted text -

- Show quoted text -

Are you talking about active damping type circuits?
(The few I've seen always seem to have some other trade off...
like less noise but over a reduced bandwidth.)

Indeed I do, and yes, there are trade offs. I've mentioned
this a few times on s.e.d., in threads on LNAs. I've made some
with input resistances that appear to have only one third of
the noise a real resistor of that value would have.

It's possible to do much better still if you're willing to
put up with a narrow bandwidth.

Jeroen Belleman