Two feedback paths (again)

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

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? ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
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I love to cook with wine.     Sometimes I even put it in the food.

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

Voltage controlled current source?

--

John Devereux

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

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

--

John Devereux

I don't remember how I came about it... I didn't invent it. I think I ran across an AppNote that mentioned it, tried it... sonnuva gun ;-) ...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.

 |

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.

In real life or in spice?

George H.

    ...Jim Thompson    |    mens     |   |     et      |  |       |

ide quoted text -

: ?

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

Well, in real life in LTSpice.

You mean LTSpice isn't real life?

Another one is those handy 1E9H inductors as noiseless loads :)

--

John Devereux

tes:

rites:

or?

go

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.

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

Or to integrate first and then differentiate in the second stage.

Cheers

Phil Hobbs

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Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

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

|  |

Thanks for the component types. Please note that the new circuit has no shown dc feedback path for the opamp.

If you don't mind i recommend the following version in a LTSpice asc file:

Version 4 SHEET 1 1672 680 WIRE 64 0 -160 0 WIRE 384 0 64 0 WIRE 640 0 384 0 WIRE 704 0 640 0 WIRE 928 0 704 0 WIRE 1168 0 992 0 WIRE 1600 0 1168 0 WIRE 704 16 704 0 WIRE 384 32 384 0 WIRE 64 48 64 0 WIRE 352 48 320 48 WIRE -160 64 -160 0 WIRE 528 64 416 64 WIRE 640 64 608 64 WIRE 352 80 288 80 WIRE 736 112 704 112 WIRE 192 128 64 128 WIRE 736 144 736 112 WIRE 736 160 736 144 WIRE 992 160 736 160 WIRE 1120 160 1088 160 WIRE 1168 160 1120 160 WIRE 1216 160 1168 160 WIRE 1344 160 1216 160 WIRE 320 192 320 48 WIRE 464 192 320 192 WIRE 736 192 736 160 WIRE 736 192 528 192 WIRE 1344 192 1344 160 WIRE 288 208 288 80 WIRE 288 208 192 208 WIRE 736 224 736 192 WIRE 928 224 736 224 WIRE 192 240 192 208 WIRE 736 240 736 224 WIRE 1600 240 1600 0 WIRE 64 256 64 128 WIRE 1216 256 1216 160 WIRE 320 288 320 192 WIRE 464 288 320 288 WIRE 576 288 544 288 WIRE 1344 288 1344 256 WIRE 1040 304 1040 224 WIRE 1040 304 928 304 WIRE -160 352 -160 144 WIRE 928 352 928 304 WIRE 320 368 320 288 WIRE 1344 416 1344 368 WIRE 576 448 576 288 WIRE 1120 448 1120 160 WIRE 1120 448 576 448 WIRE -160 512 -160 432 WIRE 64 512 64 320 WIRE 64 512 -160 512 WIRE 192 512 192 320 WIRE 192 512 64 512 WIRE 320 512 320 448 WIRE 320 512 192 512 WIRE 384 512 384 96 WIRE 384 512 320 512 WIRE 736 512 736 320 WIRE 736 512 384 512 WIRE 928 512 928 416 WIRE 928 512 736 512 WIRE 1216 512 1216 336 WIRE 1216 512 928 512 WIRE 1344 512 1344 496 WIRE 1344 512 1216 512 WIRE 1600 512 1600 320 WIRE 1600 512 1344 512 WIRE 192 544 192 512 FLAG 192 544 0 FLAG 1168 160 vout FLAG 640 0 vsupply FLAG 736 144 vq1e FLAG 1168 0 vsripple SYMBOL res 48 32 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL res 176 112 R0 SYMATTR InstName R2 SYMATTR Value 75k SYMBOL res 176 224 R0 SYMATTR InstName R3 SYMATTR Value 25k SYMBOL res 720 224 R0 SYMATTR InstName R4 SYMATTR Value 10k SYMBOL res 912 208 R0 SYMATTR InstName R5 SYMATTR Value 500 SYMBOL res 624 48 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 500 SYMBOL cap 912 352 R0 SYMATTR InstName C1 SYMATTR Value 100? SYMATTR SpiceLine V=35 SYMBOL cap 528 176 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C2 SYMATTR Value 10n SYMATTR SpiceLine V=50 SYMBOL res 448 304 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R7 SYMATTR Value 10k SYMBOL npn2 640 16 R0 SYMATTR InstName Q1 SYMATTR Value TIP31C SYMBOL npn2 992 224 R270 SYMATTR InstName Q2 SYMATTR Value TIP31C SYMBOL voltage -160 48 R0 WINDOW 123 0 0 Left 0 WINDOW 39 24 132 Left 0 SYMATTR SpiceLine Rser=0.1 SYMATTR InstName V1 SYMATTR Value 12 SYMBOL References\\LT1009 64 288 R0 SYMATTR InstName U2 SYMBOL res 1200 240 R0 SYMATTR InstName R8 SYMATTR Value 100 SYMBOL res 304 352 R0 SYMATTR InstName R9 SYMATTR Value 2.5k SYMBOL voltage 1344 272 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value SFFM(0 1 20k 0.3 1k) SYMBOL cap 1328 192 R0 SYMATTR InstName C3 SYMATTR Value 1m SYMATTR SpiceLine V=25 SYMBOL res 1328 400 R0 SYMATTR InstName R10 SYMATTR Value 250 SYMBOL voltage -160 336 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value SFFM(0 0.1 10k 0.3 500) SYMBOL cap 992 -16 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C4 SYMATTR Value 100? SYMBOL res 1584 224 R0 SYMATTR InstName R11 SYMATTR Value 1k SYMBOL Opamps\\LT1013 384 0 R0 SYMATTR InstName U1 TEXT -192 568 Left 0 !.tran 0 10m 0 1u

--
There is a little vout ripple due to the load disturance generator i 
included.  The source disrurbance generator does not seem to affect 
matters much. 

?-)

alysis.pdf

A vccs will do it just fine.

And that has different costs. DC is lost unless you do feed-beside for that. Think 'scope vertical amplifiers.

?-)

|  | o

See above for DC feed back.

:

Yeah That's just about what I have.... I used a much bigger C2. I figured the DC path has a 500ms time constant so the AC path should turn on at about the same frequency.

Thanks for the LTspice circuit BTW.

George H.

|    |  |

totally.

has no

file:

Ok. Similar to the LTSpice circuit i posted. It is a useful tool, but the very incomplete libraries (even of LTC parts) can be very frustrating.

?-)

That goddamn Greek mu again?

-- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

Here's the original schematic...

< >

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