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Nulling a long-tailed pair

Discussion in 'Electronic Design' started by Spehro Pefhany, Sep 26, 2012.

  1. Okay, I have a long tailed pair (actually a Gilbert cell differential
    output), presumably diffused resistors to Vdd, and there's a DC offset
    and tempco of that offset. Current is about 1mA on each side.

    I tried nulling the differential output voltage by adding a high
    resistance to the lower side to Vdd, and the tempco seems about the
    same magnitude, but the sign flips.

    Is this to be expected? Lucky chance? Experimental error?
     
  2. The resistors and collectors are connected on-chip so I can't get to
    them individually. I suppose I could make a small signal measurement
    of the resistors by assuming the collectors are high-Z and putting a
    small AC signal on the collectors but it might be hard to get that
    number accurately.
    The drift appears to be roughly PTAT relative to the offset.
     
  3. whit3rd

    whit3rd Guest

    The offset voltage of a differential op amp can be trimmed by imbalancing the
    currents in the two input transistors. The temperature coefficient of
    an op amp can be trimmed by balancing the collector currents
    (Hillbiber, an old Fairchild engineer, had a scheme for doing this
    on a uA741 through the trim pins), but the offset voltage suffers.

    Somehow, the uA725 input circuit was designed so that zero tempco
    occurred at zero offset voltage. It was the go-to op amp for many
    years, for microvolt (like, thermocouple) applications.
     
  4. Yes, I remember that pretty well. In this case, I'd be willing to add
    a couple of trims if I can reduce the TCVos by an order of magnitude,
    provided it doesn't require days of measurement in the thermal
    chamber.
     
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