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variable high resistance

Discussion in 'Electronic Design' started by Walter Harley, Jan 15, 2006.

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  1. Something I was working on recently got me wondering:

    Suppose I want to have an adjustable resistance, that will be linear (that
    is, ohmic) for AC voltages less than 1Vp and audio frequencies. The
    resistance needs to vary from about 50k to 5MEG, controlled by some other
    (analog) voltage or current. The relationship between control signal and
    resistance does not need to be particularly linear or repeatable from part
    to part.

    So far, easy. Now, the slightly-harder part: the power supply is a pair of
    3V lithium cells, and to maximize battery life it would be desirable to draw
    < 100uA or so.

    If it weren't for the current spec, I could use an FET optocoupler like an
    H11F3, I think (not sure if they're ohmic up to 1V at lower resistances, but
    the datasheet seems to suggest that at these high resistances they'd be
    okay). But to get one of those down to 50k takes >1mA through the LED.

    With higher voltage rails I could just use a JFET. But with 3V rails,
    wouldn't the Vgs, and the unit-to-unit variation in Vgs, bite me? With only
    2V between Vss and max negative signal I'm not sure I can keep the channel
    from turning on.

    I suppose one idea would be to use a charge pump to generate a more-negative
    voltage rail. Or are there JFETs that will work with these low voltages,
    without needing to be individually selected for threshold voltage? Or maybe
    a low-power MOSFET? Any other ideas?

    Thanks for any suggestions!

  2. Guest

    logic level fets

  3. Guest

    For a job the other week, I'd graphed the LED current versus resistance
    for a couple of cheap, Silonix, MSL 37V51 opto-resistors, (Farnell
    part 316-8785).

    (rough values)
    LED current CdS resistance
    0 >10M
    8uA 5M
    10uA 2M
    50uA 100k
    100uA 40k
    500uA 8k
    1ma 4k5
    5ma 2k
    10mA 1k

  4. Fred Bloggs

    Fred Bloggs Guest

    Something I was working on recently got me wondering:
    There are JFETs with guaranteed pinch-off less than 1.0V available. Just
    exactly what kind of circuit did you have in mind?- an attenuator?- a VGA?
  5. That would work, then.

    The circuit is as unsophisticated as you can imagine: it's just a variable
    load for an electromagnetic pickup in an electric bass. The issue is that,
    because of the high inductance and DC resistance of these pickups, their
    frequency response and overall tone (including other aspects such as the
    amount to which the drag of the magnetic field damps the strings' motion,
    etc.) depends strongly on the load they see.

    Electric bassists (and guitarists) like to able to affect their "tone" in
    various ways via controls on the instrument. Most modern basses either feed
    the pickups into a passive tone network, or into an opamp-based buffer which
    then drives an active EQ section of some sort. Neither solution covers the
    range of desirable tones; some basses have switches to turn the buffering on
    or off, or some bassists simply use different instruments depending on what
    sound they want.

    Anyway, I'd been wondering whether making the loading continuously variable,
    rather than just switched on or off, would be musically useful. But I
    wanted to be able to get the max impedance higher than what a commonly
    available pot would do - above 1M, the selection of pots gets small. Thus
    the question.

    For the actual project at hand, it looks like there's not much sonic
    difference when the load goes higher than 1M, meaning I don't need an active
    solution at all. So it was more just a matter of curiousity - "if I needed
    this, how would I do it."

    Much appreciated,
  6. Thanks, John. That's less current than I thought they would need.

    But what's the response time like? ISTR the CdS photocells take a long time
    to recover to high resistances - I need something that will track pretty
    close to its final value within 0.1 seconds or less, preferably more like
    ..05 seconds.
  7. [...]
    Slow, yes indeed!.
    Step from 1k to 1M takes 400ms.
  8. Ken Smith

    Ken Smith Guest

    How about a semi-digital method?

    ! !
    ! A1 ----- A2 !
    X --!>----! Ref !---!>-- Y
    ! !
    N ==============! !

    A1 and A2 are good quality buffering amplifiers. The circuits between X
    and Y are arranged so that it is non-inverting. When N = 0, the input
    looks like a 50K. When N is full scale, the input impedance is very high.

    If you want a knob the user turns, replace the DAC with a pot the user

  9. That was the first thing I tried; but it turned into an oscillator (a 21Hz
    square wave, as it happened). But I had some other stuff going on in the
    circuit too, that undoubtedly aggravated the situation - if I need to get
    this working maybe I'll eliminate the extras and give it another try.
  10. Joseph2k

    Joseph2k Guest

    I would consider what is called a transconductance amplifier. I do not know
    how well they do above about 50 kOhms equivalent though. Generalized
    immitance converters may be able to do this also. I see that you have some
    reason for not blindly settling on conductive plastic potentiometors.
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