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Fast switching current mirrors

Discussion in 'Electronic Basics' started by David Moreno, Jun 25, 2004.

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  1. David Moreno

    David Moreno Guest

    Hi !

    I'm currently working in a fast switching current mirror to charge
    linearly a capacitor. The circuit is a normal current mirror with
    a multiplexor attached to the base of the second transistor and a
    generator to switch the current throught it base.

    In the simulation I've found that when you rise the frequency (at
    50MHz or so)
    the current mirror is not fast enought to switch, and a delay appears
    makes the current throught the capacitor to lower. I'm intriguing in
    because the transistor that I'm using are known to work up to 5-6GHz,
    and the
    frecuency of the switching is well below that.

    In the practice the results are simply that the current mirrored is
    the same as the origin current, and the diference increases when you
    the frecuency.

    And finally the question: Some way to make this work faster or better
    the response at high frecuencies ?

    Thanks a lot ! =D

    * E:\ALIENFILES\electronica\current mirror\espejo bft93.asc
    X§Q1 N003 N003 N001 BFT93
    X§Q2 Uc N004 N001 BFT93
    C1 Uc 0 47pF
    V1 N001 0 5
    I1 N003 0 PULSE(0 1mA 0 0.1ns 0.1ns 5ns 10ns)
    V2 N002 0 PULSE(0 5 0 0.1ns 0.1ns 5ns 10ns)
    S1 N003 N004 N002 0 miswitch
    S2 N004 N001 0 N002 miswitch "E:\ALIENFILES\electronica\spice\bft93.spice"
    ..tran 240n
    ..model miswitch SW(Ron=1. Roff=1MEG Vt=2.5V Vh=0 Lser=0 Vser=0)
    ..ic V(Uc)=0V


    * Filename: BFT93_SPICE.PRM
    * Date : September 1995
    * 1: COLLECTOR; 2: BASE; 3: EMITTER;
    ..SUBCKT BFT93 1 2 3
    Q1 6 5 7 7 BFT93
    * SOT23 parasitic model
    Lb 4 5 .4nH
    Le 7 8 .83nH
    L1 2 4 .35nH
    L2 1 6 .17nH
    L3 3 8 .35nH
    Ccb 4 6 0.071pF
    Cbe 4 8 0.002pF
    Cce 6 8 0.071pF
    * Filename: BFT93.PRM Date:
    March 1992
    + IS = 8.35127E-016
    + BF = 4.85648E+001
    + NF = 1.00043E+000
    + VAF = 1.90118E+001
    + IKF = 1.46824E-001
    + ISE = 9.09455E-014
    + NE = 1.74928E+000
    + BR = 1.21832E+001
    + NR = 9.97694E-001
    + VAR = 3.37492E+000
    + IKR = 6.74270E-003
    + ISC = 2.34297E-014
    + NC = 1.44993E+000
    + RB = 1.00000E+001
    + IRB = 1.00000E-006
    + RBM = 1.00000E+001
    + RE = 2.00000E-001
    + RC = 3.80000E+000
    + EG = 1.11000E+000
    + XTI = 3.00000E+000
    + CJE = 1.57034E-012
    + VJE = 6.00000E-001
    + MJE = 3.82204E-001
    + TF = 1.48531E-011
    + XTF = 2.20970E+000
    + VTF = 2.98987E+000
    + ITF = 1.43721E-002
    + CJC = 1.99525E-012
    + VJC = 5.84499E-001
    + MJC = 2.81320E-001
  2. The ft is the frequency that the current gain is unity. The switching
    speed of a transiser can be orders below this. Ft is only a guide.
    You havent posted the actuall circuit (LTSpice?), but from the text it
    looks like your switching the base. I am not going to wade through a
    text schematic foir a better evaluation.

    The fastest way to switch (usually) is to current steer the *output*
    current, not the base current. That is, keep the mirror always on and
    divert the output from the required output to a dummy output. A mos is a
    reasonable way to do this.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  3. David Moreno

    David Moreno Guest

    Ok, Which parameter do you think is most important then for a switching
    current ?
    Yes, LTSPICE. By post do you mean in binaries, no ?
    Yep, sorry U_U. Next time I'll post the file.
    Yes, but when you do that the switching element injects a charge
    in the capacitor that discharge it parcially. Serial switching is not
    really recomendable.
    I've to try this Spice also =D
  4. John Larkin

    John Larkin Guest

    I've done this a bunch of times. Start with a precise, slow current
    source, maybe a high-beta PNP transistor inside an opamp feedback
    loop. That dumps current into a PNP differential pair which steers the
    current into your capacitor or dumps it to ground; wiggle the diff
    pair bases to steer; I'm usually using differential ECL here. For even
    more fun, use the "dumped" current to turn on a transistor that
    discharges the cap when the current source is switched "off". Charge
    injection is minimal.

    You need high-beta transistors to avoid base current:temperature
    errors. Microwave PNPs are tempting, but tend to have low betas and
    like to oscillate in this setup. BCX71K is a nice part, medium-fast
    and high beta. I've done a nice 13 ns linear ramp this way, with a few
    additional tricks.

    What are you building? How fast?

  5. The Ft is good guide. There is no one parameter. Everything is a trade
    No, I meant the LTSpice full text schematic file.
    Of course it is. It is a very useful technique and used a lot, e.g.
    current steered DACs. Whether or not there is significant charge
    injection depends on the particular application. I have certainly done
    this in a charge pump for a PLL where this method was the only one that
    achieved the required speed and accuracy.

    *Whenever* you do a switch, there will be always be some level of charge
    injection. For example, if you switch the base there will be feedthrough
    via Cbc.

    Best Regards,

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  6. David Moreno

    David Moreno Guest

    Hi !
    That's exactly the next circuit that I'm goind to work into. I'll put
    two series resistor to excite the base of the second transistor, and
    some square voltage signal (arount 1Vpp) in the base of first

    I've simulated this topology and It seems to work well, but when you
    raise the frecuency the circuit seems to no respond well (too slow).
    That transistor is too slow for my purposes (around 100MHZ) but I'll
    it a Try with BFT93. Thanks for the Help ! =D
    I'm currently working in Hardware Neural Networks. I'm using this
    mirror to store contributions in a capacitor (as a local memory). I've
    a working circuit at 1MHz or so, but now I'm trying to increase the
    working frecuency.

    Un saludo.
  7. David Moreno

    David Moreno Guest

    There is no shortcut then to select the adecuate transistor. If I
    by the Ft parameter then my transistor are ok for my purposes.
    Ah, It was directly in the message. I've posted also the transistor
    SPICE model, look down in the text of the original post.

    That's something I try to reduce.
    Sorry, I'll try to fix it: Is not really recomendable if you really
    mind about the current injection.
    You are absolutely right.
    At which frecuency ?. I had a very hard time finding analog switches
    as fast as 100MHz, finally found them in Texas Instruments. And the
    charge is not negligble at all if you put them serrially with your
    load (a capacitor in my case).
    But is something that, in the practical circuit, isn't as bad as it
    sounds. Finally I raised the current, and the circuit worked pretty
    well but what if I want to use lower currents ?.
    Thanks for your help ! =)
  8. John Larkin

    John Larkin Guest

    Has there ever been a neural network that actually did something
    useful? I know a few nn enthusiasts, scientists and not engineers, who
    keep suggesting it be used to solve signal processing problems where
    I'm certain it could never work. And so far, it hasn't

    Seems awfully unscientific to me.

  9. John Larkin

    John Larkin Guest

    Oh, this box

    uses differential PNP current steering to create interruptable linear
    ramps; it's based on Pepper's fiendishly clever (but fatally flawed)
    delay vernier patent. I think the clock period here is 80 or maybe 120
    MHz, and they start/stop ramps on clock edges, with picosecond

  10. Nominal 100Mhz, but the switching speed had to be << 1ns.
    This was a 0.18u i.c. cmos process:)
    Its the trade off thing again g(power, sigma, frequency) = 0

    Lower power means something *has* to give, either speed or accuracy.
    There is no such thing as a free lunch.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  11. John Larkin

    John Larkin Guest

    Then there's the very zen-like distributed amplifier that puts
    transistors in parallel without putting them in parallel. This
    transcends Gm/c limits; doesn't work with bipolars, though.

  12. I am only vaguely familiar with this method, but I very much doubt it
    gets around what I posted. Its not a direct Gm/c limit. Its a combined
    power, speed, accuracy limit. The "fundamental" physics limit, in
    principle allows any speed, providing one sacrifices power or accuracy,
    or both. adding devices will increase the current.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  13. David Moreno

    David Moreno Guest

    There are a few, ie:
    Well, I've already build a prototype that is able of recognize
    images with a high level of noise in them. Now I'm more concerned
    to build a basic fast node, then I'll se how to apply it to image
    Why do you say that ?. Neural networks has been a scientific threat
    for about 60 years =).

    Un saludo
  14. David Moreno

    David Moreno Guest

    Wow, the skies of electronic design ... =). Right now I'm
    using FiberGlass and Taconic Boards.
    I'll take a look at this, I hope this will answer me some questions. Thanks !
    I want free lunch ! X)

    Un saludo
  15. Yes. John Larkin is one such example.

    I know a few nn enthusiasts, scientists and not engineers, who
    You mean you always fail to notice when the blond bit gives you the come
    on message?

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  16. John Larkin

    John Larkin Guest

    Are you implying that I might be useful? I thought I was purely

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