Odd Circuit, OP Amp over OP Amp

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

Reply to
Rich K.
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[snip]

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

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| James E.Thompson                                 |    mens     | 
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Reply to
Jim Thompson

What's the point?

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

Loop-gain-bandwidth improvement.

Do try to keep up with the technology. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Reply to
Jim Thompson

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

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

We've seen this before, it is a minimal phase passband phase deviation sche me. Recalling that a phase lag in the feedback network requires a compensat ing phase lead in the forward amplifier response, the idea is to obtain a f lat phase response over as much as the passband as possible ( hence the mat ching OAs with identical gains).

Reply to
bloggs.fredbloggs.fred

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

Reply to
bloggs.fredbloggs.fred

Don't bug us with YOUR problems.

Reply to
Robert Baer

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

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

Somewhere in my files, now all boxed up for the move :-(, I have an IEEE paper on several configurations that improve effective GBW for integrators.

I'll post when I find it. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Reply to
Jim Thompson

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

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John Larkin                  Highland Technology Inc 
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Reply to
John Larkin

The scheme dates back to when such OpAmps did not exist, or at least not at a reasonable price. So the need for greater GBW was attained by using two slower OpAmps.

Did you not note the date, 1987, in Blogg's post?

IIRC, the paper I will post, when I find it, shows configurations using up to four OpAmps. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Reply to
Jim Thompson

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)

Reply to
jurb6006

See the section on "Error take-off":

formatting link

Reply to
bitrex

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

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

It does violate the first rule of low-distortion opamp design, "always invert."

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

OK, but the ADI paper shows a roughly 6 dB gain peaking as the cost. And the opamp GBWs have to be matched to 1-2%. I don't think they mention distortion or resistor tolerances.

If you cascaded two inverting stages, each with lower gain, there would be two opportunities for gain peaking, sort of like their 3rd order version.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

I've never used the circuit myself, but there are situations (e.g. your constant-phase bandpass filter) where low phase error is really what you want. I'd be sort of surprised if the GBWs of monolithic duals weren't matched to a percent or two.

Cheers

Phil

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

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.

Reply to
George Herold

Le Thu, 10 Oct 2013 13:52:59 -0400, Phil Hobbs a écrit:

Exactly. I used that exact circuit coupled with modern fast opamps for a

*precision* 100kHz IF amplifier in a high accuracy VNA IF and detector board. Works beautifully. The ppm level sync detector was fun too...

Note that in this case, the HF peaking wasn't important, being out of band, hence filtered...

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Thanks, 
Fred.
Reply to
Fred Bartoli

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