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Odd Circuit, OP Amp over OP Amp

Discussion in 'Electronic Design' started by Rich K., Oct 9, 2013.

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  1. Rich K.

    Rich K. Guest

    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
     
  2. Guest

    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).
     
  3. Guest

    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) .
     
  4. Robert Baer

    Robert Baer Guest

    Don't bug us with YOUR problems.
     
  5. Phil Hobbs

    Phil Hobbs Guest

    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
     
  6. Guest

    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)
     
  7. Phil Hobbs

    Phil Hobbs Guest

    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
     
  8. 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.
     
  9. Rich K.

    Rich K. Guest

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

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

    RichK.
     
  10. Guest

    Dunno- have never worked with them.
     
  11. Rich K.

    Rich K. Guest

    Thanks all. Much food for thought.
     
  12. 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.
     
  13. Guest

    "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 ?
     
  14. Is it something to do with common centroid design?
     
  15. 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.
     
  16. Tim Williams

    Tim Williams Guest

    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
     
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