Odd Circuit, OP Amp over OP Amp

[Rich K.]

Why do you think the op. used this topology over the simple circuit 1
example? I thank you for your opinions in advance.

Attached: LTSpice file.

------------------------------------------------------------------------
Version 4
SHEET 1 880 680
WIRE 176 -384 48 -384
WIRE 320 -384 240 -384
WIRE 48 -288 48 -384
WIRE 176 -288 48 -288
WIRE 320 -288 320 -384
WIRE 320 -288 256 -288
WIRE 48 -176 48 -288
WIRE 192 -176 48 -176
WIRE 48 -160 48 -176
WIRE 320 -160 320 -288
WIRE 320 -160 256 -160
WIRE 528 -160 320 -160
WIRE 192 -144 112 -144
WIRE 224 -96 224 -128
WIRE 48 -48 48 -80
WIRE 112 48 112 -144
WIRE 112 48 0 48
WIRE 224 64 224 32
WIRE 192 80 176 80
WIRE 352 96 256 96
WIRE 432 96 352 96
WIRE 528 96 432 96
WIRE 0 112 0 48
WIRE 0 112 -160 112
WIRE 192 112 0 112
WIRE 208 112 192 112
WIRE 432 112 432 96
WIRE -160 160 -160 112
WIRE 0 160 0 112
WIRE 224 160 224 128
WIRE 352 176 128 176
WIRE 432 176 352 176
WIRE 352 208 352 176
WIRE 0 272 0 240
WIRE 224 288 224 272
WIRE 176 304 176 80
WIRE 192 304 176 304
WIRE 288 320 256 320
WIRE 128 336 128 176
WIRE 192 336 128 336
WIRE 224 384 224 352
WIRE 176 464 176 304
WIRE 288 464 288 320
WIRE 288 464 256 464
WIRE 176 512 176 464
FLAG 176 592 0
FLAG 352 288 0
FLAG 528 96 out1
FLAG 224 160 -vcc
FLAG 224 384 -vcc
FLAG 224 32 +vcc
FLAG 224 272 +vcc
FLAG -96 480 0
FLAG 48 480 0
FLAG 48 400 -vcc
FLAG -96 400 +vcc
FLAG -160 240 0
FLAG 0 272 0
FLAG 48 -48 0
FLAG 528 -160 out2
FLAG 224 -96 -vcc
FLAG 224 -192 +vcc
SYMBOL Opamps\\LT1022 224 256 R0
SYMATTR InstName U1
SYMBOL Opamps\\LT1022 224 32 R0
SYMATTR InstName U2
SYMBOL res 336 80 R0
SYMATTR InstName R1
SYMATTR Value 10k
SYMBOL res 336 192 R0
SYMATTR InstName R2
SYMATTR Value 1k
SYMBOL res 160 496 R0
SYMATTR InstName R4
SYMATTR Value 1k
SYMBOL res 272 448 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R5
SYMATTR Value 10k
SYMBOL voltage -96 384 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V2
SYMATTR Value 9
SYMBOL voltage 48 496 R180
WINDOW 0 24 96 Left 2
WINDOW 3 24 16 Left 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 9
SYMBOL voltage -160 144 R0
WINDOW 3 -118 141 Left 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR Value SINE(0 0.001 100000)
SYMATTR InstName V3
SYMBOL res -16 144 R0
SYMATTR InstName R3
SYMATTR Value 50
SYMBOL cap 416 112 R0
SYMATTR InstName C1
SYMATTR Value 4.7pf
SYMBOL Opamps\\LT1022 224 -224 R0
SYMATTR InstName U3
SYMBOL res 272 -304 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R6
SYMATTR Value 10k
SYMBOL res 64 -64 R180
WINDOW 0 36 76 Left 2
WINDOW 3 36 40 Left 2
SYMATTR InstName R7
SYMATTR Value 1k
SYMBOL cap 240 -400 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C2
SYMATTR Value 4.7pf
TEXT -192 616 Left 2 !.tran 0.001
TEXT 584 -168 Left 2 ;Circuit 1
TEXT 584 96 Left 2 ;Circuit 2
TEXT 368 -88 Left 2 ;The designer used this in a 100kz amp.
TEXT 368 -32 Left 2 ;The orignal op amps were TL084's

 

[[email protected]]

It may be just a scheme to improve _net_ loop-gain-bandwidth-product.



I have a paper somewhere on such multi-OpAmp schemes. I'll post when

I find it.



...Jim Thompson

--

| James E.Thompson | mens |

| Analog Innovations | et |

| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |

| San Tan Valley, AZ 85142 Skype: Contacts Only | |

| Voice480)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.

We've seen this before, it is a minimal phase passband phase deviation scheme. Recalling that a phase lag in the feedback network requires a compensating phase lead in the forward amplifier response, the idea is to obtain a flat phase response over as much as the passband as possible ( hence the matching OAs with identical gains).

 

[[email protected]]

On Wednesday, October 9, 2013 7:43:39 PM UTC-4, Jim Thompson wrote:

See Analog Devices App Note AN-107 Active Feedback Improves Amplifier Phase Accuracy - James Wong (1987) .

 

[Robert Baer]

Why the hell can't you get a killfile to work?

Don't bug us with YOUR problems.

 

[Phil Hobbs]

See Analog Devices App Note AN-107 Active Feedback Improves Amplifier
Phase Accuracy - James Wong (1987) .

That's the trick where they used a dual, and put the input of the other
op amp in series with the feedback? It was in a PMI app note back
around then. I think it also wound up in the OP227 datasheet, or maybe
it was the OP270.

Cute idea that I've never actually needed to use.

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

 

[[email protected]]

Seems to me this would also improve linearity by "mirroring" whatever non-linearity exists.

I wonder if such a scheme might be useful for fanatic level, goldenears audiophiles....

BTW, I ain't one. The people who claim to hear the difference in 0.007 % and 0.005 % THD are more likely psychic than have that much aural accuity. However, if there is money to be made I'm all ears. (pun intended)

 

[Phil Hobbs]

If you want less error, why not buy a faster opamp, or cascade two amps, each
with gain of sqrt(11)?

It makes the phase error second order, which is a bigger win than either
of those, if low phase error is what you care about. I haven't read it
in some years, but iirc the phase error was in the millidegrees out to
1/10 of the 3 dB cutoff.

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

 

[George Herold]

On Wednesday, October 9, 2013 7:43:39 PM UTC-4, Jim Thompson wrote:



See Analog Devices App Note AN-107 Active Feedback Improves Amplifier Phase Accuracy - James Wong (1987) .

Interesting, thanks. (another old trick that I didn't know.)

Say would this improve (reduce) the Q-enhancement one sees in State-variable filters? (Or other integrating filter topologies)

George H.

 

[Rich K.]

On Wednesday, October 9, 2013 7:43:39 PM UTC-4, Jim Thompson wrote:

See Analog Devices App Note AN-107 Active Feedback Improves Amplifier
Phase Accuracy - James Wong (1987) .

-----------------------------------snip---------------------------------

Spot on! Thanks Fred. Now to crank up LT Spice and play around a bit.

RichK.

 

[[email protected]]

Say would this improve (reduce) the Q-enhancement one sees in State-variable filters? (Or other integrating filter topologies)

Dunno- have never worked with them.

 

[Rich K.]

Don't bug us with YOUR problems.

Thanks all. Much food for thought.

 

[George Herold]

The easy way to cure that, AND dissipation factor problems as well...
<http://www.analog-innovations.com/SED/StateVariableFilter(P+1).pdf>

Interesting, Thanks!.. I've never had much of an issue with cap dissipation.. (I just use expensive caps :^)

One 'issue' with your circuit is it's got a lot more R's to change if one wants to change the Q or freqeucny. (Hmm can I get a Q higher than 2?)

You should write a book with your circuit ideas, implementations. I'd pay a few hundred for it... maybe you can get some cute young thing to help with editing...(as long as the wife doesn't object. :^)

George H.

 

[[email protected]]

"if you are
using a quad, so you can easily get a complex zero as well. "

I wonder why there are so few with three, or five in the package. Why not seven ?

 

[John Devereux]

using a quad, so you can easily get a complex zero as well. "

I wonder why there are so few with three, or five in the package. Why not seven ?

Is it something to do with common centroid design?

 

[George Herold]

Why do you think the op. used this topology over the simple circuit 1

example? I thank you for your opinions in advance.

Attached: LTSpice file.

SHEET 1 880 680

WIRE 176 -384 48 -384

[snip, full listing at Message-ID: <[email protected]>]

TEXT 368 -88 Left 2 ;The designer used this in a 100kz amp.

TEXT 368 -32 Left 2 ;The orignal op amps were TL084's

Getting back to actually looking at it, I'm not sure if it does
_anything_ at all.

I cut out the cap's and then pushed the freq up around 1MHz or so and it showed the some Bandwidth improvement. (But I didn't check any numbers... just played.)

George H.

 

[Tim Williams]

Always found it strange that there have been so many chips in DIP14 (and
SO-14 and others since) with scads of N/C pins. uA723, LM319, even a
MCP4922 DAC I'm playing with. They'd fit quite nicely in 10 pin packages,
but no one ever invested in that, DIPs have always gone 4, 6, 8, 14, 16,
....

I have seen MSOP and DFN styles with nontraditional pin counts. I guess
miniaturization is finally forcing frugal pin counts on such things.

Tim