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How to control a constant current source?

Discussion in 'Electronic Design' started by Fox zhou, Oct 26, 2004.

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  1. Fox zhou

    Fox zhou Guest

    Dear all,

    I'd like to charge a capacitor with a constant current source in short
    period of time(1ns-10ns). At the end of the time, I want to stop the
    charging. I propose to use a switch to control the constant current
    source. Turn on at the start and turn off at the end.

    But where can I find the corresponding switch? (1GHz)
    Does any one know there are the controllable constant current source
    chips can complete this task?

    --Fox
     
  2. yigiter

    yigiter Guest

    you can use a rgulator for constant current source. for example in 7805
    datasheet, you can find a sample circuit. for switching, i have not got
    much idea but maybe you can use a microcontroller to adjust the time.
     
  3. Rich Grise

    Rich Grise Guest

    Yeah, there ya go.

    Submit a PIC circuit with a 7805 that outputs a 10 nsec ramp with
    1 nsec resoolution, and switching times of 1 psec.

    OK.

    It won't be from me.

    Good Luck!
    Rich
     
  4. John Larkin

    John Larkin Guest


    The usual way to do this is a PNP differential pair steering a source
    current pumped into the emitters. A typical timing ramp might be
    10-100 ns, 5-10 mA, maybe a few volts of swing, as much as the
    downstream amps or comparators can stand. The current source can be an
    active source (another PNP inside a feedback loop) or just a resistor
    to a highish voltage if extreme accuracy isn't needed. BXC71's work
    well; Ft isn't a big issue here if you drive the bases hard. You can
    use faster PNPs, like the NEC parts, but they tend to oscillate.

    A ramp cap can be reset by an open-drain CMOS part, a regular CMOS and
    a schottky diode, or by a gaasfet. Keep in mind that every
    semiconductor on the charging node is a nonlinear capacitor. 2%
    linearity is easy, 0.1% requires serious technique and tricks.

    A really fast ramp can use a very fast current source gated by two
    schottky diodes operating as a current divertor. You can get packaged
    dual schottkies down to about 0.25 pF. Careful here, schottkies are
    leaky. Really fast ramps (say, 10 ns or less total ramp time) are
    tricky, as parasitics will cause ringing and the ramp will be ugly.


    John
     
  5. John Larkin

    John Larkin Guest

    oops, BCX71.

    John
     
  6. Bill Sloman

    Bill Sloman Guest

    The usual way of doing this is to feed the constant current into the
    common emitter of a long-tailed pair, and put the capacitor in series
    with the collector of one of pair. You can discharge the capacitor
    after you've measure the voltage with some kind of switch - I've used
    FETs, MOSFETs and a second long-tailed pair of complementary
    transistors in various different examples.

    For this sort of speed you want to build the fast-switching
    long-tailed pair with RF bipolar transistors - when I was doing this
    back in the 1990s, we mostly used NPN BFR92 parts, switching a roughly
    10mA current.

    These are 5GHz parts. Farnell now list NPN 10GHz parts for the same
    sort of price. Farnell list two 5GHz PNP parts - the BFT92 and the
    BFT93 - which we found handy as fast recharge sources.

    The controllable constant current source for the tail is easy enough -
    we tended to use LM317 three-terminal volage regulators as the basic
    current sources (see that data sheet for the circuit) but they look
    capacitative at high frequencies, so you need the highest possible
    resistance in series to stop this driving the the transistors of the
    long-tailed pair into oscillation.

    You can also use "constant current diodes" - FETs with the gate tied
    to source - for lowish currents (5mA or lower), but they are just as
    capacitative.
     
  7. nyffeler

    nyffeler Guest

    Are you looking for a time to amplitude converter?
    If so, have a look at
    http://ej.iop.org/links/q24/R3eIq,maJdzdv1f2yjwktw/e003n2.pdf
    or a commercial one on
    http://www.canberra.com/products/692.asp
     
  8. Fox zhou

    Fox zhou Guest

  9. Oliver Betz

    Oliver Betz Guest

    (nyffeler) wrote:

    [...]
    is this a URL with session ID? Or only accessible for registered
    users? It doesn't work here.

    Any other way to get the article?

    TIA,

    Oliver
     
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